LM2576-5 [MOTOROLA]
Easy Switcher 3.0A Step-Down Voltage Regulator; 轻松切换3.0A降压稳压器型号: | LM2576-5 |
厂家: | MOTOROLA |
描述: | Easy Switcher 3.0A Step-Down Voltage Regulator |
文件: | 总28页 (文件大小:288K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
Order this document by LM2576/D
EASY SWITCHER
3.0 A STEP–DOWN
VOLTAGE REGULATOR
The LM2576 series of regulators are monolithic integrated circuits ideally
suited for easy and convenient design of a step–down switching regulator
(buck converter). All circuits of this series are capable of driving a 3.0 A load
with excellent line and load regulation. These devices are available in fixed
output voltages of 3.3 V, 5.0 V, 12 V, 15 V, and an adjustable output version.
These regulators were designed to minimize the number of external
components to simplify the power supply design. Standard series of
inductors optimized for use with the LM2576 are offered by several different
inductor manufacturers.
SEMICONDUCTOR
TECHNICAL DATA
Since the LM2576 converter is a switch–mode power supply, its efficiency
is significantly higher in comparison with popular three–terminal linear
regulators, especially with higher input voltages. In many cases, the power
dissipated is so low that no heatsink is required or its size could be reduced
dramatically.
A standard series of inductors optimized for use with the LM2576 are
available from several different manufacturers. This feature greatly simplifies
the design of switch–mode power supplies.
The LM2576 features include a guaranteed ±4% tolerance on output
voltage within specified input voltages and output load conditions, and ±10%
on the oscillator frequency (±2% over 0°C to 125°C). External shutdown is
included, featuring 80 µA (typical) standby current. The output switch
includes cycle–by–cycle current limiting, as well as thermal shutdown for full
protection under fault conditions.
T SUFFIX
PLASTIC PACKAGE
CASE 314D
1
Pin 1. V
in
2. Output
5
3. Ground
4. Feedback
5. ON/OFF
TV SUFFIX
PLASTIC PACKAGE
CASE 314B
1
Features
5
• 3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions
Heatsink surface
connected to Pin 3.
• Adjustable Version Output Voltage Range, 1.23 to 37 V ±4% Maximum
Over Line and Load Conditions
• Guaranteed 3.0 A Output Current
• Wide Input Voltage Range
D2T SUFFIX
PLASTIC PACKAGE
CASE 936A
• Requires Only 4 External Components
• 52 kHz Fixed Frequency Internal Oscillator
• TTL Shutdown Capability, Low Power Standby Mode
• High Efficiency
• Uses Readily Available Standard Inductors
• Thermal Shutdown and Current Limit Protection
1
2
(D PAK)
5
Heatsink surface (shown as terminal 6 in case outline
drawing) is connected to Pin 3.
DEVICE TYPE/NOMINAL OUTPUT VOLTAGE
Applications
LM2576–3.3
LM2576–5
LM2576–12
LM2576–15
LM2576–ADJ
3.3 V
5.0 V
12 V
• Simple High–Efficiency Step–Down (Buck) Regulator
• Efficient Pre–Regulator for Linear Regulators
• On–Card Switching Regulators
• Positive to Negative Converter (Buck–Boost)
• Negative Step–Up Converters
15 V
1.23 V to 37 V
ORDERING INFORMATION
• Power Supply for Battery Chargers
Operating
Temperature Range
Device
Package
LM2576T–XX
Straight Lead
Vertical Mount
Surface Mount
LM2576TV–XX T = –40° to +125°C
J
LM2576D2T–XX
XX = Voltage Option, i.e. 3.3, 5, 12, 15 V; and ADJ for
Adjustable Output.
Motorola, Inc. 1999
Rev 1, 07/1999
LM2576
Figure 1. Block Diagram and Typical Application
Typical Application (Fixed Output Voltage Versions)
Feedback
4
L1
100
7.0 V – 40 V
Unregulated
DC Input
+V
in
LM2576
µ
H
Output
2
1
5.0 V Regulated
C
in
Output 3.0 A Load
D1
1N5822
100 µF
C
out
1000
3
Gnd
5
ON/OFF
µF
Representative Block Diagram and Typical Application
+V
in
ON/OFF
Unregulated
DC Input
3.1 V Internal
Regulator
Output
Voltage Versions
R2
ON/OFF
(Ω)
1
5
C
3.3 V
5.0 V
12 V
15 V
1.7 k
3.1 k
8.84 k
11.3 k
in
4
Feedback
Current
Limit
For adjustable version
R1 = open, R2 = 0
R2
Fixed Gain
Error Amplifier
Ω
Comparator
Driver
Regulated
Output
R1
1.0 k
Latch
Freq
Shift
L1
V
out
Output
18 kHz
1.0 Amp
Switch
2
Gnd
1.235 V
C
D1
out
Band–Gap
Reference
Thermal
Shutdown
52 kHz
Oscillator
3
Reset
Load
ABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyond
which damage to the device may occur.)
Rating
Symbol
Value
45
Unit
V
Maximum Supply Voltage
V
in
ON/OFF Pin Input Voltage
–
–0.3 V ≤ V ≤ +V
V
in
Output Voltage to Ground (Steady–State)
–
–1.0
V
Power Dissipation
Case 314B and 314D (TO–220, 5–Lead)
Thermal Resistance, Junction–to–Ambient
Thermal Resistance, Junction–to–Case
P
Internally Limited
W
D
R
R
65
5.0
°C/W
°C/W
W
θJA
θJC
2
Case 936A (D PAK)
P
D
Internally Limited
Thermal Resistance, Junction–to–Ambient
Thermal Resistance, Junction–to–Case
R
θJA
R
θJC
70
5.0
°C/W
°C/W
Storage Temperature Range
T
–65 to +150
2.0
°C
stg
Minimum ESD Rating (Human Body Model:
–
kV
C = 100 pF, R = 1.5 kΩ)
Lead Temperature (Soldering, 10 seconds)
Maximum Junction Temperature
–
260
150
°C
°C
T
J
NOTE: ESD data available upon request.
2
MOTOROLA ANALOG IC DEVICE DATA
LM2576
OPERATING RATINGS (Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed
specifications and test conditions, see the Electrical Characteristics.)
Rating
Operating Junction Temperature Range
Supply Voltage
Symbol
Value
–40 to +125
40
Unit
°C
T
J
V
in
V
SYSTEM PARAMETERS ([Note 1] Test Circuit Figure 15)
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V = 12 V for the 3.3 V, 5.0 V, and Adjustable version, V = 25 V for
in in
the 12 V version, and V = 30 V for the 15 V version. I
junction temperature range that applies [Note 2], unless otherwise noted.)
= 500 mA. For typical values T = 25°C, for min/max values T is the operating
in
Load
J
J
Characteristics
Symbol
Min
Typ
Max
Unit
LM2576–3.3 ([Note 1] Test Circuit Figure 15)
Output Voltage (V = 12 V, I
in
= 0.5 A, T = 25°C)
V
out
3.234
3.3
3.366
V
V
Load
J
Output Voltage (6.0 V ≤ V ≤ 40 V, 0.5 A ≤ I
in
≤ 3.0 A)
V
out
Load
T = 25°C
T = –40 to +125°C
J
3.168
3.135
3.3
–
3.432
3.465
J
Efficiency (V = 12 V, I
in
= 3.0 A)
η
–
75
–
%
Load
LM2576–5 ([Note 1] Test Circuit Figure 15)
Output Voltage (V = 12 V, I = 0.5 A, T = 25°C)
V
out
4.9
5.0
5.1
V
V
in
Load
J
Output Voltage (8.0 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A)
V
out
in
Load
T = 25°C
T = –40 to +125°C
J
4.8
4.75
5.0
–
5.2
5.25
J
Efficiency (V = 12 V, I
in
= 3.0 A)
η
–
77
–
%
Load
LM2576–12 ([Note 1] Test Circuit Figure 15)
Output Voltage (V = 25 V, I = 0.5 A, T = 25°C)
V
out
11.76
12
12.24
V
V
in
Load
J
Output Voltage (15 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A)
V
out
in
Load
T = 25°C
T = –40 to +125°C
J
11.52
11.4
12
–
12.48
12.6
J
Efficiency (V = 15 V, I
in
= 3.0 A)
η
–
88
–
%
Load
LM2576–15 ([Note 1] Test Circuit Figure 15)
Output Voltage (V = 30 V, I = 0.5 A, T = 25°C)
V
out
14.7
15
15.3
V
V
in
Load
J
Output Voltage (18 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A)
V
out
in
Load
T = 25°C
T = –40 to +125°C
J
14.4
14.25
15
–
15.6
15.75
J
Efficiency (V = 18 V, I
in
= 3.0 A)
η
–
88
–
%
Load
LM2576 ADJUSTABLE VERSION ([Note 1] Test Circuit Figure 15)
Feedback Voltage (V = 12 V, I
in
= 0.5 A, V
= 5.0 V, T = 25°C)
V
out
1.217
1.23
1.243
V
V
Load
out
J
Feedback Voltage (8.0 V ≤ V ≤ 40 V, 0.5 A ≤ I
in
≤ 3.0 A, V
out
= 5.0 V)
V
out
Load
T = 25°C
T = –40 to +125°C
J
1.193
1.18
1.23
–
1.267
1.28
J
Efficiency (V = 12 V, I
in
= 3.0 A, V
out
= 5.0 V)
η
–
77
–
%
Load
NOTES: 1. External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the
LM2576 is used as shown in the Figure 15 test circuit, system performance will be as shown in system parameters section.
2. Tested junction temperature range for the LM2576:
T
= –40°C
T
= +125°C
low
high
3
MOTOROLA ANALOG IC DEVICE DATA
LM2576
DEVICE PARAMETERS
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V = 12 V for the 3.3 V, 5.0 V, and Adjustable version, V = 25 V for
in in
the 12 V version, and V = 30 V for the 15 V version. I
junction temperature range that applies [Note 2], unless otherwise noted.)
= 500 mA. For typical values T = 25°C, for min/max values T is the operating
in
Load
J
J
Characteristics
Symbol
Min
Typ
Max
Unit
ALL OUTPUT VOLTAGE VERSIONS
Feedback Bias Current (V
out
= 5.0 V [Adjustable Version Only])
I
b
nA
T = 25°C
–
–
25
–
100
200
J
T = –40 to +125°C
J
Oscillator Frequency [Note 3]
T = 25°C
f
kHz
V
osc
–
47
42
52
–
–
–
58
63
J
T = 0 to +125°C
J
T = –40 to +125°C
J
Saturation Voltage (I
= 3.0 A [Note 4])
V
sat
out
T = 25°C
T = –40 to +125°C
J
–
–
1.5
–
1.8
2.0
J
Max Duty Cycle (“on”) [Note 5]
Current Limit (Peak Current [Notes 3 and 4])
T = 25°C
DC
94
98
–
%
A
I
CL
4.2
3.5
5.8
–
6.9
7.5
J
T = –40 to +125°C
J
Output Leakage Current [Notes 6 and 7], T = 25°C
Output = 0 V
Output = –1.0 V
I
mA
mA
µA
V
J
L
–
–
0.8
6.0
2.0
20
Quiescent Current [Note 6]
I
Q
T = 25°C
–
–
5.0
–
9.0
11
J
T = –40 to +125°C
J
Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”))
I
stby
T = 25°C
–
–
80
–
200
400
J
T = –40 to +125°C
J
ON/OFF Pin Logic Input Level (Test Circuit Figure 15)
V
= 0 V
V
IH
out
T = 25°C
2.2
2.4
1.4
–
–
–
J
T = –40 to +125°C
J
V
= Nominal Output Voltage
V
IL
out
T = 25°C
–
–
1.2
–
1.0
0.8
J
T = –40 to +125°C
J
ON/OFF Pin Input Current (Test Circuit Figure 15)
µA
ON/OFF Pin = 5.0 V (“off”), T = 25°C
I
I
–
–
15
0
30
5.0
J
IH
IL
ON/OFF Pin = 0 V (“on”), T = 25°C
J
NOTES: 3. The oscillator frequency reduces to approximately 18 kHz in the event of an output short or an overload which causes the regulated output voltage to
drop approximately 40% from the nominal output voltage. This self protection feature lowers the average dissipation of the IC by lowering the
minimum duty cycle from 5% down to approximately 2%.
4. Output (Pin 2) sourcing current. No diode, inductor or capacitor connected to output pin.
5. Feedback (Pin 4) removed from output and connected to 0 V.
6. Feedback (Pin 4) removed from output and connected to +12 V for the Adjustable, 3.3 V, and 5.0 V versions, and +25 V for the 12 V and 15 V
versions, to force the output transistor “off”.
7. V = 40 V.
in
4
MOTOROLA ANALOG IC DEVICE DATA
LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
Figure 2. Normalized Output Voltage
Figure 3. Line Regulation
1.0
0.8
0.6
1.4
V
I
= 20 V
1.2
1.0
0.8
0.6
in
I
T
= 500 mA
Load
= 25°C
= 500 mA
Load
Normalized at T = 25
J
°C
J
0.4
0.2
3.3 V, 5.0 V and ADJ
0
–0.2
–0.4
0.4
0.2
0
12 V and 15 V
–0.2
–0.4
–0.6
–0.6
–0.8
–1.0
–50
–25
0
25
50
75
100
125
0
5.0
10
15
V , INPUT VOLTAGE (V)
in
20
25
30
35
40
T , JUNCTION TEMPERATURE (
°
C)
J
Figure 4. Dropout Voltage
Figure 5. Current Limit
2.0
1.5
1.0
0.5
0
6.5
6.0
5.5
V
= 25 V
I
Load
= 3.0 A
in
5.0
4.5
4.0
I
= 500 mA
Load
L1 = 150 µH
R
= 0.1
Ω
ind
–50
–25
0
25
50
75
100
125
–50
–25
0
25
50
75
100
125
T , JUNCTION TEMPERATURE (
°C)
T , JUNCTION TEMPERATURE (°C)
J
J
Figure 6. Quiescent Current
Figure 7. Standby Quiescent Current
20
18
200
V
= 5.0 V
V
= 5.0 V
180
160
140
120
100
80
out
ON/OFF
Measured at
Ground Pin
16
T
= 25°C
J
14
V
= 40 V
in
I
= 3.0 A
Load
12
10
V
= 12 V
in
60
I
= 200 mA
Load
8.0
6.0
4.0
40
20
0
–50
0
5.0
10
15
20
25
30
35
40
–25
0
25
50
75
100
125
V
, INPUT VOLTAGE (V)
T , JUNCTION TEMPERATURE (°C)
in
J
5
MOTOROLA ANALOG IC DEVICE DATA
LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
Figure 8. Standby Quiescent Current
Figure 9. Switch Saturation Voltage
200
1.6
180
160
140
120
100
80
1.4
1.2
T
= 25
°C
–40°C
J
1.0
0.8 25
°C
0.6
125°C
60
40
0.4
0.2
0
20
0
0
5
10
15
20
25
30
35
40
0
0.5
1.0
1.5
2.0
2.5
3.0
V
, INPUT VOLTAGE (V)
SWITCH CURRENT (A)
in
Figure 10. Oscillator Frequency
Figure 11. Minimum Operating Voltage
8.0
6.0
4.0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
Adjustable Version Only
V
= 12 V
in
Normalized at
25
°C
2.0
0
–2.0
V
1.23 V
= 500 mA
–4.0
out
1.5
1.0
0.5
0
I
Load
–6.0
–8.0
–10
–50
–25
0
25
50
75
100
125
–50
–25
0
25
50
75
100
125
T , JUNCTION TEMPERATURE (
°C)
T , JUNCTION TEMPERATURE (°C)
J
J
Figure 12. Feedback Pin Current
100
80
Adjustable Version Only
60
40
20
0
–20
–40
–60
–80
–100
–50
–25
0
25
50
75
100
125
T , JUNCTION TEMPERATURE (
°C)
J
6
MOTOROLA ANALOG IC DEVICE DATA
LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
Figure 13. Switching Waveforms
Figure 14. Load Transient Response
50 V
0
A
B
100 mV
Output
0
4.0 A
Voltage
Change
2.0 A
0
– 100 mV
3.0 A
4.0 A
2.0 A
0
C
D
Load
Current
2.0 A
1.0 A
0
5
µs/DIV
100 µs/DIV
Vout = 15 V
A: Output Pin Voltage, 10 V/DIV
B: Inductor Current, 2.0 A/DIV
C: Inductor Current, 2.0 A/DIV, AC–Coupled
D: Output Ripple Voltage, 50mV/dDIV, AC–Coupled
Horizontal Time Base: 5 µs/DIV
7
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Figure 15. Typical Test Circuit
Fixed Output Voltage Versions
Feedback
4
V
in
LM2576
L1
100
Fixed Output
µ
H
1
V
Output
2
out
3
Gnd
5
ON/OFF
7.0 V – 40 V
Unregulated
DC Input
C
in
C
out
1000
100 µF
D1
MBR360
µF
Load
C
C
D1
L1
R1
R2
–
–
–
–
–
–
100 µF, 75 V, Aluminium Electrolytic
1000 µF, 25 V, Aluminium Electrolytic
Schottky, MBR360
100 µH, Pulse Eng. PE–92108
2.0 k, 0.1%
in
out
6.12 k, 0.1%
Adjustable Output Voltage Versions
Feedback
4
V
in
LM2576
Adjustable
L1
100
V
out
µ
H
1
Output
5,000 V
2
3
Gnd
5
ON/OFF
7.0 V – 40 V
Unregulated
DC Input
R2
C
in
100
C
out
1000
µF
D1
MBR360
µF
Load
R1
R2
R1
V
V
1.0
out
ref
V
out
R2
R1
– 1.0
V
ref
Where V = 1.23 V, R1
ref
between 1.0 k and 5.0 k
PCB LAYOUT GUIDELINES
As in any switching regulator, the layout of the printed
circuit board is very important. Rapidly switching currents
associated with wiring inductance, stray capacitance and
parasitic inductance of the printed circuit board traces can
generate voltage transients which can generate
electromagnetic interferences (EMI) and affect the desired
operation. As indicated in the Figure 15, to minimize
inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.
For best results, single–point grounding (as indicated) or
ground plane construction should be used.
On the other hand, the PCB area connected to the Pin 2
(emitter of the internal switch) of the LM2576 should be kept
to a minimum in order to minimize coupling to sensitive
circuitry.
Another sensitive part of the circuit is the feedback. It is
important to keep the sensitive feedback wiring short. To
assure this, physically locate the programming resistors near
to the regulator, when using the adjustable version of the
LM2576 regulator.
8
MOTOROLA ANALOG IC DEVICE DATA
LM2576
PIN FUNCTION DESCRIPTION
Pin
Symbol
Description (Refer to Figure 1)
1
V
in
This pin is the positive input supply for the LM2576 step–down switching regulator. In order to minimize
voltage transients and to supply the switching currents needed by the regulator, a suitable input bypass
capacitor must be present (C in Figure 1).
in
2
Output
This is the emitter of the internal switch. The saturation voltage V
It should be kept in mind that the PCB area connected to this pin should be kept to a minimum in order to
minimize coupling to sensitive circuitry.
of this output switch is typically 1.5 V.
sat
3
4
Gnd
Circuit ground pin. See the information about the printed circuit board layout.
Feedback
This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the
internal resistor divider network R2, R1 and applied to the non–inverting input of the internal error amplifier.
In the Adjustable version of the LM2576 switching regulator this pin is the direct input of the error amplifier
and the resistor network R2, R1 is connected externally to allow programming of the output voltage.
5
ON/OFF
It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total
input supply current to approximately 80 µA. The threshold voltage is typically 1.4 V. Applying a voltage
above this value (up to +V ) shuts the regulator off. If the voltage applied to this pin is lower than 1.4 V or
in
if this pin is left open, the regulator will be in the “on” condition.
DESIGN PROCEDURE
Buck Converter Basics
This period ends when the power switch is once again
turned on. Regulation of the converter is accomplished by
varying the duty cycle of the power switch. It is possible to
describe the duty cycle as follows:
The LM2576 is a “Buck” or Step–Down Converter which is
the most elementary forward–mode converter. Its basic
schematic can be seen in Figure 16.
t
The operation of this regulator topology has two distinct
time periods. The first one occurs when the series switch is
on, the input voltage is connected to the input of the inductor.
The output of the inductor is the output voltage, and the
rectifier (or catch diode) is reverse biased. During this period,
since there is a constant voltage source connected across
the inductor, the inductor current begins to linearly ramp
upwards, as described by the following equation:
on
T
d
, where T is the period of switching.
For the buck converter with ideal components, the duty
cycle can also be described as:
V
out
d
V
in
Figure 17 shows the buck converter, idealized waveforms
of the catch diode voltage and the inductor current.
V
– V
t
on
out
in
I
L(on)
L
Figure 17. Buck Converter Idealized Waveforms
During this “on” period, energy is stored within the core
material in the form of magnetic flux. If the inductor is properly
designed, there is sufficient energy stored to carry the
requirements of the load during the “off” period.
V
on(SW)
Figure 16. Basic Buck Converter
Power
Switch
Off
Power
Switch
On
Power
Switch
Off
Power
Switch
On
Power
Switch
L
V
(FWD)
D
C
V
D
out
R
Load
in
Time
The next period is the “off” period of the power switch.
When the power switch turns off, the voltage across the
inductor reverses its polarity and is clamped at one diode
voltage drop below ground by the catch diode. The current
now flows through the catch diode thus maintaining the load
current loop. This removes the stored energy from the
inductor. The inductor current during this time is:
I
pk
I
(AV)
Load
I
min
Power
Switch
Power
Switch
Diode
Diode
V
– V
L
t
Time
out
D
off
I
L(off)
9
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step–by–step
design procedure and some examples are provided.
Procedure
Example
Given Parameters:
= Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)
Given Parameters:
= 5.0 V
V
out
V
out
V
I
= Maximum Input Voltage
= Maximum Load Current
V
I
= 15 V
= 3.0 A
in(max)
in(max)
Load(max)
Load(max)
1. Controller IC Selection
1. Controller IC Selection
According to the required input voltage, output voltage and
current, select the appropriate type of the controller IC output
voltage version.
According to the required input voltage, output voltage, current
polarity and current value, use the LM2576–5 controller IC
2. Input Capacitor Selection (C
)
2. Input Capacitor Selection (C )
in
in
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
A 100 µF, 25 V aluminium electrolytic capacitor located near to
the input and ground pins provides sufficient bypassing.
input pin +V and ground pin Gnd. This capacitor should be
in
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.
3. Catch Diode Selection (D1)
3. Catch Diode Selection (D1)
A. Since the diode maximum peak current exceeds the
regulator maximum load current the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design the diode should have a
current rating equal to the maximum current limit of the
LM2576 to be able to withstand a continuous output short
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
A. For this example the current rating of the diode is 3.0 A.
B. Use a 20 V 1N5820 Schottky diode, or any of the suggested
fast recovery diodes shown in Table 1.
4. Inductor Selection (L1)
4. Inductor Selection (L1)
A. According to the required working conditions, select the
correct inductor value using the selection guide from
Figures 18 to 22.
A. Use the inductor selection guide shown in Figures 19.
B. From the appropriate inductor selection guide, identify the
inductance region intersected by the Maximum Input
Voltage line and the Maximum Load Current line. Each
region is identified by an inductance value and an inductor
code.
B. From the selection guide, the inductance area intersected
by the 15 V line and 3.0 A line is L100.
C. Select an appropriate inductor from the several different
manufacturers part numbers listed in Table 2.
The designer must realize that the inductor current rating
must be higher than the maximum peak current flowing
through the inductor. This maximum peak current can be
calculated as follows:
C. Inductor value required is 100 µH. From Table 2, choose
an inductor from any of the listed manufacturers.
V
–V
t
on
out
2L
in
I
I
Load(max)
p(max)
where t is the “on” time of the power switch and
on
V
out
1.0
t
x
on
V
f
osc
in
For additional information about the inductor, see the
inductor section in the “Application Hints” section of
this data sheet.
10
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Procedure (Fixed Output Voltage Version) (continued)In order to simplify the switching regulator design, a step–by–step
design procedure and some examples are provided.
Procedure
Example
5. Output Capacitor Selection (C
)
5. Output Capacitor Selection (C
)
out
out
= 680 µF to 2000 µF standard aluminium electrolytic.
A. Since the LM2576 is a forward–mode switching regulator
with voltage mode control, its open loop 2–pole–1–zero
frequency characteristic has the dominant pole–pair
determined by the output capacitor and inductor values. For
stable operation and an acceptable ripple voltage,
(approximately 1% of the output voltage) a value between
680 µF and 2000 µF is recommended.
A. C
out
B. Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating at least 8.0 V is appropriate, and a 10 V or
16 V rating is recommended.
B. Capacitor voltage rating = 20 V.
Procedure (Adjustable Output Version: LM2576–ADJ)
Procedure
Example
Given Parameters:
Given Parameters:
V
= Regulated Output Voltage
V
= 8.0 V
out
out
V
I
= Maximum DC Input Voltage
= Maximum Load Current
V
I
= 25 V
= 2.5 A
in(max)
in(max)
Load(max)
Load(max)
1. Programming Output Voltage
1. Programming Output Voltage (selecting R1 and R2)
To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:
Select R1 and R2:
R2
R1
V
1.23 1.0
Select R1 = 1.8 kΩ
out
R2
R1
V
V
1.0
where V = 1.23 V
ref
out
ref
V
out
8.0 V
1.8 k
R2
R1
1.0
1.0
Resistor R1 can be between 1.0 k and 5.0 kΩ. (For best
temperature coefficient and stability with time, use 1% metal
film resistors).
V
1.23 V
ref
R2 = 9.91 kΩ, choose a 9.88 k metal film resistor.
V
out
R2
R1
– 1.0
V
ref
2. Input Capacitor Selection (C
)
2. Input Capacitor Selection (C )
in
in
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
A 100 µF, 150 V aluminium electrolytic capacitor located near
the input and ground pin provides sufficient bypassing.
input pin +V and ground pin Gnd This capacitor should be
in
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.
For additional information see input capacitor section in the
“Application Hints” section of this data sheet.
3. Catch Diode Selection (D1)
3. Catch Diode Selection (D1)
A. Since the diode maximum peak current exceeds the
regulator maximum load current the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design, the diode should have a
current rating equal to the maximum current limit of the
LM2576 to be able to withstand a continuous output short.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
A. For this example, a 3.0 A current rating is adequate.
B. Use a 30 V 1N5821 Schottky diode or any
suggested fast recovery diode in the Table 1.
11
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Procedure (Adjustable Output Version: LM2576–ADJ) (continued)
Procedure
Example
A. Calculate E x T [V x µs] constant:
8.0
4. Inductor Selection (L1)
4. Inductor Selection (L1)
A. Use the following formula to calculate the inductor Volt x
microsecond [V x µs] constant:
V
6
10
F[Hz]
1000
52
out
(
)
E x T
25 – 8.0 x
x
80 [V x s]
E x T
V
– V
x
[V x s]
out
in
25
V
in
B. Match the calculated E x T value with the corresponding
number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 22. This E x T constant is a
measure of the energy handling capability of an inductor and
is dependent upon the type of core, the core area, the
number of turns, and the duty cycle.
B. E x T = 80 [V x µs]
C. Next step is to identify the inductance region intersected by
the E x T value and the maximum load current value on the
horizontal axis shown in Figure 25.
C. I
= 2.5 A
Inductance Region = H150
Load(max)
D. From the inductor code, identify the inductor value. Then
select an appropriate inductor from Table 2.
D. Proper inductor value = 150 µH
Choose the inductor from Table 2.
The inductor chosen must be rated for a switching
frequency of 52 kHz and for a current rating of 1.15 x I
The inductor current rating can also be determined by
calculating the inductor peak current:
.
Load
V
– V
t
on
out
2L
in
I
I
Load(max)
p(max)
where t is the “on” time of the power switch and
on
V
out
1.0
t
x
on
V
f
osc
in
For additional information about the inductor, see the
inductor section in the “External Components” section of
this data sheet.
5. Output Capacitor Selection (C
out
)
5. Output Capacitor Selection (C
)
out
A. Since the LM2576 is a forward–mode switching regulator
with voltage mode control, its open loop 2–pole–1–zero
frequency characteristic has the dominant pole–pair
determined by the output capacitor and inductor values.
A.
25
8 x 150
C
13,300 x
332.5 µF
out
To achieve an acceptable ripple voltage, select
= 680 µF electrolytic capacitor.
C
out
For stable operation, the capacitor must satisfy the
following requirement:
V
in(max)
C
13,300
[µF]
out
V
x L [µH]
out
B. Capacitor values between 10 µF and 2000 µF will satisfy
the loop requirements for stable operation. To achieve an
acceptable output ripple voltage and transient response, the
output capacitor may need to be several times larger than
the above formula yields.
C. Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating of at least 8.0 V is appropriate, and a 10 V
or 16 V rating is recommended.
12
MOTOROLA ANALOG IC DEVICE DATA
LM2576
LM2576 Series Buck Regulator Design Procedures (continued)
Indicator Value Selection Guide (For Continuous Mode Operation)
Figure 18. LM2576–3.3
Figure 19. LM2576–5
60
60
40
L680
40
20
15
H1000
H680
H470
L330
H330
H220
H150
L470
L330
L220
L150
20
15
L680
10
L470
8.0
12
7.0
L220
10
L100
L68
L150
9.0
6.0
L100
1.5
8.0
L47
2.0
L68
L47
5.0
0.3
7.0
0.4
0.5 0.6
0.8
1.0
1.5
2.5 3.0
0.3
0.4
0.5 0.6
0.8 1.0 1.2
2.0
2.5 3.0
I , MAXIMUM LOAD CURRENT (A)
L
I , MAXIMUM LOAD CURRENT (A)
L
Figure 20. LM2576–12
Figure 21. LM2576–15
60
60
40
35
30
40
35
30
H1500
L680
H1500
25
H1000
L470
H1000
H680
H470
25
H680
H470
H330
20
18
H220
H150
H330
H220
22
H150
20
19
L680
0.4
L470
16
15
L330
L330
L220
L220
L150
L150
18
L100
L100
L68
L68
14
17
0.3
0.4
0.5 0.6
0.8 1.0
1.5
2.0
2.5 3.0
0.3
0.5 0.6
0.8 1.0
1.5
2.0
2.5 3.0
I , MAXIMUM LOAD CURRENT (A)
L
I , MAXIMUM LOAD CURRENT (A)
L
Figure 22. LM2576–ADJ
300
250
H2000
200
H1500
H1000
150
H680
L220
H470
L150
H330
H220
100
90
80
70
60
50
45
40
H150
L680
0.4
L470
L330
L100
1.5
35
L68
2.0
30
L47
25
20
0.3
0.5 0.6
0.8 1.0
2.5 3.0
I , MAXIMUM LOAD CURRENT (A)
L
13
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Table 1. Diode Selection Guide
Schottky
Fast Recovery
3.0 A
4.0 – 6.0 A
3.0 A
4.0 – 6.0 A
Through
Hole
Surface
Mount
Through
Hole
Surface
Mount
Through
Hole
Surface
Mount
Through
Hole
Surface
Mount
V
R
20 V
1N5820
MBR320P
SR302
SK32
1N5823
SR502
SB520
30 V
1N5821
MBR330
SR303
SK33
30WQ03
1N5824
SR503
SB530
50WQ03
MUR320
31DF1
HER302
MURS320T3
MURD320
30WF10
MUR420
HER602
MURD620CT
50WF10
31DQ03
40 V
1N5822
MBR340
SR304
SK34
30WQ04
MBRS340T3
MBRD340
1N5825
SR504
SB540
MBRD640CT
50WQ04
(all diodes
rated
(all diodes
rated
(all diodes
rated
(all diodes
rated
to at least
100 V)
to at least
100 V)
to at least
100 V)
to at least
100 V)
31DQ04
50 V
60 V
MBR350
31DQ05
SR305
SK35
30WQ05
SB550
50WQ05
MBR360
DQ06
MBRS360T3
MBRD360
50SQ080
MBRD660CT
SR306
NOTE: Diofes listed inbold are available from Motorola.
Table 2. Inductor Selection by Manufacturer’s Part Number
Inductor
Code
Inductor
Value
Tech 39
Schott Corp.
Pulse Eng.
Renco
L47
L68
47 µH
77 212
671 26980
PE–53112
PE–92114
PE–92108
PE–53113
PE–52626
PE–52627
PE–53114
PE–52629
PE–53115
PE–53116
PE–53117
PE–53118
PE–53119
PE–53120
PE–53121
PE–53122
RL2442
68 µH
77 262
77 312
77 360
77 408
77 456
*
671 26990
671 27000
671 27010
671 27020
671 27030
671 27040
671 27050
671 27060
671 27070
671 27080
671 27090
671 27100
671 27110
671 27120
671 27130
RL2443
RL2444
RL1954
RL1953
RL1952
RL1951
RL1950
RL2445
RL2446
RL2447
RL1961
RL1960
RL1959
RL1958
RL2448
L100
L150
L220
L330
L470
L680
H150
H220
H330
H470
H680
H1000
H1500
100 µH
150 µH
220 µH
330 µH
470 µH
680 µH
150 µH
220 µH
330 µH
470 µH
680 µH
1000 µH
1500 µH
2200 µH
77 506
77 362
77 412
77 462
*
77 508
77 556
*
H2200
*
NOTE: * Contact Manufacturer
14
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers
Phone
Fax
+ 1–619–674–8100
+ 1–619–674–8262
Pulse Engineering, Inc.
Pulse Engineering, Inc. Europe
Renco Electronics, Inc.
Tech 39
Phone
Fax
+ 353–9324–107
+ 353–9324–459
Phone
Fax
+ 1–516–645–5828
+ 1–516–586–5562
Phone
Fax
+ 33–1–4115–1681
+ 33–1–4709–5051
Phone
Fax
+ 1–612–475–1173
+ 1–612–475–1786
Schott Corporation
EXTERNAL COMPONENTS
An aluminium electrolytic capacitor’s ESR value is related
Input Capacitor (C
The Input Capacitor Should Have a Low ESR
)
in
to many factors such as the capacitance value, the voltage
rating, the physical size and the type of construction. In most
cases, the higher voltage electrolytic capacitors have lower
ESR value. Often capacitors with much higher voltage
ratings may be needed to provide low ESR values that, are
required for low output ripple voltage.
For stable operation of the switch mode converter a low
ESR (Equivalent Series Resistance) aluminium or solid
tantalum bypass capacitor is needed between the input pin
and the ground pin, to prevent large voltage transients from
appearing at the input. It must be located near the regulator
and use short leads. With most electrolytic capacitors, the
capacitance value decreases and the ESR increases with
lower temperatures. For reliable operation in temperatures
below –25°C larger values of the input capacitor may be
needed. Also paralleling a ceramic or solid tantalum
capacitor will increase the regulator stability at cold
temperatures.
The Output Capacitor Requires an ESR Value
That Has an Upper and Lower Limit
As mentioned above, a low ESR value is needed for low
output ripple voltage, typically 1% to 2% of the output voltage.
But if the selected capacitor’s ESR is extremely low (below
0.05 Ω), there is a possibility of an unstable feedback loop,
resulting in oscillation at the output. This situation can occur
when a tantalum capacitor, that can have a very low ESR, is
used as the only output capacitor.
RMS Current Rating of C
in
The important parameter of the input capacitor is the RMS
current rating. Capacitors that are physically large and have
large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage
electrolytic capacitor will be physically larger than a lower
voltage capacitor, and thus be able to dissipate more heat to
the surrounding air, and therefore will have a higher RMS
current rating. The consequence of operating an electrolytic
capacitor beyond the RMS current rating is a shortened
operating life. In order to assure maximum capacitor
operating lifetime, the capacitor’s RMS ripple current rating
should be:
At Low Temperatures, Put in Parallel Aluminium
Electrolytic Capacitors with Tantalum Capacitors
Electrolytic capacitors are not recommended for
temperatures below –25°C. The ESR rises dramatically at
cold temperatures and typically rises 3 times at –25°C and as
much as 10 times at –40°C. Solid tantalum capacitors have
much better ESR spec at cold temperatures and are
recommended for temperatures below –25°C. They can be
also used in parallel with aluminium electrolytics. The value
of the tantalum capacitor should be about 10% or 20% of the
total capacitance. The output capacitor should have at least
50% higher RMS ripple current rating at 52 kHz than the
peak–to–peak inductor ripple current.
I
> 1.2 x d x I
rms
Load
where d is the duty cycle, for a buck regulator
V
t
Catch Diode
on
T
out
d
Locate the Catch Diode Close to the LM2576
The LM2576 is a step–down buck converter; it requires a
fast diode to provide a return path for the inductor current
when the switch turns off. This diode must be located close to
the LM2576 using short leads and short printed circuit traces
to avoid EMI problems.
V
in
|V
|
t
on
T
out
and d
for a buck boost regulator.
|V
|
V
out
in
Output Capacitor (C
out
)
For low output ripple voltage and good stability, low ESR
output capacitors are recommended. An output capacitor has
two main functions: it filters the output and provides regulator
loop stability. The ESR of the output capacitor and the
peak–to–peak value of the inductor ripple current are the
main factors contributing to the output ripple voltage value.
Standard aluminium electrolytics could be adequate for some
applications but for quality design, low ESR types are
recommended.
Use a Schottky or a Soft Switching
Ultra–Fast Recovery Diode
Since the rectifier diodes are very significant sources of
losses within switching power supplies, choosing the rectifier
that best fits into the converter design is an important
process. Schottky diodes provide the best performance
15
MOTOROLA ANALOG IC DEVICE DATA
LM2576
because of their fast switching speed and low forward
voltage drop.
different design load currents are selected. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, because
the inductor value and size can be kept relatively low.
Consequently, the percentage of inductor peak–to–peak
current increases. This discontinuous mode of operation is
perfectly acceptable for this type of switching converter. Any
buck regulator will be forced to enter discontinuous mode if
the load current is light enough.
They provide the best efficiency especially in low output
voltage applications (5.0 V and lower). Another choice could
be Fast–Recovery, or Ultra–Fast Recovery diodes. It has to
be noted, that some types of these diodes with an abrupt
turnoff characteristic may cause instability or EMI troubles.
A fast–recovery diode with soft recovery characteristics
can better fulfill some quality, low noise design requirements.
Table 1 provides a list of suitable diodes for the LM2576
regulator. Standard 50/60 Hz rectifier diodes, such as the
1N4001 series or 1N5400 series are NOT suitable.
Figure 23. Continuous Mode Switching Current
Waveforms
Inductor
The magnetic components are the cornerstone of all
switching power supply designs. The style of the core and
the winding technique used in the magnetic component’s
design has a great influence on the reliability of the overall
power supply.
Using an improper or poorly designed inductor can cause
high voltage spikes generated by the rate of transitions in
current within the switching power supply, and the possibility
of core saturation can arise during an abnormal operational
mode. Voltage spikes can cause the semiconductors to enter
avalanche breakdown and the part can instantly fail if enough
energy is applied. It can also cause significant RFI (Radio
Frequency Interference) and EMI (Electro–Magnetic
Interference) problems.
2.0 A
Inductor
Current
Waveform
0 A
2.0 A
Power
Switch
Current
Waveform
0 A
HORIZONTAL TIME BASE: 5.0 µs/DIV
Continuous and Discontinuous Mode of Operation
The LM2576 step–down converter can operate in both the
continuous and the discontinuous modes of operation. The
regulator works in the continuous mode when loads are
relatively heavy, the current flows through the inductor
continuously and never falls to zero. Under light load
conditions, the circuit will be forced to the discontinuous
mode when inductor current falls to zero for certain period of
time (see Figure 23 and Figure 24). Each mode has
distinctively different operating characteristics, which can
affect the regulator performance and requirements. In many
cases the preferred mode of operation is the continuous
mode. It offers greater output power, lower peak currents in
the switch, inductor and diode, and can have a lower output
ripple voltage. On the other hand it does require larger
inductor values to keep the inductor current flowing
continuously, especially at low output load currents and/or
high input voltages.
Selecting the Right Inductor Style
Some important considerations when selecting a core
type are core material, cost, the output power of the power
supply, the physical volume the inductor must fit within, and
the amount of EMI (Electro–Magnetic Interference) shielding
that the core must provide. The inductor selection guide
covers different styles of inductors, such as pot core, E–core,
toroid and bobbin core, as well as different core materials
such as ferrites and powdered iron from different
manufacturers.
For high quality design regulators the toroid core seems to
be the best choice. Since the magnetic flux is contained
within the core, it generates less EMI, reducing noise
problems in sensitive circuits. The least expensive is the
bobbin core type, which consists of wire wound on a ferrite
rod core. This type of inductor generates more EMI due to the
fact that its core is open, and the magnetic flux is not
contained within the core.
To simplify the inductor selection process, an inductor
selection guide for the LM2576 regulator was added to this
data sheet (Figures 18 through 22). This guide assumes that
the regulator is operating in the continuous mode, and
selects an inductor that will allow a peak–to–peak inductor
ripple current to be a certain percentage of the maximum
design load current. This percentage is allowed to change as
When multiple switching regulators are located on the
same printed circuit board, open core magnetics can cause
interference between two or more of the regulator circuits,
especially at high currents due to mutual coupling. A toroid,
pot core or E–core (closed magnetic structure) should be
used in such applications.
16
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Do Not Operate an Inductor Beyond its
Maximum Rated Current
Figure 24. Discontinuous Mode Switching Current
Waveforms
Exceeding an inductor’s maximum current rating may
cause the inductor to overheat because of the copper wire
losses, or the core may saturate. Core saturation occurs
when the flux density is too high and consequently the cross
sectional area of the core can no longer support additional
lines of magnetic flux.
This causes the permeability of the core to drop, the
inductance value decreases rapidly and the inductor begins
to look mainly resistive. It has only the DC resistance of the
winding. This can cause the switch current to rise very rapidly
and force the LM2576 internal switch into cycle–by–cycle
current limit, thus reducing the DC output load current. This
can also result in overheating of the inductor and/or the
LM2576. Different inductor types have different saturation
characteristics, and this should be kept in mind when
selecting an inductor.
0.4 A
Inductor
Current
Waveform
0 A
0.4 A
Power
Switch
Current
Waveform
0 A
HORIZONTAL TIME BASE: 5.0 µs/DIV
GENERAL RECOMMENDATIONS
to smooth the output by means of an additional LC filter (20 µH,
100 µF), that can be added to the output (see Figure 34) to
further reduce the amount of output ripple and transients.
With such a filter it is possible to reduce the output ripple
voltage transients 10 times or more. Figure 25 shows the
difference between filtered and unfiltered output waveforms
of the regulator shown in Figure 34.
The lower waveform is from the normal unfiltered output of
the converter, while the upper waveform shows the output
ripple voltage filtered by an additional LC filter.
Output Voltage Ripple and Transients
Source of the Output Ripple
Since the LM2576 is a switch mode power supply
regulator, its output voltage, if left unfiltered, will contain a
sawtooth ripple voltage at the switching frequency. The
output ripple voltage value ranges from 0.5% to 3% of the
output voltage. It is caused mainly by the inductor sawtooth
ripple current multiplied by the ESR of the output capacitor.
Short Voltage Spikes and How to Reduce Them
The regulator output voltage may also contain short
voltage spikes at the peaks of the sawtooth waveform (see
Figure 25). These voltage spikes are present because of the
fast switching action of the output switch, and the parasitic
inductance of the output filter capacitor. There are some
other important factors such as wiring inductance, stray
capacitance, as well as the scope probe used to evaluate
these transients, all these contribute to the amplitude of these
spikes. To minimize these voltage spikes, low inductance
capacitors should be used, and their lead lengths must be
kept short. The importance of quality printed circuit board
layout design should also be highlighted.
Heatsinking and Thermal Considerations
The Through–Hole Package TO–220
The LM2576 is available in two packages, a 5–pin
TO–220(T, TV) and a 5–pin surface mount D PAK(D2T).
Although the TO–220(T) package needs a heatsink under
most conditions, there are some applications that require no
heatsink to keep the LM2576 junction temperature within the
allowed operating range. Higher ambient temperatures
require some heat sinking, either to the printed circuit (PC)
board or an external heatsink.
2
2
The Surface Mount Package D PAK and its Heatsinking
2
The other type of package, the surface mount D PAK, is
designed to be soldered to the copper on the PC board. The
copper and the board are the heatsink for this package and
the other heat producing components, such as the catch
diode and inductor. The PC board copper area that the
Figure 25. Output Ripple Voltage Waveforms
Voltage spikes
caused by
switching action
of the output
switch and the
parasitic
Filtered
Output
Voltage
2
package is soldered to should be at least 0.4 in (or
2
260 mm ) and ideally should have 2 or more square inches
inductance of the
2
output capacitor
(1300 mm ) of 0.0028 inch copper. Additional increases of
2
2
copper area beyond approximately 6.0 in (4000 mm ) will
not improve heat dissipation significantly. If further thermal
improvements are needed, double sided or multilayer PC
boards with large copper areas should be considered. In
order to achieve the best thermal performance, it is highly
recommended to use wide copper traces as well as large
areas of copper in the printed circuit board layout. The only
exception to this is the OUTPUT (switch) pin, which should
not have large areas of copper (see page 8 ’PCB Layout
Guideline’).
Unfiltered
Output
Voltage
HORIZONTAL TIME BASE: 5.0 µs/DIV
Minimizing the Output Ripple
In order to minimize the output ripple voltage it is possible
to enlarge the inductance value of the inductor L1 and/or to
use a larger value output capacitor. There is also another way
17
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Thermal Analysis and Design
If the actual operating temperature is greater than the
selected safe operating junction temperature, then a larger
heatsink is required.
The following procedure must be performed to determine
whether or not a heatsink will be required. First determine:
1. P
maximum regulator power dissipation in the
application.
maximum ambient temperature in the
application.
maximum allowed junction temperature
(125°C for the LM2576). For a conservative
design, the maximum junction temperature
should not exceed 110°C to assure safe
operation. For every additional +10°C
temperature rise that the junction must
withstand, the estimated operating lifetime of
the component is halved.
D(max)
Some Aspects That can Influence Thermal Design
It should be noted that the package thermal resistance and
the junction temperature rise numbers are all approximate,
and there are many factors that will affect these numbers,
such as PC board size, shape, thickness, physical position,
location, board temperature, as well as whether the
surrounding air is moving or still.
Other factors are trace width, total printed circuit copper
area, copper thickness, single– or double–sided, multilayer
board, the amount of solder on the board or even colour of
the traces.
2. T
)
A(max
J(max)
3. T
The size, quantity and spacing of other components on
the board can also influence its effectiveness to dissipate
the heat.
4. R
5. R
package thermal resistance junction–case.
package thermal resistance junction–ambient.
θJC
θJA
(Refer to Absolute Maximum Ratings on page 2 of this data
sheet or R and R values).
θJC θJA
Figure 26. Inverting Buck–Boost Develops –12 V
The following formula is to calculate the approximate total
power dissipated by the LM2576:
12 to 40 V
Feedback
Unregulated
DC Input
L1
68
+V
4
in
1
P
= (V x I ) + d x I
in
x V
sat
LM2576–12
µH
D
Q
Load
Output
C
µ
in
where d is the duty cycle and for buck converter
2
100
F
D1
1N5822
C
3
Gnd
5
ON/OFF
out
2200
V
V
t
on
T
O
in
µF
d
,
I
(quiescent current) and V
LM2576 data sheet,
is minimum input voltage applied,
is the regulator output voltage,
is the load current.
can be found in the
–12 V @ 0.7 A
Regulated
Output
Q
sat
V
V
I
in
O
Load
ADDITIONAL APPLICATIONS
Inverting Regulator
The dynamic switching losses during turn–on and turn–off
An inverting buck–boost regulator using the LM2576–12 is
shown in Figure 26. This circuit converts a positive input
voltage to a negative output voltage with a common ground
by bootstrapping the regulators ground to the negative output
voltage. By grounding the feedback pin, the regulator senses
the inverted output voltage and regulates it.
can be neglected if proper type catch diode is used.
Packages Not on a Heatsink (Free–Standing)
For a free–standing application when no heatsink is used,
the junction temperature can be determined by the following
expression:
T = (R
) (P ) + T
In this example the LM2576–12 is used to generate a
–12 V output. The maximum input voltage in this case
cannot exceed +28 V because the maximum voltage
appearing across the regulator is the absolute sum of the
input and output voltages and this must be limited to a
maximum of 40 V.
This circuit configuration is able to deliver approximately
0.7 A to the output when the input voltage is 12 V or higher. At
lighter loads the minimum input voltage required drops to
approximately 4.7 V, because the buck–boost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
J
θJA
D A
where (R
θJA
)(P ) represents the junction temperature rise
D
caused by the dissipated power and T is the maximum
A
ambient temperature.
Packages on a Heatsink
If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, than a heatsink is required. The junction
temperature will be calculated as follows:
T = P (R
+ R
+ R
) + T
θSA A
J
D
θJA
θCS
where
R
R
R
is the thermal resistance junction–case,
is the thermal resistance case–heatsink,
is the thermal resistance heatsink–ambient.
θJC
θCS
θSA
18
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Since the switch currents in this buck–boost configuration
are higher than in the standard buck converter topology, the
available output current is lower.
Figure 27. Inverting Buck–Boost Regulator
with Delayed start–up
This type of buck–boost inverting regulator can also
require a larger amount of start–up input current, even for
light loads. This may overload an input power source with a
current limit less than 5.0 A.
Such an amount of input start–up current is needed for at
least 2.0 ms or more. The actual time depends on the output
voltage and size of the output capacitor.
Because of the relatively high start–up currents required
by this inverting regulator topology, the use of a delayed
start–up or an undervoltage lockout circuit is recommended.
Using a delayed start–up arrangement, the input capacitor
can charge up to a higher voltage before the switch–mode
regulator begins to operate.
12 V to 25 V
Unregulated
DC Input
Feedback
+V
L1
68
in
4
LM2576–12
µH
Output
1
C
µF
in
C1
100
2
0.1 µF
/50 V
5
ON/OFF 3 Gnd
C
2200
/16 V
out
D1
1N5822
R1
47 k
µF
R2
47 k
–12 V @ 700 m A
Regulated
Output
The high input current needed for start–up is now partially
supplied by the input capacitor C .
in
It has been already mentioned above, that in some
situations, the delayed start–up or the undervoltage lockout
features could be very useful. A delayed start–up circuit
applied to a buck–boost converter is shown in Figure 27,
Figure 33 in the “Undervoltage Lockout” section describes an
undervoltage lockout feature for the same converter
topology.
Figure 28. Inverting Buck–Boost Regulator Shutdown
Circuit Using an Optocoupler
+V
in
+V
in
LM2576–XX
1
C
R1
F 47 k
in
100
µ
Design Recommendations:
The inverting regulator operates in a different manner than
the buck converter and so a different design procedure has to
5
ON/OFF
3
Gnd
Shutdown
Input
5.0 V
0
Off
be used to select the inductor L1 or the output capacitor C
.
R3
470
out
On
The output capacitor values must be larger than what is
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of µF).
R2
47 k
–V
out
MOC8101
The recommended range of inductor values for the
inverting converter design is between 68 µH and 220 µH. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
NOTE: This picture does not show the complete circuit.
The following formula is used to obtain the peak inductor
current:
With the inverting configuration, the use of the ON/OFF pin
requires some level shifting techniques. This is caused by the
fact, that the ground pin of the converter IC is no longer at
ground. Now, the ON/OFF pin threshold voltage (1.3 V
approximately) has to be related to the negative output
voltage level. There are many different possible shut down
methods, two of them are shown in Figures 28 and 29.
I
(V
|V |)
V
x t
on
Load in
O
in
2L
I
peak
on
V
1
in
|V |
O
1.0
where t
x
, and f
52 kHz.
osc
V
|V |
f
osc
in
O
Under normal continuous inductor current operating
conditions, the worst case occurs when V is minimal.
in
19
MOTOROLA ANALOG IC DEVICE DATA
LM2576
currents require a large value output capacitor (in the range
of thousands of µF). The recommended range of inductor
values for the negative boost regulator is the same as for
inverting converter design.
Figure 29. Inverting Buck–Boost Regulator Shutdown
Circuit Using a PNP Transistor
Shutdown
+V
Another important point is that these negative boost
converters cannot provide current limiting load protection in
the event of a short in the output so some other means, such
as a fuse, may be necessary to provide the load protection.
Off
Input
0
On
R2
5.6 k
Delayed Start–up
+V
in
+V
in
1
There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
start–up current. In such cases, if the input power source is
limited, this delayed start–up feature becomes very useful.
To provide a time delay between the time when the input
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 31 can be used. As the input
voltage is applied, the capacitor C1 charges up, and the
voltage across the resistor R2 falls down. When the voltage
on the ON/OFF pin falls below the threshold value 1.3 V, the
regulator starts up. Resistor R1 is included to limit the
maximum voltage applied to the ON/OFF pin. It reduces the
power supply noise sensitivity, and also limits the capacitor
C1 discharge current, but its use is not mandatory.
LM2576–XX
C
in
100
µF
Q1
2N3906
5
ON/OFF
3
Gnd
R1
12 k
–V
out
NOTE: This picture does not show the complete circuit.
Negative Boost Regulator
This example is a variation of the buck–boost topology and
it is called negative boost regulator. This regulator
experiences relatively high switch current, especially at low
input voltages. The internal switch current limiting results in
lower output load current capability.
The circuit in Figure 30 shows the negative boost
configuration. The input voltage in this application ranges
from –5.0 V to –12 V and provides a regulated –12 V output.
If the input voltage is greater than –12 V, the output will rise
above –12 V accordingly, but will not damage the regulator.
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
respectively) ripple voltage exists, a long delay time can
cause some problems by coupling the ripple into the ON/OFF
pin, the regulator could be switched periodically on and off
with the line (or double) frequency.
Figure 31. Delayed start–up Circuitry
+V
in
+V
in
LM2576–XX
1
Figure 30. Negative Boost Regulator
C1
0.1
5
ON/OFF
3
Gnd
µF
C
in
100 µF
C
out
2200
Low Esr
R1
47 k
4
µ
F
R2
47 k
V
in
Feedback
Output
2
LM2576–12
1
C
in
1N5820
3
5
Gnd
ON/OFF
100 µF
NOTE: This picture does not show the complete circuit.
V
= –12 V
out
Undervoltage Lockout
Typical Load Current
Some applications require the regulator to remain off until
the input voltage reaches a certain threshold level. Figure 32
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck–boost converter is
shown in Figure 33. Resistor R3 pulls the ON/OFF pin high
and keeps the regulator off until the input voltage reaches a
100 µH
400 mA for V = –5.2 V
V
in
in
750 mA for V = –7.0 V
in
–5.0 V to –12 V
Design Recommendations:
The same design rules as for the previous inverting
buck–boost converter can be applied. The output capacitor
C
must be chosen larger than would be required for a what
out
standard buck converter. Low input voltages or high output
20
MOTOROLA ANALOG IC DEVICE DATA
LM2576
predetermined threshold level with respect to the ground
Under normal continuous inductor current operating
Pin 3, which is determined by the following expression:
conditions, the worst case occurs when V is minimal.
in
R2
R1
( )
Q1
V
V
1.0
V
th
Z1
BE
Figure 33. Undervoltage Lockout Circuit for
Buck–Boost Converter
Figure 32. Undervoltage Lockout Circuit for
Buck Converter
+V
in
+V
in
LM2576–XX
1
+V
in
+V
in
LM2576–XX
C
100
in
R2
15 k
R3
47 k
1
5
ON/OFF
3
Gnd
µF
C
100
R2
10 k
R3
47 k
in
µF
5
ON/OFF
3
Gnd
Z1
1N5242B
V
≈ 13 V
th
Z1
1N5242B
Q1
2N3904
R1
15 k
Q1
2N3904
V
out
R1
10 k
V
≈ 13 V
th
NOTE: This picture does not show the complete circuit.
NOTE: This picture does not show the complete circuit.
Adjustable Output, Low–Ripple Power Supply
A 3.0 A output current capability power supply that
features an adjustable output voltage is shown in Figure 34.
This regulator delivers 3.0 A into 1.2 V to 35 V output. The
input voltage ranges from roughly 3.0 V to 40 V. In order to
achieve a 10 or more times reduction of output ripple, an
additional L–C filter is included in this circuit.
The following formula is used to obtain the peak inductor
current:
I
(V
|V |)
V
x t
on
Load in
O
in
2L
I
peak
on
V
1
in
|V |
O
1.0
where t
x
, and f
52 kHz.
osc
V
|V |
f
osc
in
O
Figure 34. 1.2 to 35 V Adjustable 3.0 A Power Supply with Low Output Ripple
Feedback
4
40 V Max
Unregulated
DC Input
+V
in
L1
150
L2
LM2574–Adj
Output
Voltage
µ
H
20
µ
H
1
Output
2
1.2 to 35 V @ 3.0 A
C
µ
R2
50 k
in
3
Gnd
5
ON/OFF
100
F
C
D1
1N5822
out
2200
C1
100 µF
µF
R1
1.21 k
Optional Output
Ripple Filter
21
MOTOROLA ANALOG IC DEVICE DATA
LM2576
THE LM2576–5 STEP–DOWN VOLTAGE REGULATOR WITH 5.0 V @ 3.0 A OUTPUT POWER CAPABILITY.
TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUT
Figure 35. Schematic Diagram of the LM2576–5 Step–Down Converter
Feedback
4
+V
in
Unregulated
DC Input
+V = 7.0 to 40 V
L1
150
LM2576–5
µ
H
1
Output
2
Regulated Output
= 5.0 V @ 3.0 A
in
V
out1
3
Gnd
5
ON/OFF
C1
µF
ON/OFF
100
C
1000
/16 V
out
D1
1N5822
/50 V
µF
Gnd
in
Gnd
out
C1
C2
D1
L1
–
–
–
–
100 µF, 50 V, Aluminium Electrolytic
1000 µF, 16 V, Aluminium Electrolytic
3.0 A, 40 V, Schottky Rectifier, 1N5822
150 µH, RL2444, Renco Electronics
Figure 36. Printed Circuit Board Layout
Component Side
Figure 37. Printed Circuit Board Layout
Copper Side
LM2576
U1
D1
+
C2
C1
V
ou
t
+
ON/OFF
L1
+V
in
Gnd
in
Gnd
out
NOTE: Not to scale.
NOTE: Not to scale.
22
MOTOROLA ANALOG IC DEVICE DATA
LM2576
THE LM2576–ADJ STEP–DOWN VOLTAGE REGULATOR WITH 8.0 V @ 1.0 A OUTPUT POWER
CAPABILITY. TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUT
Figure 38. Schematic Diagram of the 8.0 V @ 3.0 A Step–Down Converter Using the LM2576–ADJ
4
Feedback
Unregulated
DC Input
+V
in
L1
150
LM2576–ADJ
Regulated
Output Filtered
µ
H
1
+V = 10 V to 40 V
in
Output
2
V
= 8.0 V @ 3.0 A
out2
R2
10 k
3
Gnd
5
ON/OFF
C1
100 µF
/50 V
C2
1000
/16 V
D1
1N5822
µF
R1
1.8 k
ON/OFF
R2
R1
V
V
1.0
out
= 1.23 V
ref
V
ref
R1 is between 1.0 k and 5.0 k
C1
C2
D1
L1
R1
R2
–
–
–
–
–
–
100 µF, 50 V, Aluminium Electrolytic
1000 µF, 16 V, Aluminium Electrolytic
3.0 A, 40 V, Schottky Rectifier, 1N5822
150 µH, RL2444, Renco Electronics
1.8 kΩ, 0.25 W
10 kΩ, 0.25 W
Figure 39. Printed Circuit Board Layout
Component Side
Figure 40. Printed Circuit Board Layout
Copper Side
LM2576
U1
D1
R1
R2
ON/OFF
C1
+
+
C2
V
out
+V
in
L1
Gnd
in
Gnd
out
NOTE: Not to scale.
NOTE: Not to scale.
References
• National Semiconductor LM2576 Data Sheet and Application Note
• National Semiconductor LM2595 Data Sheet and Application Note
• Marty Brown “Practical Switching Power Supply Design”, Academic Press, Inc., San Diego 1990
• Ray Ridley “High Frequency Magnetics Design”, Ridley Engineering, Inc. 1995
23
MOTOROLA ANALOG IC DEVICE DATA
LM2576
OUTLINE DIMENSIONS
T SUFFIX
PLASTIC PACKAGE
CASE 314D–03
ISSUE D
SEATING
–T–
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI
PLANE
C
Y14.5M, 1982.
–Q–
2. CONTROLLING DIMENSION: INCH.
3. DIMENSION D DOES NOT INCLUDE
INTERCONNECT BAR (DAMBAR) PROTRUSION.
DIMENSION D INCLUDING PROTRUSION SHALL
NOT EXCEED 10.92 (0.043) MAXIMUM.
B
E
U
INCHES
MIN MAX
0.613 14.529 15.570
MILLIMETERS
MIN MAX
A
S
DIM
A
B
C
D
E
G
H
J
K
L
Q
U
S
L
0.572
0.390
0.170
0.025
0.048
0.415
0.180
0.038
0.055
9.906 10.541
1
2
3
4 5
4.318
0.635
1.219
4.572
0.965
1.397
K
0.067 BSC
1.702 BSC
0.087
0.015
1.020
0.320
0.140
0.105
0.543
0.112
0.025
2.210
0.381
2.845
0.635
1.065 25.908 27.051
0.365
0.153
0.117
8.128
3.556
2.667
9.271
3.886
2.972
J
G
0.582 13.792 14.783
H
D 5 PL
M
M
0.356 (0.014)
T Q
TV SUFFIX
PLASTIC PACKAGE
CASE 314B–05
ISSUE J
NOTES:
C
1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
B
–P–
OPTIONAL
CHAMFER
Q
F
2. CONTROLLING DIMENSION: INCH.
3. DIMENSION D DOES NOT INCLUDE
INTERCONNECT BAR (DAMBAR) PROTRUSION.
DIMENSION D INCLUDING PROTRUSION SHALL
NOT EXCEED 0.043 (1.092) MAXIMUM.
E
A
INCHES
MIN MAX
0.613 14.529 15.570
MILLIMETERS
MIN MAX
U
DIM
A
B
C
D
E
L
S
V
0.572
0.390
0.170
0.025
0.048
0.850
W
0.415
0.180
0.038
0.055
9.906 10.541
K
4.318
0.635
1.219
4.572
0.965
1.397
F
0.935 21.590 23.749
G
H
J
K
L
N
Q
S
0.067 BSC
0.166 BSC
1.702 BSC
4.216 BSC
0.015
0.900
0.320
0.025
0.381
0.635
1.100 22.860 27.940
0.365
5X J
8.128
8.128 BSC
3.556
9.271
3.886
G
M
0.24 (0.610)
T
H
0.320 BSC
5X D
0.140
–––
0.153
0.620
––– 15.748
N
M
M
0.10 (0.254)
T P
U
V
W
0.468
–––
0.090
0.505 11.888 12.827
0.735
0.110
––– 18.669
2.286 2.794
SEATING
PLANE
–T–
24
MOTOROLA ANALOG IC DEVICE DATA
LM2576
OUTLINE DIMENSIONS
D2T SUFFIX
PLASTIC PACKAGE
CASE 936A–02
2
(D PAK)
ISSUE A
NOTES:
1
DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
–T–
TERMINAL 6
2
3
CONTROLLING DIMENSION: INCH.
TAB CONTOUR OPTIONAL WITHIN DIMENSIONS
A AND K.
DIMENSIONS U AND V ESTABLISH A MINIMUM
MOUNTING SURFACE FOR TERMINAL 6.
DIMENSIONS A AND B DO NOT INCLUDE MOLD
FLASH OR GATE PROTRUSIONS. MOLD FLASH
AND GATE PROTRUSIONS NOT TO EXCEED
0.025 (0.635) MAXIMUM.
OPTIONAL
CHAMFER
A
E
U
4
5
S
K
V
B
H
1
2
3
4 5
INCHES
MILLIMETERS
MIN MAX
9.804 10.236
M
L
DIM
A
B
C
D
E
MIN
MAX
0.403
0.368
0.180
0.036
0.055
0.386
0.356
0.170
0.026
0.045
9.042
4.318
0.660
1.143
9.347
4.572
0.914
1.397
D
P
N
M
0.010 (0.254)
T
G
R
G
H
K
L
M
N
P
R
S
U
V
0.067 BSC
0.539
0.050 REF
1.702 BSC
0.579 13.691 14.707
1.270 REF
0.000
0.088
0.018
0.058
5
0.010
0.102
0.026
0.078
0.000
0.254
2.591
0.660
1.981
2.235
0.457
1.473
5
C
REF
REF
0.116 REF
0.200 MIN
0.250 MIN
2.946 REF
5.080 MIN
6.350 MIN
25
MOTOROLA ANALOG IC DEVICE DATA
LM2576
NOTES
26
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regarding
the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and
specificallydisclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters which may be provided in Motorola
datasheetsand/orspecificationscananddovaryindifferentapplicationsandactualperformancemayvaryovertime. Alloperatingparameters,including“Typicals”
must be validated for each customer application by customer’s technical experts. Motorola does not convey any license under its patent rights nor the rights of
others. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other
applicationsintended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injury
ordeathmayoccur. ShouldBuyerpurchaseoruseMotorolaproductsforanysuchunintendedorunauthorizedapplication,BuyershallindemnifyandholdMotorola
and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees
arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that
Motorola was negligent regarding the design or manufacture of the part. Motorola and
Opportunity/Affirmative Action Employer.
are registered trademarks of Motorola, Inc. Motorola, Inc. is an Equal
27
MOTOROLA ANALOG IC DEVICE DATA
LM2576
Mfax is a trademark of Motorola, Inc.
JAPAN: Nippon Motorola Ltd.: SPD, Strategic Planning Office, 4–32–1,
How to reach us:
USA/EUROPE/Locations Not Listed: Motorola Literature Distribution;
P.O. Box 5405, Denver, Colorado 80217. 1–303–675–2140 or 1–800–441–2447 Nishi–Gotanda, Shinagawa–ku, Tokyo 141, Japan. 81–3–5487–8488
Customer Focus Center: 1–800–521–6274
Mfax : RMFAX0@email.sps.mot.com – TOUCHTONE 1–602–244–6609
ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; 8B Tai Ping Industrial Park,
Motorola Fax Back System
– US & Canada ONLY 1–800–774–1848 51 Ting Kok Road, Tai Po, N.T., Hong Kong. 852–26629298
– http://sps.motorola.com/mfax/
HOME PAGE: http://motorola.com/sps/
LM2576/D
◊
相关型号:
LM2576-5.0BUT&R
Switching Regulator, Voltage-mode, 7.5A, 63kHz Switching Freq-Max, PSSO5, TO-263, 5 PIN
MICROCHIP
LM2576-5.0WUTR
7.5A SWITCHING REGULATOR, 63kHz SWITCHING FREQ-MAX, PSSO5, ROHS COMPLIANT, TO-263, 5 PIN
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