LM2576-ADJMWC [NSC]
IC 7.5 A SWITCHING REGULATOR, 63 kHz SWITCHING FREQ-MAX, UUC, WAFER, Switching Regulator or Controller;
| 型号： | LM2576-ADJMWC |
| 厂家： | 
National Semiconductor |
| 描述： | IC 7.5 A SWITCHING REGULATOR, 63 kHz SWITCHING FREQ-MAX, UUC, WAFER, Switching Regulator or Controller 稳压器 |
| 文件： | 总24页 (文件大小：744K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
August 2004
LM2576/LM2576HV Series
SIMPLE SWITCHER® 3A Step-Down Voltage Regulator
General Description
Features
n 3.3V, 5V, 12V, 15V, and adjustable output versions
n Adjustable version output voltage range,
1.23V to 37V (57V for HV version) 4% max over
line and load conditions
The LM2576 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving 3A load with
excellent line and load regulation. These devices are avail-
able in fixed output voltages of 3.3V, 5V, 12V, 15V, and an
adjustable output version.
n Guaranteed 3A output current
n Wide input voltage range, 40V up to 60V for
HV version
n Requires only 4 external components
n 52 kHz fixed frequency internal oscillator
n TTL shutdown capability, low power standby mode
n High efficiency
n Uses readily available standard inductors
n Thermal shutdown and current limit protection
n P+ Product Enhancement tested
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation and a fixed-frequency oscillator.
The LM2576 series offers a high-efficiency replacement for
popular three-terminal linear regulators. It substantially re-
duces the size of the heat sink, and in some cases no heat
sink is required.
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.
Applications
n Simple high-efficiency step-down (buck) regulator
n Efficient pre-regulator for linear regulators
n On-card switching regulators
Other features include a guaranteed 4% tolerance on out-
put voltage within specified input voltages and output load
conditions, and 10% on the oscillator frequency. External
shutdown is included, featuring 50 µA (typical) standby cur-
rent. The output switch includes cycle-by-cycle current limit-
ing, as well as thermal shutdown for full protection under
fault conditions.
n Positive to negative converter (Buck-Boost)
Typical Application (Fixed Output Voltage
Versions)
01147601
FIGURE 1.
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation
DS011476
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Block Diagram
01147602
3.3V R2 = 1.7k
5V, R2 = 3.1k
12V, R2 = 8.84k
15V, R2 = 11.3k
For ADJ. Version
R1 = Open, R2 = 0Ω
Patent Pending
Ordering Information
Temperature
Range
Output Voltage
12
NS Package Package
Type
3.3
5.0
15
ADJ
Number
−40˚C ≤ TA LM2576HVS-3.3 LM2576HVS-5.0 LM2576HVS-12 LM2576HVS-15 LM2576HVS-ADJ
≤ 125˚C
TS5B
TO-263
LM2576S-3.3
LM2576S-5.0
LM2576S-12
LM2576S-15
LM2576S-ADJ
LM2576HVSX-3.3 LM2576HVSX-5.0 LM2576HVSX-12 LM2576HVSX-15 LM2576HVSX-ADJ
TS5B
Tape & Reel
LM2576SX-3.3
LM2576HVT-3.3 LM2576HVT-5.0 LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ
LM2576T-3.3 LM2576T-5.0 LM2576T-12 LM2576T-15 LM2576T-ADJ
LM2576HVT-3.3 LM2576HVT-5.0 LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ
LM2576SX-5.0
LM2576SX-12
LM2576SX-15
LM2576SX-ADJ
T05A
TO-220
T05D
Flow LB03
LM2576T-3.3
Flow LB03
Flow LB03
LM2576T-5.0
Flow LB03
Flow LB03
LM2576T-12
Flow LB03
Flow LB03
LM2576T-15
Flow LB03
Flow LB03
LM2576T-ADJ
Flow LB03
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2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Minimum ESD Rating
(C = 100 pF, R = 1.5 kΩ)
Lead Temperature
2 kV
(Soldering, 10 Seconds)
260˚C
Maximum Supply Voltage
LM2576
45V
63V
Operating Ratings
LM2576HV
Temperature Range
LM2576/LM2576HV
Supply Voltage
LM2576
ON /OFF Pin Input Voltage
Output Voltage to Ground
(Steady State)
−0.3V ≤ V ≤ +VIN
−40˚C ≤ TJ ≤ +125˚C
−1V
Internally Limited
−65˚C to +150˚C
150˚C
40V
60V
Power Dissipation
LM2576HV
Storage Temperature Range
Maximum Junction Temperature
LM2576-3.3, LM2576HV-3.3
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol
Parameter
Conditions
LM2576-3.3
Units
(Limits)
LM2576HV-3.3
Typ
3.3
3.3
3.3
75
Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
VOUT
VOUT
VOUT
η
Output Voltage
VIN = 12V, ILOAD = 0.5A
V
Circuit of Figure 2
3.234
3.366
V(Min)
V(Max)
V
Output Voltage
LM2576
6V ≤ VIN ≤ 40V, 0.5A ≤ ILOAD ≤ 3A
Circuit of Figure 2
3.168/3.135
3.432/3.465
V(Min)
V(Max)
V
Output Voltage
LM2576HV
6V ≤ VIN ≤ 60V, 0.5A ≤ ILOAD ≤ 3A
Circuit of Figure 2
3.168/3.135
3.450/3.482
V(Min)
V(Max)
%
Efficiency
VIN = 12V, ILOAD = 3A
LM2576-5.0, LM2576HV-5.0
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with Figure 2 boldface type apply over full Operating Tem-
perature Range.
Symbol
Parameter
Conditions
LM2576-5.0
Units
(Limits)
LM2576HV-5.0
Typ
5.0
5.0
5.0
Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
VOUT
VOUT
VOUT
Output Voltage
VIN = 12V, ILOAD = 0.5A
V
Circuit of Figure 2
4.900
5.100
V(Min)
V(Max)
V
Output Voltage
LM2576
0.5A ≤ ILOAD ≤ 3A,
8V ≤ VIN ≤ 40V
4.800/4.750
5.200/5.250
V(Min)
V(Max)
V
Circuit of Figure 2
0.5A ≤ ILOAD ≤ 3A,
8V ≤ VIN ≤ 60V
Output Voltage
LM2576HV
4.800/4.750
5.225/5.275
V(Min)
V(Max)
Circuit of Figure 2
3
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LM2576-5.0, LM2576HV-5.0
Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25˚C, and those with Figure 2 boldface type apply over full Operating Tem-
perature Range.
Symbol
Parameter
Conditions
LM2576-5.0
Units
(Limits)
LM2576HV-5.0
Typ
Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
η
Efficiency
VIN = 12V, ILOAD = 3A
77
%
LM2576-12, LM2576HV-12
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol
Parameter
Conditions
LM2576-12
Units
(Limits)
LM2576HV-12
Typ
12
Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
VOUT
VOUT
VOUT
η
Output Voltage
VIN = 25V, ILOAD = 0.5A
V
Circuit of Figure 2
11.76
12.24
V(Min)
V(Max)
V
Output Voltage
LM2576
0.5A ≤ ILOAD ≤ 3A,
15V ≤ VIN ≤ 40V
12
11.52/11.40
12.48/12.60
V(Min)
V(Max)
V
Circuit of Figure 2
0.5A ≤ ILOAD ≤ 3A,
15V ≤ VIN ≤ 60V
Output Voltage
LM2576HV
12
11.52/11.40
12.54/12.66
V(Min)
V(Max)
%
Circuit of Figure 2
VIN = 15V, ILOAD = 3A
Efficiency
88
LM2576-15, LM2576HV-15
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol
Parameter
Conditions
LM2576-15
Units
(Limits)
LM2576HV-15
Typ
15
Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
VOUT
VOUT
VOUT
η
Output Voltage
VIN = 25V, ILOAD = 0.5A
V
Circuit of Figure 2
14.70
15.30
V(Min)
V(Max)
V
Output Voltage
LM2576
0.5A ≤ ILOAD ≤ 3A,
18V ≤ VIN ≤ 40V
15
14.40/14.25
15.60/15.75
V(Min)
V(Max)
V
Circuit of Figure 2
0.5A ≤ ILOAD ≤ 3A,
18V ≤ VIN ≤ 60V
Output Voltage
LM2576HV
15
14.40/14.25
15.68/15.83
V(Min)
V(Max)
%
Circuit of Figure 2
VIN = 18V, ILOAD = 3A
Efficiency
88
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4
LM2576-ADJ, LM2576HV-ADJ
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol
Parameter
Conditions
LM2576-ADJ
Units
(Limits)
LM2576HV-ADJ
Typ
1.230
1.230
1.230
77
Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
VOUT
VOUT
VOUT
η
Feedback Voltage
VIN = 12V, ILOAD = 0.5A
VOUT = 5V,
V
1.217
1.243
V(Min)
V(Max)
V
Circuit of Figure 2
Feedback Voltage
LM2576
0.5A ≤ ILOAD ≤ 3A,
8V ≤ VIN ≤ 40V
1.193/1.180
1.267/1.280
V(Min)
V(Max)
V
VOUT = 5V, Circuit of Figure 2
0.5A ≤ ILOAD ≤ 3A,
Feedback Voltage
LM2576HV
8V ≤ VIN ≤ 60V
1.193/1.180
1.273/1.286
V(Min)
V(Max)
%
VOUT = 5V, Circuit of Figure 2
VIN = 12V, ILOAD = 3A, VOUT = 5V
Efficiency
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version, VIN = 25V for the 12V version, and VIN
= 30V for the 15V version. ILOAD = 500 mA.
Symbol
Parameter
Conditions
LM2576-XX
Units
(Limits)
LM2576HV-XX
Typ
Limit
(Note 2)
DEVICE PARAMETERS
Ib
Feedback Bias Current
VOUT = 5V (Adjustable Version Only)
(Note 11)
50
52
100/500
47/42
nA
kHz
fO
Oscillator Frequency
kHz
(Min)
kHz
58/63
(Max)
V
VSAT
DC
Saturation Voltage
Max Duty Cycle (ON)
Current Limit
IOUT = 3A (Note 4)
(Note 5)
1.4
98
1.8/2.0
V(Max)
%
93
%(Min)
A
ICL
(Notes 4, 11)
5.8
4.2/3.5
6.9/7.5
2
A(Min)
A(Max)
mA(Max)
mA
IL
Output Leakage Current
Quiescent Current
(Notes 6, 7): Output = 0V
Output = −1V
Output = −1V
7.5
5
30
10
mA(Max)
mA
IQ
(Note 6)
mA(Max)
µA
ISTBY
Standby Quiescent
Current
ON /OFF Pin = 5V (OFF)
50
200
µA(Max)
5
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All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version, VIN = 25V for the 12V version, and VIN
= 30V for the 15V version. ILOAD = 500 mA.
Symbol
Parameter
Conditions
LM2576-XX
Units
(Limits)
LM2576HV-XX
Typ
Limit
(Note 2)
DEVICE PARAMETERS
θJA
Thermal Resistance
T Package, Junction to Ambient (Note 8)
T Package, Junction to Ambient (Note 9)
T Package, Junction to Case
65
45
2
θJA
˚C/W
θJC
θJA
S Package, Junction to Ambient (Note 10)
50
ON /OFF CONTROL Test Circuit Figure 2
VIH
VIL
IIH
ON /OFF Pin
Logic Input Level
ON /OFF Pin Input
Current
VOUT = 0V
1.4
1.2
12
2.2/2.4
1.0/0.8
V(Min)
V(Max)
µA
VOUT = Nominal Output Voltage
ON /OFF Pin = 5V (OFF)
30
µA(Max)
µA
IIL
ON /OFF Pin = 0V (ON)
0
10
µA(Max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. 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.
Note 2: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods.
Note 3: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the
LM2576/LM2576HV is used as shown in the Figure 2 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 4: Output pin sourcing current. No diode, inductor or capacitor connected to output.
Note 5: Feedback pin removed from output and connected to 0V.
Note 6: Feedback pin removed from output and connected to +12V for the Adjustable, 3.3V, and 5V versions, and +25V for the 12V and 15V versions, to force the
output transistor OFF.
Note 7: V = 40V (60V for high voltage version).
IN
Note 8: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 1
board with minimum copper area.
⁄
2
inch leads in a socket, or on a PC
inch leads soldered to a PC board
Note 9: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 1
⁄
4
containing approximately 4 square inches of copper area surrounding the leads.
Note 10: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using
0.5 square inches of copper area, θ is 50˚C/W, with 1 square inch of copper area, θ is 37˚C/W, and with 1.6 or more square inches of copper area, θ is 32˚C/W.
JA
JA
JA
Note 11: The oscillator frequency reduces to approximately 11 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 power dissipation of the IC by lowering the minimum duty cycle
from 5% down to approximately 2%.
Typical Performance Characteristics
(Circuit of Figure 2)
Normalized Output Voltage
Line Regulation
01147627
01147628
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6
Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Dropout Voltage
Current Limit
01147629
01147631
01147633
01147630
Standby
Quiescent Current
Quiescent Current
01147632
Switch Saturation
Voltage
Oscillator Frequency
01147634
7
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Efficiency
Minimum Operating Voltage
01147635
01147636
01147638
01147637
Quiescent Current
vs Duty Cycle
Feedback Voltage
vs Duty Cycle
01147637
Quiescent Current
vs Duty Cycle
Minimum Operating Voltage
01147636
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8
Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Feedback Voltage
vs Duty Cycle
Feedback Pin Current
01147638
01147604
Maximum Power Dissipation
(TO-263) (See Note 10)
Switching Waveforms
01147606
01147624
V
= 15V
OUT
A: Output Pin Voltage, 50V/div
B: Output Pin Current, 2A/div
C: Inductor Current, 2A/div
D: Output Ripple Voltage, 50 mV/div,
AC-Coupled
Horizontal Time Base: 5 µs/div
Load Transient Response
01147605
9
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Single-point grounding (as indicated) or ground plane con-
struction should be used for best results. When using the
Adjustable version, physically locate the programming resis-
tors near the regulator, to keep the sensitive feedback wiring
short.
Test Circuit and Layout Guidelines
As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.
Fixed Output Voltage Versions
01147607
C
C
D
— 100 µF, 75V, Aluminum Electrolytic
IN
— 1000 µF, 25V, Aluminum Electrolytic
OUT
— Schottky, MBR360
— 100 µH, Pulse Eng. PE-92108
— 2k, 0.1%
1
L
1
R
1
R
2
— 6.12k, 0.1%
Adjustable Output Voltage Version
01147608
where V
= 1.23V, R1 between 1k and 5k.
REF
FIGURE 2.
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10
LM2576 Series Buck Regulator
Design Procedure
PROCEDURE (Fixed Output Voltage Versions)
Given: VOUT = Regulated Output Voltage (3.3V, 5V, 12V,
or 15V) VIN(Max) = Maximum Input Voltage ILOAD(Max) =
Maximum Load Current
EXAMPLE (Fixed Output Voltage Versions)
Given: VOUT = 5V VIN(Max) = 15V ILOAD(Max) = 3A
1. Inductor Selection (L1) A. Select the correct Inductor
value selection guide from Figures 3, 4, 5 or Figure 6.
(Output voltages of 3.3V, 5V, 12V or 15V respectively).
For other output voltages, see the design procedure for
the adjustable version. B. From the inductor value
selection guide, identify the inductance region intersected
by VIN(Max) and ILOAD(Max), and note the inductor code
for that region. C. Identify the inductor value from the
inductor code, and select an appropriate inductor from
the table shown in Figure 3. Part numbers are listed for
three inductor manufacturers. The inductor chosen must
be rated for operation at the LM2576 switching frequency
(52 kHz) and for a current rating of 1.15 x ILOAD. For
additional inductor information, see the inductor section
in the Application Hints section of this data sheet.
2. Output Capacitor Selection (COUT) A. The value of
the output capacitor together with the inductor defines
the dominate pole-pair of the switching regulator loop.
For stable operation and an acceptable output ripple
voltage, (approximately 1% of the output voltage) a value
between 100 µF and 470 µF is recommended. B. The
capacitor’s voltage rating should be at least 1.5 times
greater than the output voltage. For a 5V regulator, a
rating of at least 8V is appropriate, and a 10V or 15V
rating is recommended. Higher voltage electrolytic
capacitors generally have lower ESR numbers, and for
this reason it may be necessary to select a capacitor
rated for a higher voltage than would normally be
needed.
1. Inductor Selection (L1) A. Use the selection guide
shown in Figure 4. B. From the selection guide, the
inductance area intersected by the 15V line and 3A line
is L100. C. Inductor value required is 100 µH. From the
table in Figure 3. Choose AIE 415-0930, Pulse
Engineering PE92108, or Renco RL2444.
2. Output Capacitor Selection (COUT) A. COUT = 680
µF to 2000 µF standard aluminum electrolytic.
B.Capacitor voltage rating = 20V.
3. Catch Diode Selection (D1) A.The catch-diode
current rating must be at least 1.2 times greater than the
maximum load current. Also, if the power supply design
must withstand a continuous output short, the diode
should have a current rating equal to the maximum
current limit of the LM2576. The most stressful condition
for this diode is an overload or shorted output condition.
B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.
3. Catch Diode Selection (D1) A.For this example, a 3A
current rating is adequate. B. Use a 20V 1N5823 or
SR302 Schottky diode, or any of the suggested
fast-recovery diodes shown in Figure 8.
4. Input Capacitor (CIN) An aluminum or tantalum
electrolytic bypass capacitor located close to the
regulator is needed for stable operation.
4. Input Capacitor (CIN) A 100 µF, 25V aluminum
electrolytic capacitor located near the input and ground
pins provides sufficient bypassing.
11
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LM2576 Series Buck Regulator
Design Procedure (Continued)
INDUCTOR VALUE SELECTION GUIDES (For
Continuous Mode Operation)
01147611
FIGURE 5. LM2576(HV)-12
01147609
FIGURE 3. LM2576(HV)-3.3
01147612
FIGURE 6. LM2576(HV)-15
01147610
FIGURE 4. LM2576(HV)-5.0
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12
LM2576 Series Buck Regulator Design Procedure (Continued)
01147613
FIGURE 7. LM2576(HV)-ADJ
PROCEDURE (Adjustable Output Voltage Versions)
Given: VOUT = Regulated Output Voltage VIN(Max) =
Maximum Input Voltage ILOAD(Max) = Maximum Load
Current F = Switching Frequency (Fixed at 52 kHz)
1. Programming Output Voltage (Selecting R1 and R2,
as shown in Figure 2) Use the following formula to select
the appropriate resistor values.
EXAMPLE (Adjustable Output Voltage Versions)
Given: VOUT = 10V VIN(Max) = 25V ILOAD(Max) = 3A F =
52 kHz
1. Programming Output Voltage (Selecting R1 and R2)
R1 can be between 1k and 5k. (For best temperature
coefficient and stability with time, use 1% metal film resis-
tors)
R2 = 1k (8.13 − 1) = 7.13k, closest 1% value is 7.15k
13
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LM2576 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions)
EXAMPLE (Adjustable Output Voltage Versions)
2. Inductor Selection (L1) A. Calculate the inductor Volt
• microsecond constant, E • T (V • µs), from the
following formula:
2. Inductor Selection (L1) A. Calculate E • T (V • µs)
B. E • T = 115 V • µs C. ILOAD(Max) = 3A D. Inductance
Region = H150 E. Inductor Value = 150 µH Choose from
AIE part #415-0936 Pulse Engineering part #PE-531115,
or Renco part #RL2445.
B. Use the E • T value from the previous formula and
match it with the E • T number on the vertical axis of the
Inductor Value Selection Guide shown in Figure 7. C. On
the horizontal axis, select the maximum load current. D.
Identify the inductance region intersected by the E • T
value and the maximum load current value, and note the
inductor code for that region. E. Identify the inductor value
from the inductor code, and select an appropriate inductor
from the table shown in Figure 9. Part numbers are listed
for three inductor manufacturers. The inductor chosen
must be rated for operation at the LM2576 switching fre-
quency (52 kHz) and for a current rating of 1.15 x ILOAD
.
For additional inductor information, see the inductor sec-
tion in the application hints section of this data sheet.
3. Output Capacitor Selection (COUT) A. The value of
the output capacitor together with the inductor defines
the dominate pole-pair of the switching regulator loop.
For stable operation, the capacitor must satisfy the
following requirement:
3. Output Capacitor Selection (COUT)
However, for acceptable output ripple voltage select COUT
≥ 680 µF COUT = 680 µF electrolytic capacitor
The above formula yields capacitor values between 10 µF
and 2200 µF that will satisfy the loop requirements for
stable operation. But to achieve an acceptable output
ripple voltage, (approximately 1% of the output voltage)
and transient response, the output capacitor may need to
be several times larger than the above formula yields. B.
The capacitor’s voltage rating should be at last 1.5 times
greater than the output voltage. For a 10V regulator, a
rating of at least 15V or more is recommended. Higher
voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select
a capacitor rate for a higher voltage than would normally be
needed.
4. Catch Diode Selection (D1) A. The catch-diode
current rating must be at least 1.2 times greater than the
maximum load current. Also, if the power supply design
must withstand a continuous output short, the diode
should have a current rating equal to the maximum
current limit of the LM2576. The most stressful condition
for this diode is an overload or shorted output. See diode
selection guide in Figure 8. B. The reverse voltage rating
of the diode should be at least 1.25 times the maximum
input voltage.
4. Catch Diode Selection (D1) A. For this example, a
3.3A current rating is adequate. B. Use a 30V 31DQ03
Schottky diode, or any of the suggested fast-recovery
diodes in Figure 8.
5. Input Capacitor (CIN) An aluminum or tantalum
electrolytic bypass capacitor located close to the
regulator is needed for stable operation.
5. Input Capacitor (CIN) A 100 µF aluminum electrolytic
capacitor located near the input and ground pins
provides sufficient bypassing.
To further simplify the buck regulator design procedure, Na-
tional Semiconductor is making available computer design
software to be used with the SIMPLE SWITCHER line of
switching regulators. Switchers Made Simple (Version 3.3)
is available on a (31⁄
2") diskette for IBM compatible comput-
ers from a National Semiconductor sales office in your area.
www.national.com
14
VR
20V
Schottky
Fast Recovery
4A–6A
3A
1N5820
4A–6A
1N5823
3A
MBR320P
SR302
30V
40V
1N5821
MBR330
31DQ03
SR303
50WQ03
1N5824
The following
diodes are all
rated to 100V
The following
diodes are all
rated to 100V
1N5822
MBR340
31DQ04
SR304
MBR340
50WQ04
1N5825
50WF10
MUR410
HER602
31DF1
HER302
50V
60V
MBR350
31DQ05
SR305
50WQ05
MBR360
DQ06
50WR06
50SQ060
SR306
FIGURE 8. Diode Selection Guide
Inductor
Code
Inductor
Value
Schott
Pulse Eng.
Renco
(Note 14)
RL2442
RL2443
RL2444
RL1954
RL1953
RL1952
RL1951
RL1950
RL2445
RL2446
RL2447
RL1961
RL1960
RL1959
RL1958
RL2448
(Note 12)
(Note 13)
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
L47
47 µH
68 µH
671 26980
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
L68
L100
L150
L220
L330
L470
L680
H150
H220
H330
H470
H680
H1000
H1500
H2200
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
Note 12: Schott Corporation, (612) 475-1173, 1000 Parkers Lake Road, Wayzata, MN 55391.
Note 13: Pulse Engineering, (619) 674-8100, P.O. Box 12235, San Diego, CA 92112.
Note 14: Renco Electronics Incorporated, (516) 586-5566, 60 Jeffryn Blvd. East, Deer Park, NY 11729.
FIGURE 9. Inductor Selection by Manufacturer’s Part Number
If the operating temperature range includes temperatures
below −25˚C, the input capacitor value may need to be
larger. With most electrolytic capacitors, the capacitance
value decreases and the ESR increases with lower tempera-
tures and age. Paralleling a ceramic or solid tantalum ca-
pacitor will increase the regulator stability at cold tempera-
tures. For maximum capacitor operating lifetime, the
capacitor’s RMS ripple current rating should be greater than
Application Hints
INPUT CAPACITOR (CIN
)
To maintain stability, the regulator input pin must be by-
passed with at least a 100 µF electrolytic capacitor. The
capacitor’s leads must be kept short, and located near the
regulator.
15
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rapidly. Different inductor types have different saturation
characteristics, and this should be kept in mind when select-
ing an inductor.
Application Hints (Continued)
The inductor manufacturer’s data sheets include current and
energy limits to avoid inductor saturation.
INDUCTOR RIPPLE CURRENT
When the switcher is operating in the continuous mode, the
inductor current waveform ranges from a triangular to a
sawtooth type of waveform (depending on the input voltage).
For a given input voltage and output voltage, the peak-to-
peak amplitude of this inductor current waveform remains
constant. As the load current rises or falls, the entire saw-
tooth current waveform also rises or falls. The average DC
value of this waveform is equal to the DC load current (in the
buck regulator configuration).
INDUCTOR SELECTION
All switching regulators have two basic modes of operation:
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulator per-
formance and requirements.
If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will change to a discontinuous mode of operation.
This is a perfectly acceptable mode of operation. Any buck
switching regulator (no matter how large the inductor value
is) will be forced to run discontinuous if the load current is
light enough.
The LM2576 (or any of the SIMPLE SWITCHER family) can
be used for both continuous and discontinuous modes of
operation.
OUTPUT CAPACITOR
The inductor value selection guides in Figure 3 through
Figure 7 were designed for buck regulator designs of the
continuous inductor current type. When using inductor val-
ues shown in the inductor selection guide, the peak-to-peak
inductor ripple current will be approximately 20% to 30% of
the maximum DC current. With relatively heavy load cur-
rents, the circuit operates in the continuous mode (inductor
current always flowing), but under light load conditions, the
circuit will be forced to the discontinuous mode (inductor
current falls to zero for a period of time). This discontinuous
mode of operation is perfectly acceptable. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, primarily
because of the lower inductor values required for the discon-
tinuous mode.
An output capacitor is required to filter the output voltage and
is needed for loop stability. The capacitor should be located
near the LM2576 using short pc board traces. Standard
aluminum electrolytics are usually adequate, but low ESR
types are recommended for low output ripple voltage and
good stability. The ESR of a capacitor depends on many
factors, some which are: the value, the voltage rating, physi-
cal size and the type of construction. In general, low value or
low voltage (less than 12V) electrolytic capacitors usually
have higher ESR numbers.
The amount of output ripple voltage is primarily a function of
the ESR (Equivalent Series Resistance) of the output ca-
pacitor and the amplitude of the inductor ripple current
(∆IIND). See the section on inductor ripple current in Applica-
tion Hints.
The selection guide chooses inductor values suitable for
continuous mode operation, but if the inductor value chosen
is prohibitively high, the designer should investigate the
possibility of discontinuous operation. The computer design
software Switchers Made Simple will provide all component
values for discontinuous (as well as continuous) mode of
operation.
The lower capacitor values (220 µF–1000 µF) will allow
typically 50 mV to 150 mV of output ripple voltage, while
larger-value capacitors will reduce the ripple to approxi-
mately 20 mV to 50 mV.
Output Ripple Voltage = (∆IIND) (ESR of COUT
)
To further reduce the output ripple voltage, several standard
electrolytic capacitors may be paralleled, or a higher-grade
capacitor may be used. Such capacitors are often called
“high-frequency,” “low-inductance,” or “low-ESR.” These will
reduce the output ripple to 10 mV or 20 mV. However, when
operating in the continuous mode, reducing the ESR below
0.03Ω can cause instability in the regulator.
Tantalum capacitors can have a very low ESR, and should
be carefully evaluated if it is the only output capacitor. Be-
cause of their good low temperature characteristics, a tan-
talum can be used in parallel with aluminum electrolytics,
with the tantalum making up 10% or 20% of the total capaci-
tance.
Inductors are available in different styles such as pot core,
toriod, E-frame, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least
expensive, the bobbin core type, consists of wire wrapped
on a ferrite rod core. This type of construction makes for an
inexpensive inductor, but since the magnetic flux is not com-
pletely contained within the core, it generates more electro-
magnetic interference (EMI). This EMI can cause problems
in sensitive circuits, or can give incorrect scope readings
because of induced voltages in the scope probe.
The inductors listed in the selection chart include ferrite pot
core construction for AIE, powdered iron toroid for Pulse
Engineering, and ferrite bobbin core for Renco.
The capacitor’s ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.
An inductor should not be operated beyond its maximum
rated current because it may saturate. When an inductor
begins to saturate, the inductance decreases rapidly and the
inductor begins to look mainly resistive (the DC resistance of
the winding). This will cause the switch current to rise very
www.national.com
16
HEAT SINK/THERMAL CONSIDERATIONS
Application Hints (Continued)
CATCH DIODE
In many cases, only a small heat sink is required to keep the
LM2576 junction temperature within the allowed operating
range. For each application, to determine whether or not a
heat sink will be required, the following must be identified:
Buck regulators require a diode to provide a return path for
the inductor current when the switch is off. This diode should
be located close to the LM2576 using short leads and short
printed circuit traces.
1. Maximum ambient temperature (in the application).
2. Maximum regulator power dissipation (in application).
Because of their fast switching speed and low forward volt-
age drop, Schottky diodes provide the best efficiency, espe-
cially in low output voltage switching regulators (less than
5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery
diodes are also suitable, but some types with an abrupt
turn-off characteristic may cause instability and EMI prob-
lems. A fast-recovery diode with soft recovery characteristics
is a better choice. Standard 60 Hz diodes (e.g., 1N4001 or
1N5400, etc.) are also not suitable. See Figure 8 for Schot-
tky and “soft” fast-recovery diode selection guide.
3. Maximum allowed junction temperature (125˚C for the
LM2576). For a safe, conservative design, a tempera-
ture approximately 15˚C cooler than the maximum tem-
peratures should be selected.
4. LM2576 package thermal resistances θJA and θJC
.
Total power dissipated by the LM2576 can be estimated as
follows:
PD = (VIN)(IQ) + (VO/VIN)(ILOAD)(VSAT
)
where IQ (quiescent current) and VSAT can be found in the
Characteristic Curves shown previously, VIN is the applied
minimum input voltage, VO is the regulated output voltage,
and ILOAD is the load current. The dynamic losses during
turn-on and turn-off are negligible if a Schottky type catch
diode is used.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply will contain a
sawtooth ripple voltage at the switcher frequency, typically
about 1% of the output voltage, and may also contain short
voltage spikes at the peaks of the sawtooth waveform.
When no heat sink is used, the junction temperature rise can
be determined by the following:
The output ripple voltage is due mainly to the inductor saw-
tooth ripple current multiplied by the ESR of the output
capacitor. (See the inductor selection in the application
hints.)
∆TJ = (PD) (θJA
)
To arrive at the actual operating junction temperature, add
the junction temperature rise to the maximum ambient tem-
perature.
The voltage spikes are present because of the the fast
switching action of the output switch, and the parasitic induc-
tance of the output filter capacitor. To minimize these voltage
spikes, special low inductance capacitors can be used, and
their lead lengths must be kept short. Wiring inductance,
stray capacitance, as well as the scope probe used to evalu-
ate these transients, all contribute to the amplitude of these
spikes.
TJ = ∆TJ + TA
If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, then a heat sink is required.
When using a heat sink, the junction temperature rise can be
determined by the following:
An additional small LC filter (20 µH & 100 µF) can be added
to the output (as shown in Figure 15) to further reduce the
amount of output ripple and transients. A 10 x reduction in
output ripple voltage and transients is possible with this filter.
∆TJ = (PD) (θJC + θinterface + θHeat sink
The operating junction temperature will be:
)
TJ = TA + ∆TJ
As above, if the actual operating junction temperature is
greater than the selected safe operating junction tempera-
ture, then a larger heat sink is required (one that has a lower
thermal resistance).
FEEDBACK CONNECTION
The LM2576 (fixed voltage versions) feedback pin must be
wired to the output voltage point of the switching power
supply. When using the adjustable version, physically locate
both output voltage programming resistors near the LM2576
to avoid picking up unwanted noise. Avoid using resistors
greater than 100 kΩ because of the increased chance of
noise pickup.
Included on the Switcher Made Simple design software is a
more precise (non-linear) thermal model that can be used to
determine junction temperature with different input-output
parameters or different component values. It can also calcu-
late the heat sink thermal resistance required to maintain the
regulators junction temperature below the maximum operat-
ing temperature.
ON /OFF INPUT
For normal operation, the ON /OFF pin should be grounded
or driven with a low-level TTL voltage (typically below 1.6V).
To put the regulator into standby mode, drive this pin with a
high-level TTL or CMOS signal. The ON /OFF pin can be
safely pulled up to +VIN without a resistor in series with it.
The ON /OFF pin should not be left open.
Additional Applications
INVERTING REGULATOR
Figure 10 shows a LM2576-12 in a buck-boost configuration
to generate a negative 12V output from a positive input
voltage. This circuit bootstraps the regulator’s ground pin to
the negative output voltage, then by grounding the feedback
pin, the regulator senses the inverted output voltage and
regulates it to −12V.
GROUNDING
To maintain output voltage stability, the power ground con-
nections must be low-impedance (see Figure 2). For the
5-lead TO-220 and TO-263 style package, both the tab and
pin 3 are ground and either connection may be used, as they
are both part of the same copper lead frame.
For an input voltage of 12V or more, the maximum available
output current in this configuration is approximately 700 mA.
At lighter loads, the minimum input voltage required drops to
approximately 4.7V.
17
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Additional Applications (Continued)
The switch currents in this buck-boost configuration are
higher than in the standard buck-mode design, thus lowering
the available output current. Also, the start-up input current
of the buck-boost converter is higher than the standard
buck-mode regulator, and this may overload an input power
source with a current limit less than 5A. Using a delayed
turn-on or an undervoltage lockout circuit (described in the
next section) would allow the input voltage to rise to a high
enough level before the switcher would be allowed to turn
on.
Because of the structural differences between the buck and
the buck-boost regulator topologies, the buck regulator de-
sign procedure section can not be used to to select the
inductor or the output capacitor. The recommended range of
inductor values for the buck-boost design is between 68 µH
and 220 µH, and the output capacitor values must be larger
than what is normally required for buck designs. Low input
voltages or high output currents require a large value output
capacitor (in the thousands of micro Farads).
01147615
Typical Load Current
400 mA for V = −5.2V
IN
750 mA for V = −7V
IN
Note: Heat sink may be required.
FIGURE 11. Negative Boost
The peak inductor current, which is the same as the peak
switch current, can be calculated from the following formula:
Because of the boosting function of this type of regulator, the
switch current is relatively high, especially at low input volt-
ages. Output load current limitations are a result of the
maximum current rating of the switch. Also, boost regulators
can not provide current limiting load protection in the event of
a shorted load, so some other means (such as a fuse) may
be necessary.
Where fosc = 52 kHz. Under normal continuous inductor
current operating conditions, the minimum VIN represents
the worst case. Select an inductor that is rated for the peak
current anticipated.
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off
until the input voltage reaches a certain threshold. An und-
ervoltage lockout circuit which accomplishes this task is
shown in Figure 12, while Figure 13 shows the same circuit
applied to a buck-boost configuration. These circuits keep
the regulator off until the input voltage reaches a predeter-
mined level.
VTH ≈ VZ1 + 2VBE(Q1)
01147614
FIGURE 10. Inverting Buck-Boost Develops −12V
Also, the maximum voltage appearing across the regulator is
the absolute sum of the input and output voltage. For a −12V
output, the maximum input voltage for the LM2576 is +28V,
or +48V for the LM2576HV.
The Switchers Made Simple (version 3.0) design software
can be used to determine the feasibility of regulator designs
using different topologies, different input-output parameters,
different components, etc.
01147616
Note: Complete circuit not shown.
NEGATIVE BOOST REGULATOR
Another variation on the buck-boost topology is the negative
boost configuration. The circuit in Figure 11 accepts an input
voltage ranging from −5V to −12V and provides a regulated
−12V output. Input voltages greater than −12V will cause the
output to rise above −12V, but will not damage the regulator.
FIGURE 12. Undervoltage Lockout for Buck Circuit
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18
ing. Increasing the RC time constant can provide longer
delay times. But excessively large RC time constants can
cause problems with input voltages that are high in 60 Hz or
120 Hz ripple, by coupling the ripple into the ON /OFF pin.
Additional Applications (Continued)
ADJUSTABLE OUTPUT, LOW-RIPPLE POWER SUPPLY
A 3A power supply that features an adjustable output voltage
is shown in Figure 15. An additional L-C filter that reduces
the output ripple by a factor of 10 or more is included in this
circuit.
01147617
Note: Complete circuit not shown (see Figure 10).
FIGURE 13. Undervoltage Lockout
for Buck-Boost Circuit
01147618
Note: Complete circuit not shown.
DELAYED STARTUP
The ON /OFF pin can be used to provide a delayed startup
feature as shown in Figure 14. With an input voltage of 20V
and for the part values shown, the circuit provides approxi-
mately 10 ms of delay time before the circuit begins switch-
FIGURE 14. Delayed Startup
01147619
FIGURE 15. 1.2V to 55V Adjustable 3A Power Supply with Low Output Ripple
Definition of Terms
BUCK REGULATOR
A switching regulator topology in which a higher voltage is
converted to a lower voltage. Also known as a step-down
switching regulator.
BUCK-BOOST REGULATOR
CATCH DIODE OR CURRENT STEERING DIODE
A switching regulator topology in which a positive voltage is
converted to a negative voltage without a transformer.
The diode which provides a return path for the load current
when the LM2576 switch is OFF.
DUTY CYCLE (D)
EFFICIENCY (η)
Ratio of the output switch’s on-time to the oscillator period.
The proportion of input power actually delivered to the load.
19
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OPERATING VOLT MICROSECOND CONSTANT (E•Top
)
Definition of Terms (Continued)
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)
The product (in VoIt•µs) of the voltage applied to the inductor
and the time the voltage is applied. This E•Top 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.
The purely resistive component of a real capacitor’s imped-
ance (see Figure 16). It causes power loss resulting in
capacitor heating, which directly affects the capacitor’s op-
erating lifetime. When used as a switching regulator output
filter, higher ESR values result in higher output ripple volt-
ages.
Connection Diagrams (Note 15)
Straight Leads
5-Lead TO-220 (T)
Top View
01147620
FIGURE 16. Simple Model of a Real Capacitor
Most standard aluminum electrolytic capacitors in the
100 µF–1000 µF range have 0.5Ω to 0.1Ω ESR. Higher-
grade capacitors (“low-ESR”, “high-frequency”, or “low-
inductance”) in the 100 µF–1000 µF range generally have
ESR of less than 0.15Ω.
01147621
LM2576T-XX or LM2576HVT-XX
NS Package Number T05A
TO-263 (S)
5-Lead Surface-Mount Package
Top View
EQUIVALENT SERIES INDUCTANCE (ESL)
The pure inductance component of a capacitor (see Figure
16). The amount of inductance is determined to a large
extent on the capacitor’s construction. In a buck regulator,
this unwanted inductance causes voltage spikes to appear
on the output.
OUTPUT RIPPLE VOLTAGE
The AC component of the switching regulator’s output volt-
age. It is usually dominated by the output capacitor’s ESR
multiplied by the inductor’s ripple current (∆IIND). The peak-
to-peak value of this sawtooth ripple current can be deter-
mined by reading the Inductor Ripple Current section of the
Application hints.
01147625
LM2576S-XX or LM2576HVS-XX
NS Package Number TS5B
LM2576SX-XX or LM2576HVSX-XX
NS Package Number TS5B, Tape and Reel
CAPACITOR RIPPLE CURRENT
RMS value of the maximum allowable alternating current at
which a capacitor can be operated continuously at a speci-
fied temperature.
Bent, Staggered Leads
5-Lead TO-220 (T)
Top View
STANDBY QUIESCENT CURRENT (ISTBY
)
Supply current required by the LM2576 when in the standby
mode (ON /OFF pin is driven to TTL-high voltage, thus
turning the output switch OFF).
INDUCTOR RIPPLE CURRENT (∆IIND
)
The peak-to-peak value of the inductor current waveform,
typically a sawtooth waveform when the regulator is operat-
ing in the continuous mode (vs. discontinuous mode).
CONTINUOUS/DISCONTINUOUS MODE OPERATION
Relates to the inductor current. In the continuous mode, the
inductor current is always flowing and never drops to zero,
vs. the discontinuous mode, where the inductor current
drops to zero for a period of time in the normal switching
cycle.
01147622
LM2576T-XX Flow LB03
or LM2576HVT-XX Flow LB03
NS Package Number T05D
INDUCTOR SATURATION
Note 15: (XX indicates output voltage option. See ordering information table
The condition which exists when an inductor cannot hold any
more magnetic flux. When an inductor saturates, the induc-
tor appears less inductive and the resistive component domi-
nates. Inductor current is then limited only by the DC resis-
tance of the wire and the available source current.
for complete part number.)
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20
Physical Dimensions inches (millimeters)
unless otherwise noted
5-Lead TO-220 (T)
Order Number LM2576T-3.3, LM2576HVT-3.3,
LM2576T-5.0, LM2576HVT-5.0, LM2576T-12,
LM2576HVT-12, LM2576T-15, LM2576HVT-15,
LM2576T-ADJ or LM2576HVT-ADJ
NS Package Number T05A
21
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Bent, Staggered 5-Lead TO-220 (T)
Order Number LM2576T-3.3 Flow LB03, LM2576T-XX Flow LB03, LM2576HVT-3.3 Flow LB03,
LM2576T-5.0 Flow LB03, LM2576HVT-5.0 Flow LB03,
LM2576T-12 Flow LB03, LM2576HVT-12 Flow LB03,
LM2576T-15 Flow LB03, LM2576HVT-15 Flow LB03,
LM2576T-ADJ Flow LB03 or LM2576HVT-ADJ Flow LB03
NS Package Number T05D
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22
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
5-Lead TO-263 (S)
Order Number LM2576S-3.3, LM2576S-5.0,
LM2576S-12,LM2576S-15, LM2576S-ADJ,
LM2576HVS-3.3, LM2576HVS-5.0, LM2576HVS-12,
LM2576HVS-15, or LM2576HVS-ADJ
NS Package Number TS5B
5-Lead TO-263 in Tape & Reel (SX)
Order Number LM2576SX-3.3, LM2576SX-5.0,
LM2576SX-12, LM2576SX-15, LM2576SX-ADJ,
LM2576HVSX-3.3, LM2576HVSX-5.0, LM2576HVSX-12,
LM2576HVSX-15, or LM2576HVSX-ADJ
NS Package Number TS5B
23
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相关型号：
LM2576BUT&R
Switching Regulator, Voltage-mode, 7.5A, 63kHz Switching Freq-Max, PSSO5, TO-263, 5 PIN
MICROCHIP
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