LM1578 [NSC]
Switching Regulator; 开关稳压器
| 型号： | LM1578 |
| 厂家： | 
National Semiconductor |
| 描述： | Switching Regulator |
| 文件： | 总18页 (文件大小：1040K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
April 1998
LM1578A/LM2578A/LM3578A
Switching Regulator
General Description
Features
n Inverting and non-inverting feedback inputs
n 1.0V reference at inputs
The LM1578A is a switching regulator which can easily be
set up for such DC-to-DC voltage conversion circuits as the
buck, boost, and inverting configurations. The LM1578A fea-
tures a unique comparator input stage which not only has
separate pins for both the inverting and non-inverting inputs,
but also provides an internal 1.0V reference to each input,
thereby simplifying circuit design and p.c. board layout. The
output can switch up to 750 mA and has output pins for its
collector and emitter to promote design flexibility. An external
current limit terminal may be referenced to either the ground
or the Vin terminal, depending upon the application. In addi-
tion, the LM1578A has an on board oscillator, which sets the
n Operates from supply voltages of 2V to 40V
n Output current up to 750 mA, saturation less than 0.9V
n Current limit and thermal shut down
n Duty cycle up to 90%
Applications
n Switching regulators in buck, boost, inverting, and
single-ended transformer configurations
n Motor speed control
<
switching frequency with a single external capacitor from
Hz to 100 kHz (typical).
1
n Lamp flasher
The LM1578A is an improved version of the LM1578, offer-
ing higher maximum ratings for the total supply voltage and
output transistor emitter and collector voltages.
Functional Diagram
DS008711-1
© 1998 National Semiconductor Corporation
DS008711
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Absolute Maximum Ratings (Note 1)
ESD Tolerance (Note 4)
2 kV
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Ratings
Ambient Temperature Range
LM1578A
Total Supply Voltage
50V
−0.3V to +50V
−1V to +50V
−55˚C ≤ TA ≤+125˚C
−40˚C ≤ TA ≤+85˚C
0˚C ≤ TA ≤+70˚C
Collector Output to Ground
Emitter Output to Ground (Note 2)
Power Dissipation (Note 3)
Output Current
LM2578A
LM3578A
Internally limited
750 mA
Junction Temperature Range
LM1578A
−55˚C ≤ TJ ≤+150˚C
−40˚C ≤ TJ ≤+125˚C
0˚C ≤ TJ ≤+125˚C
Storage Temperature
−65˚C to +150˚C
LM2578A
Lead Temperature
LM3578A
(soldering, 10 seconds)
Maximum Junction Temperature
260˚C
150˚C
Electrical Characteristics
=
These specifications apply for 2V ≤ VIN ≤ 40V (2.2V ≤ VIN ≤ 40V for TJ ≤ −25˚C), timing capacitor CT 3900 pF, and 25% ≤
=
duty cycle ≤ 75%, unless otherwise specified. Values in standard typeface are for TJ 25˚C; values in boldface type apply for
operation over the specified operating junction temperature range.
LM1578A
Limit
LM2578A/
LM3578A
Symbol
Parameter
Conditions
Typical
Units
(Note 5)
(Note 6)
(Note 11)
Limit
(Note 7)
OSCILLATOR
fOSC
Frequency
20
kHz
22.4
17.6
24
16
kHz (max)
kHz (min)
%/˚C
∆fOSC/∆T
Frequency Drift with
Temperature
−0.13
550
Amplitude
mVp-p
REFERENCE/COMPARATOR (Note 8)
=
=
VR
Input Reference
Voltage
I1 I2 0 mA and
1.0
V
=
=
±
I1 I2 1 mA 1% (Note 9)
1.035/1.050 1.050/1.070 V (max)
0.965/0.950 0.950/0.930 V (min)
%/V
=
=
∆VR/∆VIN
Input Reference Volt-
age Line Regulation
Inverting Input
I1 I2 0 mA and
0.003
0.5
=
=
±
I1 I2 1 mA 1% (Note 9)
0.01/0.02
0.01/0.02 %/V (max)
=
=
=
IINV
I1 I2 0 mA, duty cycle 25%
µA
Current
=
Level Shift Accuracy
Level Shift Current 1 mA
1.0
%
5/8
10/13
% (max)
∆VR/∆t
Input Reference
Voltage Long Term
Stability
100
ppm/1000h
OUTPUT
=
VC (sat)
Collector Saturation
Voltage
IC 750 mA pulsed, Emitter
0.7
1.4
0.1
60
V
grounded
0.85/1.2
1.6/2.1
50/100
50
0.90/1.2
1.7/2.0
200/250
50
V (max)
V
=
IO 80 mA pulsed,
VE (sat)
ICES
Emitter Saturation
Voltage
=
=
VIN VC 40V
V (max)
µA
=
=
Collector Leakage
Current
VIN VCE 40V, Emitter
grounded, Output OFF
µA (max)
V
=
=
0
BVCEO(SUS) Collector-Emitter
Sustaining Voltage
ISUST 0.2A (pulsed), VIN
V (min)
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2
Electrical Characteristics (Continued)
=
These specifications apply for 2V ≤ VIN ≤ 40V (2.2V ≤ VIN ≤ 40V for TJ ≤ −25˚C), timing capacitor CT 3900 pF, and 25% ≤
=
duty cycle ≤ 75%, unless otherwise specified. Values in standard typeface are for TJ 25˚C; values in boldface type apply for
operation over the specified operating junction temperature range.
LM1578A
Limit
LM2578A/
LM3578A
Symbol
Parameter
Conditions
Typical
Units
(Note 5)
(Note 6)
(Note 11)
Limit
(Note 7)
CURRENT LIMIT
VCL
Sense Voltage
Shutdown Level
Referred to VIN or Ground
(Note 10)
110
mV
95
80
mV (min)
mV (max)
%/˚C
140
160
∆VCL/∆T
Sense Voltage
0.3
Temperature Drift
Sense Bias Current
ICL
Referred to VIN
4.0
0.4
µA
µA
Referred to ground
DEVICE POWER CONSUMPTION
IS Supply Current
=
Output OFF, VE 0V
2.0
14
mA
3.0/3.3
3.5/4.0
mA (max)
mA
=
Output ON, IC 750 mA pulsed,
=
VE 0V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its rated operating conditions.
Note 2: For T ≥ 100˚C, the Emitter pin voltage should not be driven more than 0.6V below ground (see Application Information).
J
Note 3: At elevated temperatures, devices must be derated based on package thermal resistance. The device in the TO-99 package must be derated at 150˚C/W,
junction to ambient, or 45˚C/W, junction to case. The device in the 8-pin DIP must be derated at 95˚C/W, junction to ambient. The device in the surface-mount package
must be derated at 150˚C/W, junction-to-ambient.
Note 4: Human body model, 1.5 kΩ in series with 100 pF.
=
Note 5: Typical values are for T
25˚C and represent the most likely parametric norm.
J
Note 6: All limits guaranteed and 100% production tested at room temperature (standard type face) and at temperature extremes (bold type face). All limits are used
to calculate Average Outgoing Quality Level (AOQL).
Note 7: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). Room temperature limits are 100% production
tested. Limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate AOQL.
Note 8: Input terminals are protected from accidental shorts to ground but if external voltages higher than the reference voltage are applied, excessive current will
flow and should be limited to less than 5 mA.
Note 9:
I and I are the external sink currents at the inputs (refer to Test Circuit).
1 2
Note 10: Connection of a 10 kΩ resistor from pin 1 to pin 4 will drive the duty cycle to its maximum, typically 90%. Applying the minimum Current Limit Sense Voltage
to pin 7 will not reduce the duty cycle to less than 50%. Applying the maximum Current Limit Sense Voltage to pin 7 is certain to reduce the duty cycle below 50%.
Increasing this voltage by 15 mV may be required to reduce the duty cycle to 0%, when the Collector output swing is 40V or greater (see Ground-Referred Current
Limit Sense Voltage typical curve).
Note 11: A military RETS specification is available on request. At the time of printing, the LM1578A RETS spec complied with the boldface limits in this column. The
LM1578AH may also be procured as a Standard Military Drawing.
3
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Connection Diagram and Ordering Information
Metal Can
Dual-In-Line Package
DS008711-29
Order Number LM3578AM, LM2578AN or LM3578AN
See NS Package Number M08A or N08E
DS008711-28
Top View
Order Number LM1578AH/883 or SMD #5962-8958602
See NS Package Number H08C
Typical Performance Characteristics
Oscillator Frequency Change
with Temperature
Input Reference Voltage
Drift with Temperature
Oscillator Voltage Swing
DS008711-33
DS008711-32
DS008711-34
Collector Saturation Voltage
(Sinking Current,
Emitter Grounded)
Emitter Saturation Voltage
(Sourcing Current,
Ground Referred
Current Limit Sense Voltage
Collector at Vin
)
DS008711-37
DS008711-35
DS008711-36
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4
Typical Performance Characteristics (Continued)
Current Limit Sense Voltage
Drift with Temperature
Current Limit Response Time
for Various Over Drives
Current Limit Sense Voltage
vs Supply Voltage
DS008711-39
DS008711-38
DS008711-40
Supply Current
Supply Current
Collector Current with
Emitter Output Below Ground
DS008711-41
DS008711-42
DS008711-43
The Current Limit Sense Voltage is measured by connecting
an adjustable 0-to-1V floating power supply in series with the
current limit terminal and referring it to either the ground or
the Vin terminal. Set the duty cycle to 90% and monitor test
point TP5 while adjusting the floating power supply voltage
until the LM1578A’s duty cycle just reaches 0%. This voltage
is the Current Limit Sense Voltage.
*
Test Circuit
Parameter tests can be made using the test circuit shown.
Select the desired Vin, collector voltage and duty cycle with
adjustable power supplies. A digital volt meter with an input
resistance greater than 100 MΩ should be used to measure
the following:
Input Reference Voltage to Ground; S1 in either position.
The Supply Current should be measured with the duty cycle
=
=
=
=
Level Shift Accuracy (%) (TP3(V)/1V) x 100%; S1 at I1 I2
at 0% and S1 in the I1 I2 0 mA position.
=
1 mA
*
LM1578A specifications are measured using automated
=
=
=
Input Current (mA) (1V − Tp3 (V))/1 MΩ: S1 at I1 I2
test equipment. This circuit is provided for the customer’s
convenience when checking parameters. Due to possible
variations in testing conditions, the measured values from
these testing procedures may not match those of the factory.
0 mA.
Oscillator parameters can be measured at Tp4 using a fre-
quency counter or an oscilloscope.
5
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*
Test Circuit (Continued)
DS008711-3
±
Op amp supplies are 15V
>
DVM input resistance 100 MΩ
*
%
LM1578 max duty cycle is 90
Current Limit Sense Voltage: The voltage at the Current
Limit pin, referred to either the supply or the ground terminal,
which (via logic circuitry) will cause the output transistor to
turn OFF and resets cycle-by-cycle at the oscillator fre-
quency.
Definition of Terms
Input Reference Voltage: The voltage (referred to ground)
that must be applied to either the inverting or non-inverting
input to cause the regulator switch to change state (ON or
OFF).
Current Limit Sense Current: The bias current for the Cur-
rent Limit terminal with the applied voltage equal to the Cur-
rent Limit Sense Voltage.
Input Reference Current: The current that must be drawn
from either the inverting or non-inverting input to cause the
regulator switch to change state (ON or OFF).
Supply Current: The IC power supply current, excluding the
current drawn through the output transistor, with the oscilla-
tor operating.
Input Level Shift Accuracy: This specification determines
the output voltage tolerance of a regulator whose output con-
trol depends on drawing equal currents from the inverting
and non-inverting inputs (see the Inverting Regulator of Fig-
ure 21, and the RS-232 Line Driver Power Supply of Figure
23).
Functional Description
The LM1578A is a pulse-width modulator designed for use
as a switching regulator controller. It may also be used in
other applications which require controlled pulse-width volt-
age drive.
Level Shift Accuracy is tested by using two equal-value re-
sistors to draw current from the inverting and non-inverting
input terminals, then measuring the percentage difference in
the voltages across the resistors that produces a controlled
duty cycle at the switch output.
A control signal, usually representing output voltage, fed into
the LM1578A’s comparator is compared with an
internally-generated reference. The resulting error signal
and the oscillator’s output are fed to a logic network which
determines when the output transistor will be turned ON or
OFF. The following is a brief description of the subsections of
the LM1578A.
Collector Saturation Voltage: With the inverting input ter-
minal grounded thru a 10 kΩ resistor and the output transis-
tor’s emitter connected to ground, the Collector Saturation-
Voltage is the collector-to-emitter voltage for
collector current.
a given
Emitter Saturation Voltage: With the inverting input termi-
nal grounded thru a 10 kΩ resistor and the output transistor’s
collector connected to Vin, the Emitter Saturation Voltage is
the collector-to-emitter voltage for a given emitter current.
COMPARATOR INPUT STAGE
The LM1578A’s comparator input stage is unique in that
both the inverting and non-inverting inputs are available to
the user, and both contain a 1.0V reference. This is accom-
plished as follows: A 1.0V reference is fed into a modified
voltage follower circuit (see FUNCTIONAL DIAGRAM).
When both input pins are open, no current flows through R1
Collector
Emitter
Sustaining
Voltage:
The
collector-emitter breakdown voltage of the output transistor,
measured at a specified current.
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6
Functional Description (Continued)
Applications Information
and R2. Thus, both inputs to the comparator will have the po-
tential of the 1.0V reference, VA. When one input, for ex-
ample the non-inverting input, is pulled ∆V away from VA, a
current of ∆V/R1 will flow through R1. This same current
flows through R2, and the comparator sees a total voltage of
2∆V between its inputs. The high gain of the system, through
feedback, will correct for this imbalance and return both in-
puts to the 1.0V level.
CURRENT LIMIT
As mentioned in the functional description, the current limit
terminal may be referenced to either the Vin or the ground
terminal. Resistor R3 converts the current to be sensed into
a voltage for current limit detection.
This unusual comparator input stage increases circuit flex-
ibility, while minimizing the total number of external compo-
nents required for a voltage regulator system. The inverting
switching regulator configuration, for example, can be set up
without having to use an external op amp for feedback polar-
ity reversal (see TYPICAL APPLICATIONS).
OSCILLATOR
The LM1578A provides an on-board oscillator which can be
adjusted up to 100 kHz. Its frequency is set by a single exter-
nal capacitor, C1, as shown in Figure 1, and follows the
equation
DS008711-15
fOSC 8x10−5/C1
=
FIGURE 2. Current Limit, Ground Referred
The oscillator provides a blanking pulse to limit maximum
duty cycle to 90%, and a reset pulse to the internal circuitry.
DS008711-16
FIGURE 3. Current Limit, Vin Referred
DS008711-4
CURRENT LIMIT TRANSIENT SUPPRESSION
FIGURE 1. Value of Timing Capacitor vs
Oscillator Frequency
When noise spikes and switching transients interfere with
proper current limit operation, R1 and C1 act together as a
low pass filter to control the current limit circuitry’s response
time.
OUTPUT TRANSISTOR
The output transistor is capable of delivering up to 750 mA
with a saturation voltage of less than 0.9V. (see Collector
Saturation Voltage and Emitter Saturation Voltage curves).
Because the sense current of the current limit terminal varies
according to where it is referenced, R1 should be less
than 2 kΩ when referenced to ground, and less than 100Ω
The emitter must not be pulled more than 1V below ground
(this limit is 0.6V for TJ ≥ 100˚C). Because of this limit, an ex-
ternal transistor must be used to develop negative output
voltages (see the Inverting Regulator Typical Application).
Other configurations may need protection against violation
of this limit (see the Emitter Output section of the Applica-
tions Information).
when referenced to Vin
.
CURRENT LIMIT
The LM1578A’s current limit may be referenced to either the
ground or the Vin pins, and operates on a cycle-by-cycle ba-
sis.
The current limit section consists of two comparators: one
with its non-inverting input referenced to a voltage 110 mV
below Vin, the other with its inverting input referenced
110 mV above ground (see FUNCTIONAL DIAGRAM). The
current limit is activated whenever the current limit terminal
is pulled 110 mV away from either Vin or ground.
DS008711-17
FIGURE 4. Current Limit Transient Suppressor,
Ground Referred
7
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non-inverting input than is sunk by the parallel combination
of R1 and R2 at the inverting terminal. R3 should be one-fifth
of the value of R1 and R2 in parallel.
Applications Information (Continued)
DS008711-18
FIGURE 5. Current Limit Transient Suppressor,
Vin Referred
C.L. SENSE VOLTAGE MULTIPLICATION
DS008711-22
When a larger sense resistor value is desired, the voltage di-
vider network, consisting of R1 and R2, may be used. This
effectively multiplies the sense voltage by (1 + R1/R2). Also,
R1 can be replaced by a diode to increase current limit
sense voltage to about 800 mV (diode Vf + 110 mV).
FIGURE 8. Under-Voltage Lockout
MAXIMUM DUTY CYCLE LIMITING
The maximum duty cycle can be externally limited by adjust-
ing the charge to discharge ratio of the oscillator capacitor
with a single external resistor. Typical values are 50 µA for
the charge current, 450 µA for the discharge current, and a
voltage swing from 200 mV to 750 mV. Therefore, R1 is se-
lected for the desired charging and discharging slopes and
C1 is readjusted to set the oscillator frequency.
DS008711-19
FIGURE 6. Current Limit Sense Voltage Multiplication,
Ground Referred
DS008711-21
FIGURE 9. Maximum Duty Cycle Limiting
DUTY CYCLE ADJUSTMENT
When manual or mechanical selection of the output transis-
tor’s duty cycle is needed, the cirucit shown below may be
used. The output will turn on with the beginning of each os-
cillator cycle and turn off when the current sunk by R2 and
R3 from the non-inverting terminal becomes greater than the
current sunk from the inverting terminal.
With the resistor values as shown, R3 can be used to adjust
the duty cycle from 0% to 90%.
DS008711-20
FIGURE 7. Current Limit Sense Voltage Multiplication,
Vin Referred
When the sum of R2 and R3 is twice the value of R1, the
duty cycle will be about 50%. C1 may be a large electrolytic
capacitor to lower the oscillator frequency below 1 Hz.
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout is accomplished with few external
components. When Vin becomes lower than the zener
breakdown voltage, the output transistor is turned off. This
occurs because diode D1 will then become forward biased,
allowing resistor R3 to sink
a greater current from the
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8
Applications Information (Continued)
DS008711-30
FIGURE 12. D1 Prevents Output Transistor from
Improperly Turning ON due to D2’s Forward Voltage
DS008711-23
FIGURE 10. Duty Cycle Adjustment
REMOTE SHUTDOWN
SYNCHRONIZING DEVICES
The LM1578A may be remotely shutdown by sinking a
greater current from the non-inverting input than from the in-
verting input. This may be accomplished by selecting resistor
R3 to be approximately one-half the value of R1 and R2 in
parallel.
When several devices are to be operated at once, their oscil-
lators may be synchronized by the application of an external
signal. This drive signal should be a pulse waveform with a
minimum pulse width of 2 µs. and an amplitude from 1.5V to
2.0V. The signal source must be capable of 1.) driving ca-
pacitive loads and 2.) delivering up to 500 µA for each
LM1578A.
Capacitors C1 thru CN are to be selected for a 20% slower
frequency than the synchronization frequency.
DS008711-24
DS008711-25
FIGURE 11. Shutdown Occurs when VL is High
FIGURE 13. Synchronizing Devices
EMITTER OUTPUT
Typical Applications
When the LM1578A output transistor is in the OFF state, if
the Emitter output swings below the ground pin voltage, the
output transistor will turn ON because its base is clamped
near ground. The Collector Current with Emitter Output Be-
low Ground curve shows the amount of Collector current
drawn in this mode, vs temperature and Emitter voltage.
When the Collector-Emitter voltage is high, this current will
cause high power dissipation in the output transistor and
should be avoided.
The LM1578A may be operated in either the continuous or
the discontinuous conduction mode. The following applica-
tions (except for the Buck-Boost Regulator) are designed for
continuous conduction operation. That is, the inductor cur-
rent is not allowed to fall to zero. This mode of operation has
higher efficiency and lower EMI characteristics than the dis-
continuous mode.
BUCK REGULATOR
This situation can occur in the high-current high-voltage
buck application if the Emitter output is used and the catch
The buck configuration is used to step an input voltage down
to a lower level. Transistor Q1 in Figure 14 chops the input
DC voltage into a squarewave. This squarewave is then con-
verted back into a DC voltage of lower magnitude by the low
pass filter consisting of L1 and C1. The duty cycle, D, of the
squarewave relates the output voltage to the input voltage by
the following equation:
diode’s forward voltage drop is greater than 0.6V.
A
fast-recovery diode can be added in series with the Emitter
output to counter the forward voltage drop of the catch diode
(see Figure 2). For better efficiency of a high output current
buck regulator, an external PNP transistor should be used as
shown in Figure 16.
=
=
Vout D x Vin Vin x (ton)/(ton + toff).
9
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Step 2: Calculate the inductor Volts-sec product, E-Top, ac-
cording to the equations given from the chart. For the Buck:
Typical Applications (Continued)
=
E-Top (Vin − Vo) (Vo/Vin) (1000/fosc
)
=
=
(15 − 5) (5/15) (1000/50)
66V-µs.
with the oscillator frequency, fosc, expressed in kHz.
DS008711-5
FIGURE 14. Basic Buck Regulator
Figure 15 is a 15V to 5V buck regulator with an output cur-
rent, Io, of 350 mA. The circuit becomes discontinuous at
20% of Io(max), has 10 mV of output voltage ripple, an effi-
ciency of 75%, a load regulation of 30 mV (70 mA to 350 mA)
and a line regulation of 10 mV (12 ≤ Vin ≤ 18V).
Component values are selected as follows:
DS008711-6
=
=
R1 (Vo − 1) x R2 where R2 10 kΩ
=
=
=
R3 0.15Ω
R3 V/Isw(max)
Vin 15V
=
=
=
C1 1820 pF
Vo 5V
R3 0.15Ω
=
=
C2 220 µF
Vripple 10 mV
where:
=
=
C3 20 pF
Io 350 mA
V is the current limit sense voltage, 0.11V
=
=
L1 470 µH
fosc 50 kHz
Isw(max) is the maximum allowable current thru the output
transistor.
=
=
D1 1N5818
R1 40 kΩ
=
R2 10 kΩ
L1 is the inductor and may be found from the inductance cal-
culation chart (Figure 16) as follows:
FIGURE 15. Buck or Step-Down Regulator
=
Given Vin 15V
Step 3: Using the graph with axis labeled “Discontinuous At
% IOUT” and “IL(max, DC)” find the point where the desired
maximum inductor current, IL(max, DC) intercepts the desired
discontinuity percentage.
=
Vo 5V
=
Io(max) 350 mA
=
fOSC 50 kHz
Discontinuous at 20% of Io(max)
.
In this example, the point of interest is where the 0.35A line
Note that since the circuit will become discontinuous at 20%
of Io(max), the load current must not be allowed to fall below
70 mA.
%
intersects with the 20 line. This is nearly the midpoint of the
horizontal axis.
Step 4: This last step is merely the translation of the point
found in Step 3 to the graph directly below it. This is accom-
plished by moving straight down the page to the point which
intercepts the desired E-Top. For this example, E-Top is
66V-µs and the desired inductor value is 470 µH. Since this
example was for 20% discontinuity, the bottom chart could
have been used directly, as noted in step 3 of the chart
instructions.
Step 1: Calculate the maximum DC current through the in-
ductor, IL(max). The necessary equations are indicated at the
=
top of the chart and show that IL(max) Io(max) for the buck
=
configuration. Thus, IL(max) 350 mA.
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10
Typical Applications (Continued)
11
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where Vripple is the peak-to-peak output voltage ripple.
Typical Applications (Continued)
C3 is necessary for continuous operation and is generally in
the 10 pF to 30 pF range.
For a full line of standard inductor values, contact Pulse En-
gineering (San Diego, Calif.) regarding their PE526XX se-
ries, or A. I. E. Magnetics (Nashville, Tenn.).
D1 should be a Schottky type diode, such as the 1N5818 or
1N5819.
A more precise inductance value may be calculated for the
Buck, Boost and Inverting Regulators as follows:
BUCK WITH BOOSTED OUTPUT CURRENT
BUCK
For applications requiring a large output current, an external
transistor may be used as shown in Figure 17. This circuit
steps a 15V supply down to 5V with 1.5A of output current.
The output ripple is 50 mV, with an efficiency of 80%, a load
regulation of 40 mV (150 mA to 1.5A), and a line regulation
of 20 mV (12V ≤ Vin ≤ 18V).
=
L
Vo (Vin − Vo)/(∆IL Vin fosc
)
BOOST
=
L
Vin (Vo − Vin)/(∆IL fosc Vo)
INVERT
=
L
Vin |Vo|/[∆IL(Vin + |Vo|)fosc]
Component values are selected as outlined for the buck
regulator with a discontinuity factor of 10%, with the addition
of R4 and R5:
where ∆IL is the current ripple through the inductor. ∆IL is
usually chosen based on the minimum load current expected
of the circuit. For the buck regulator, since the inductor cur-
rent IL equals the load current IO,
=
R4 10VBE1Bf/Ip
=
R5 (Vin − V − VBE1 − Vsat) Bf/(IL(max, DC) + IR4
)
=
∆IL 2 • IO(min)
where:
=
∆IL 140 mA for this circuit. ∆IL can also be interpreted as
VBE1 is the VBE of transistor Q1.
=
∆IL 2 • (Discontinuity Factor) • IL
Vsat is the saturation voltage of the LM1578A output transis-
tor.
where the Discontinuity Factor is the ratio of the minimum
load current to the maximum load current. For this example,
the Discontinuity Factor is 0.2.
V is the current limit sense voltage.
=
Bf is the forced current gain of transistor Q1 (Bf 30 for Fig-
The remainder of the components of Figure 15 are chosen
as follows:
ure 17 ).
=
IR4 VBE1/R4
C1 is the timing capacitor found in Figure 1.
=
Ip IL(max, DC) + 0.5∆IL
2
C2 ≥ Vo (Vin − Vo)/(8fosc VinVrippleL1)
DS008711-8
=
=
R4 200Ω
Vin 15V
=
=
R5 330Ω
Vo 5V
=
=
C1 1820 pF
Vripple 50 mV
=
=
C2 330 µF
Io 1.5A
=
=
C3 20 pF
fosc 50 kHz
=
=
L1 220 µH
R1 40 kΩ
=
=
R2 10 kΩ
D1 1N5819
=
R3 0.05Ω
Q1 = D45
FIGURE 17. Buck Converter with Boosted Output Current
BOOST REGULATOR
The boost regulator converts a low input voltage into a
higher output voltage. The basic configuration is shown in
Figure 18. Energy is stored in the inductor while the transis-
tor is on and then transferred with the input voltage to the
output capacitor for filtering when the transistor is off. Thus,
=
Vo Vin + Vin(ton/toff).
www.national.com
12
R4, C3 and C4 are necessary for continuous operation and
are typically 220 kΩ, 20 pF, and 0.0022 µF respectively.
Typical Applications (Continued)
C1 is the timing capacitor found in Figure 1.
C2 ≥ Io (Vo − Vin)/(fosc Vo Vripple).
D1 is a Schottky type diode such as a IN5818 or IN5819.
L1 is found as described in the buck converter section, using
the inductance chart for Figure 16 for the boost configuration
and 20% discontinuity.
INVERTING REGULATOR
DS008711-9
Figure 20 shows the basic configuration for an inverting
regulator. The input voltage is of a positive polarity, but the
output is negative. The output may be less than, equal to, or
greater in magnitude than the input. The relationship be-
tween the magnitude of the input voltage and the output volt-
FIGURE 18. Basic Boost Regulator
The circuit of Figure 19 converts a 5V supply into a 15V sup-
ply with 150 mA of output current, a load regulation of 14 mV
(30 mA to 140 mA), and a line regulation of 35 mV (4.5V ≤
Vin ≤ 8.5V).
=
age is Vo Vin x (ton/toff).
DS008711-10
FIGURE 20. Basic Inverting Regulator
Figure 21 shows an LM1578A configured as a 5V to −15V
polarity inverter with an output current of 300 mA, a load
regulation of 44 mV (60 mA to 300 mA) and a line regulation
of 50 mV (4.5V ≤ Vin ≤ 8.5V).
DS008711-11
=
=
R4 200 kΩ
Vin 5V
=
=
R1 (|Vo| +1) R2 where R2 10 kΩ.
=
=
C1 1820 pF
Vo 15V
=
R3 V/(IL(max, DC) + 0.5 ∆IL).
=
=
C2 470 µF
Vripple 10 mV
=
R4 10VBE1Bf/(IL (max, DC) + 0.5 ∆IL)
=
=
C3 20 pF
Io 140 mA
where:
=
=
C4 0.0022 µF
fosc 50 kHz
V, VBE1, Vsat, and Bf are defined in the “Buck Converter with
Boosted Output Current” section.
=
=
L1 330 µH
R1 140 kΩ
=
=
D1 1N5818
R2 10 kΩ
=
∆IL 2(ILOAD(min))(Vin +|Vo|)/VIN
=
R3 0.15Ω
R5 is defined in the “Buck with Boosted Output Current” sec-
tion.
FIGURE 19. Boost or Step-Up Regulator
R6 serves the same purpose as R4 in the Boost Regulator
circuit and is typically 220 kΩ.
=
=
R1 (Vo − 1) R2 where R2 10 kΩ.
=
R3 V/(IL(max, DC) + 0.5 ∆IL)
C1, C3 and C4 are defined in the “Boost Regulator” section.
where:
C2 ≥ Io |Vo|/[fosc(|Vo| + Vin) Vripple
]
=
∆IL 2(ILOAD(min))(Vo/Vin
)
L1 is found as outlined in the section on buck converters, us-
ing the inductance chart of Figure 16 for the invert configura-
tion and 20% discontinuity.
∆IL is 200 mA in this example.
13
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Typical Applications (Continued)
DS008711-12
=
=
R4 190Ω
Vin 5V
=
=
R5 82Ω
Vo −15V
=
=
R6 220 kΩ
Vripple 5 mV
=
=
C1 1820 pF
Io 300 mA
=
=
C2 1000 µF
Imin 60 mA
=
=
C3 20 pF
fosc 50 kHz
=
=
C4 0.0022 µF
R1 160 kΩ
=
=
L1 150 µH
R2 10 kΩ
=
=
D1 1N5818
R3 0.01Ω
FIGURE 21. Inverting Regulator
BUCK-BOOST REGULATOR
RS-232 LINE DRIVER POWER SUPPLY
The Buck-Boost Regulator, shown in Figure 22, may step a
voltage up or down, depending upon whether or not the de-
sired output voltage is greater or less than the input voltage.
In this case, the output voltage is 12V with an input voltage
from 9V to 15V. The circuit exhibits an efficiency of 75%, with
a load regulation of 60 mV (10 mA to 100 mA) and a line
regulation of 52 mV.
The power supply, shown in Figure 23, operates from an in-
put voltage as low as 4.2V (5V nominal), and delivers an out-
±
±
put of 12V at 40 mA with better than 70% efficiency. The
±
circuit provides a load regulation of 150 mV (from 10% to
±
100% of full load) and a line regulation of 10 mV. Other no-
table features include a cycle-by-cycle current limit and an
output voltage ripple of less than 40 mVp-p.
=
=
R1 (Vo − 1) R2 where R2 10 kΩ
A unique feature of this circuit is its use of feedback from
both outputs. This dual feedback configuration results in a
sharing of the output voltage regulation by each output so
that neither side becomes unbalanced as in single feedback
systems. In addition, since both sides are regulated, it is not
necessary to use a linear regulator for output regulation.
=
R3 V/0. 75A
R4, C1, C3 and C4 are defined in the “Boost Regulator” sec-
tion.
D1 and D2 are Schottky type diodes such as the 1N5818 or
1N5819.
The feedback resistors, R2 and R3, may be selected as fol-
lows by assuming a value of 10 kΩ for R1;
=
=
R2 (Vo − 1V)/45.8 µA 240 kΩ
=
=
R3 (|Vo| +1V)/54.2 µA 240 kΩ
where:
Actually, the currents used to program the values for the
feedback resistors may vary from 40 µA to 60 µA, as long as
their sum is equal to the 100 µA necessary to establish the
1V threshold across R1. Ideally, these currents should be
equal (50 µA each) for optimal control. However, as was
done here, they may be mismatched in order to use standard
resistor values. This results in a slight mismatch of regulation
between the two outputs.
Vd is the forward voltage drop of the diodes.
Vsat is the saturation voltage of the LM1578A output transis-
tor.
Vsat1 is the saturation voltage of transistor Q1.
L1 ≥ (Vin − Vsat − Vsat1) (ton/Ip)
where:
The current limit resistor, R4, is selected by dividing the cur-
rent limit threshold voltage by the maximum peak current
=
level in the output switch. For our purposes R4 110 mV/
=
750 mA 0.15Ω. A value of 0.1Ω was used.
www.national.com
14
Typical Applications (Continued)
DS008711-14
DS008711-13
=
=
Vin 5V
R4 0.15Ω
=
R5 270
9V ≤ Vin ≤ 15V
=
±
Vo 12V
C1 820 pF
=
=
C1 1820 pF
Vo 12V
=
=
±
Io
40 mA
C2 10 pF
=
C2 220 µF
Io = 100 mA
=
=
fosc 80 kHz
C3 220 µF
=
=
C3 20 pF
Vripple 50 mV
=
=
D1, D2, D3 1N5819
R1 10 kΩ
=
=
C4 0.0022 µF
fosc 50 kHz
=
=
T1 PE-64287
R2 240 kΩ
=
=
L1 220 µH
R1 110k
=
R3 240 kΩ
=
=
D1, D2 1N5819
R2 10k
FIGURE 23. RS-232 Line Driver Power Supply
=
=
Q1 D44
R3 0.15
=
R4 220k
Capacitor C1 sets the oscillator frequency and is selected
from Figure 1.
FIGURE 22. Buck-Boost Regulator
Capacitor C2 serves as a compensation capacitor for syn-
chronous operation and a value of 10 to 50 pF should be suf-
ficient for most applications.
A minimum value for an ideal output capacitor C3, could be
=
calculated as C Io x t/∆V where Io is the load current, t is
the transistor on time (typically 0.4/fosc), and ∆V is the
peak-to-peak output voltage ripple. A larger output capacitor
than this theoretical value should be used since electrolytics
have poor high frequency performance. Experience has
shown that a value from 5 to 10 times the calculated value
should be used.
For good efficiency, the diodes must have a low forward volt-
age drop and be fast switching. 1N5819 Schottky diodes
work well.
Transformer selection should be picked for an output transis-
tor “on” time of 0.4/fosc, and a primary inductance high
enough to prevent the output transistor switch from ramping
higher than the transistor’s rating of 750 mA. Pulse Engi-
neering (San Diego, Calif.) and Renco Electronics, Inc.
(Deer Park, N.Y.) can provide further assistance in selecting
the proper transformer for a specific application need. The
transformer used in Figure 23 was a Pulse Engineering
PE-64287.
15
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16
Physical Dimensions inches (millimeters) unless otherwise noted
Metal Can Package (H)
Order Number LM1578AH/883 or SMD #5962-8958602
NS Package Number H08C
Plastic Surface-Mount Package (M)
Order Number LM3578AM
NS Package Number M08A
17
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Molded Dual-In-Line Package (N)
Order Number LM2578AN or LM3578AN
NS Package Number N08E
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DE-
VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI-
CONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or sys-
tems which, (a) are intended for surgical implant into
the body, or (b) support or sustain life, and whose fail-
ure to perform when properly used in accordance
with instructions for use provided in the labeling, can
be reasonably expected to result in a significant injury
to the user.
2. A critical component in any component of a life support
device or system whose failure to perform can be rea-
sonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
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Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com
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Response Group
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