Datasheet LM3578AM, LM2578AN, LM1578AH-883, LM3578AN Datasheet (NSC)

LM1578A/LM2578A/LM3578A Switching Regulator

LM1578A/LM2578A/LM3578A Switching Regulator

April 1995

General Description

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
features 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 V
plication. In addition, the LM1578A has an on board oscilla­tor, which sets the switching frequency with a single exter­nal capacitor from

terminal, depending upon the ap-

in

k

1 Hz to 100 kHz (typical).

The LM1578A is an improved version of the LM1578, offer­ing higher maximum ratings for the total supply voltage and
output transistor emitter and collector voltages.

Functional Diagram

Features

Y

Inverting and non-inverting feedback inputs

Y

1.0V reference at inputs

Y

Operates from supply voltages of 2V to 40V

Y

Output current up to 750 mA, saturation less than 0.9V

Y

Current limit and thermal shut down

Y

Duty cycle up to 90%

Applications

Y

Switching regulators in buck, boost, inverting, and
single-ended transformer configurations

Y

Motor speed control

Y

Lamp flasher

TL/H/8711– 1

C

1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.

TL/H/8711

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.

Total Supply Voltage 50V

Collector Output to Ground

Emitter Output to Ground (Note 2)

Power Dissipation (Note 3) Internally limited

Output Current 750 mA

Storage Temperature

Lead Temperature

(soldering, 10 seconds) 260

b

0.3V toa50V

b

1V toa50V

b

65§Ctoa150§C

Maximum Junction Temperature 150

C

§

ESD Tolerance (Note 4) 2 kV

Operating Ratings

Ambient Temperature Range

LM1578A
LM2578A

b

55§CsT

b

40§CsT

LM3578A 0

Junction Temperature Range

LM1578A

C

§

LM2578A
LM3578A 0

b

55§CsT

b

40§CsT

CsT

§

CsT

§

s

a

125§C

A

s

a

85§C

A

s

a

70§C

A

s

a

150§C

J

s

a

125§C

J

s

a

125§C

J

Electrical Characteristics

These specifications apply for 2VsV

s

duty cycles75%, unless otherwise specified. Values in standard typeface are for T

for operation over the specified operating junction temperature range.

Symbol Parameter Conditions

s

40V (2.2VsV

IN

IN

s

40V for T

s

b

25§C), timing capacitor C

J

Typical

(Note 5)

e

J

LM1578A

(Notes 6, 11)

e

3900 pF, and 25%

25§C; values in boldface type apply

T

LM2578A/

Limit

LM3578A

Limit

(Note 7)

Units

OSCILLATOR

f

OSC

Frequency 20 kHz

22.4 24 kHz (max)

17.6 16 kHz (min)

Df

/DT Frequency Drift with

OSC

Temperature

Amplitude 550 mV

b

0.13 %/§C

p-p

REFERENCE/COMPARATOR (Note 8)

e

V

R

Input Reference I
Voltage I

DVR/DVINInput Reference Volt- I

age Line Regulation I

I

INV

Inverting Input I
Current

e

I

1

1

1

1

1

0 mA and 1.0 V

2

e

e

I

1mAg1% (Note 9) 1.035/1.050 1.050/1.070 V (max)

2

e

e

I

0 mA and 0.003 %/V

2

e

e

I

1mAg1% (Note 9) 0.01/0.02 0.01/0.02 %/V (max)

2

e

e

I

0 mA, duty cyclee25% 0.5 mA

2

0.965/0.950 0.950/0.930 V (min)

Level Shift Accuracy Level Shift Currente1 mA 1.0 %

5/8 10/13 % (max)

DVR/Dt Input Reference 100 ppm/1000h

Voltage Long Term
Stability

OUTPUT

VC(sat) Collector Saturation I

Voltage grounded 0.85/1.2 0.90/1.2 V (max)

VE(sat) Emitter Saturation I

Voltage V

I

CES

BV

CEO(SUS)

Collector Leakage V
Current grounded, Output OFF 50/100 200/250 mA (max)

Collector-Emitter I
Sustaining Voltage 50 50 V (min)

e

750 mA pulsed, Emitter 0.7 V

C

e

80 mA pulsed, 1.4 V

O

e

e

V

IN

IN

SUST

40V 1.6/2.1 1.7/2.0 V (max)

C

e

e

V

40V, Emitter 0.1 mA

CE

e

0.2A (pulsed), V

e

060 V

IN

2

Electrical Characteristics (Continued)

Symbol Parameter Conditions

Typical

(Note 5)

LM1578A

Limit

(Notes 6, 11)

CURRENT LIMIT

V

CL

Sense Voltage Referred to VINor Ground 110 mV
Shutdown Level (Note 10) 95 80 mV (min)

140 160 mV (max)

DVCL/DT Sense Voltage 0.3 %/§C

Temperature Drift

I

CL

Sense Bias Current Referred to V

Referred to ground 0.4 mA

IN

4.0 mA

DEVICE POWER CONSUMPTION

I

S

Supply Current Output OFF, V

Output ON, I

e

V

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

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

junction to ambient, or 45
package must be derated at 150

Note 4: Human body model, 1.5 kX in series with 100 pF.

Note 5: Typical values are for T

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

Note 10: Connection of a 10 kX 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.

t

100§C, the Emitter pin voltage should not be driven more than 0.6V below ground (see Application Information).

J

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

§

C/W, junction-to-ambient.

§

e

J

and I2are the external sink currents at the inputs (refer to Test Circuit).

1

E

25§C and represent the most likely parametric norm.

e

0V 2.0 mA

E

e

750 mA pulsed, 14 mA

C

3.0/3.3 3.5/4.0 mA (max)

LM2578A/

LM3578A

Limit

(Note 7)

Units

§

C/W,

Connection Diagram and Ordering Information

Metal Can

Order Number LM3578AM, LM2578AN or LM3578AN

Order Number LM1578AH/883 or SMD

Top View

See NS Package Number H08C

TL/H/8711– 28

Ý

5962-8958602

3

Dual-In-Line Package

TL/H/8711– 29

See NS Package Number M08A or N08E

Typical Performance Characteristics

Oscillator Frequency Change
with Temperature

Collector Saturation Voltage
(Sinking Current,
Emitter Grounded)

Current Limit Sense Voltage
Drift with Temperature

Oscillator Voltage Swing Drift with Temperature

Emitter Saturation Voltage
(Sourcing Current,
Collector at V

Current Limit Response Time
for Various Over Drives

)

in

Input Reference Voltage

Ground Referred
Current Limit Sense Voltage

Current Limit Sense Voltage
vs Supply Voltage

Supply Current Supply Current Emitter Output Below Ground

4

Collector Current with

TL/H/8711– 2

Test Circuit*

Parameter tests can be made using the test circuit shown.
Select the desired V
adjustable power supplies. A digital volt meter with an input
resistance greater than 100 MX should be used to measure
the following:

Input Reference Voltage to Ground; S1 in either posi­tion.

Level Shift Accuracy (%)

e

e

I

I

1

2

Input Current (mA)

e

I

0 mA.

2

Oscillator parameters can be measured at T
quency counter or an oscilloscope.

, collector voltage and duty cycle with

in

e

(TP3(V)/1V)c100%; S1 at

1mA

e

(1VbTp3(V))/1 MX:S1atI

using a fre-

p4

1

e

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 V
test point T
age until the LM1578A’s duty cycle just reaches 0%. This

terminal. Set the duty cycle to 90% and monitor

in

while adjusting the floating power supply volt-

P5

voltage is the Current Limit Sense Voltage.

The Supply Current should be measured with the duty cycle
at 0% and S1 in the I

e

e

I

1

0 mA position.

2

*LM1578A specifications are measured using automated
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.

Note 1: Op amp supplies areg15V
Note 2: DVM input resistance
Note 3: *LM1578 max duty cycle is 90%

l

100 MX

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).

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).

Input Level Shift Accuracy: This specification determines
the output voltage tolerance of a regulator whose output
control depends on drawing equal currents from the invert­ing and non-inverting inputs (see the Inverting Regulator of

Figure 21
ure 23

, and the RS-232 Line Driver Power Supply of

Fig-

).

Level Shift Accuracy is tested by using two equal-value re­sistors 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.

TL/H/8711– 3

Collector Saturation Voltage: With the inverting input ter­minal grounded thru a 10 kX resistor and the output transis­tor’s emitter connected to ground, the Collector Saturation­Voltage is the collector-to-emitter voltage for a given collec­tor current.

Emitter Saturation Voltage: With the inverting input termi­nal grounded thru a 10 kX resistor and the output transis­tor’s collector connected to V
age is the collector-to-emitter voltage for a given emitter

, the Emitter Saturation Volt-

in

current.

Collector Emitter Sustaining Voltage: The collector-emit­ter breakdown voltage of the output transistor, measured at
a specified current.

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 fre­quency.

5

Definition of Terms (Continued)

Current Limit Sense Current: The bias current for the Cur-

rent Limit terminal with the applied voltage equal to the Cur­rent Limit Sense Voltage.

Supply Current: The IC power supply current, excluding the
current drawn through the output transistor, with the oscilla­tor operating.

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 volt­age drive.

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.

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 accom­plished 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
and R2. Thus, both inputs to the comparator will have the
potential of the 1.0V reference, V
example the non-inverting input, is pulled DV away from V
a current of DV/R1 will flow through R1. This same current
flows through R2, and the comparator sees a total voltage
of 2DV between its inputs. The high gain of the system,
through feedback, will correct for this imbalance and return
both inputs to the 1.0V level.

This unusual comparator input stage increases circuit flexi­bility, while minimizing the total number of external compo­nents 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 po­larity 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
external capacitor, C
equation

, as shown in

1

e

f

OSC

The oscillator provides a blanking pulse to limit maximum
duty cycle to 90%, and a reset pulse to the internal circuitry.

. When one input, for

A

Figure 1

b

5

8c10

/C

, and follows the

1

OUTPUT TRANSISTOR

The output transistor is capable of delivering up to 750 mA
with a saturation voltage of less than 0.9V. (see

Saturation Voltage

The emitter must not be pulled more than 1V below ground
(this limit is 0.6V for T
external transistor must be used to develop negative output

and

Emitter Saturation Voltage

t

100§C). Because of this limit, an

J

voltages (see the Inverting Regulator Typical Application).
Other configurations may need protection against violation
of this limit (see the Emitter Output section of the Applica­tions Information).

CURRENT LIMIT

The LM1578A’s current limit may be referenced to either
the ground or the V
basis.

pins, and operates on a cycle-by-cycle

in

The current limit section consists of two comparators: one
with its non-inverting input referenced to a voltage 110 mV
below V
110 mV above ground (see FUNCTIONAL DIAGRAM). The

, the other with its inverting input referenced

in

current limit is activated whenever the current limit terminal
is pulled 110 mV away from either V

in

Applications Information

CURRENT LIMIT

As mentioned in the functional description, the current limit
terminal may be referenced to either the V
terminal. Resistor R3 converts the current to be sensed into
a voltage for current limit detection.

,

A

FIGURE 2. Current Limit, Ground Referred

or ground.

or the ground

in

Collector

curves).

TL/H/8711– 15

FIGURE 1. Value of Timing Capacitor vs

Oscillator Frequency

FIGURE 3. Current Limit, VinReferred

TL/H/8711– 16

TL/H/8711– 4

6

Applications Information (Continued)

CURRENT LIMIT TRANSIENT SUPPRESSION

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.

Because the sense current of the current limit terminal var­ies according to where it is referenced, R1 should be less
than 2 kX when referenced to ground, and less than 100X
when referenced to V

.

in

FIGURE 4. Current Limit Transient Suppressor,

TL/H/8711– 17

Ground Referred

FIGURE 5. Current Limit Transient Suppressor,

TL/H/8711– 18

V

Referred

in

C.L. SENSE VOLTAGE MULTIPLICATION

When a larger sense resistor value is desired, the voltage
divider network, consisting of R1 and R2, may be used. This
effectively multiplies the sense voltage by (1
Also, R1 can be replaced by a diode to increase current limit
sense voltage to about 800 mV (diode V

a

f

a

R1/R2).

110 mV).

FIGURE 7. Current Limit Sense Voltage Multiplication,

TL/H/8711– 20

V

Referred

in

UNDER-VOLTAGE LOCKOUT

Under-voltage lockout is accomplished with few external
components. When V
breakdown voltage, the output transistor is turned off. This

becomes lower than the zener

in

occurs because diode D1 will then become forward biased,
allowing resistor R3 to sink a greater current from the non­inverting 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.

FIGURE 8. Under-Voltage Lockout

TL/H/8711– 22

MAXIMUM DUTY CYCLE LIMITING

The maximum duty cycle can be externally limited by adjust­ing the charge to discharge ratio of the oscillator capacitor
with a single external resistor. Typical values are 50 mA for
the charge current, 450 mA for the discharge current, and a
voltage swing from 200 mV to 750 mV. Therefore, R1 is
selected for the desired charging and discharging slopes
and C1 is readjusted to set the oscillator frequency.

FIGURE 6. Current Limit Sense Voltage Multiplication,

TL/H/8711– 19

Ground Referred

7

Applications Information (Continued)

FIGURE 9. Maximum Duty Cycle Limiting

TL/H/8711– 21

DUTY CYCLE ADJUSTMENT

When manual or mechanical selection of the output transis­tor’s duty cycle is needed, the cirucit shown below may be
used. The output will turn on with the beginning of each
oscillator 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%.

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.

TL/H/8711– 23

FIGURE 10. Duty Cycle Adjustment

REMOTE SHUTDOWN

The LM1578A may be remotely shutdown by sinking a
greater current from the non-inverting input than from the
inverting input. This may be accomplished by selecting re­sistor R3 to be approximately one-half the value of R1 and
R2 in parallel.

FIGURE 11. Shutdown Occurs when VLis High

TL/H/8711– 24

EMITTER OUTPUT

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

low Ground

Collector Current with Emitter Output Be-

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.

This situation can occur in the high-current high-voltage
buck application if the Emitter output is used and the catch
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.

TL/H/8711– 30

FIGURE 12. D1 Prevents Output Transistor from

Improperly Turning ON due to D2’s Forward Voltage

8

Applications Information (Continued)

SYNCHRONIZING DEVICES

When several devices are to be operated at once, their os­cillators 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 ms. and an amplitude from 1.5V
to 2.0V. The signal source must be capable of 1.) driving
capacitive loads and 2.) delivering up to 500 mA for each
LM1578A.

Capacitors C1 thru CN are to be selected for a 20% slower
frequency than the synchronization frequency.

FIGURE 13. Synchronizing Devices

TL/H/8711– 25

Typical Applications

The LM1578A may be operated in either the continuous or
the discontinuous conduction mode. The following applica­tions (except for the Buck-Boost Regulator) are designed
for continuous conduction operation. That is, the inductor
current is not allowed to fall to zero. This mode of operation
has higher efficiency and lower EMI characteristics than the
discontinuous mode.

BUCK REGULATOR

The buck configuration is used to step an input voltage
down to a lower level. Transistor Q1 in
input DC voltage into a squarewave. This squarewave is
then converted 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:

DcV

e

V

in

in

e

V

out

FIGURE 14. Basic Buck Regulator

Figure 15

rent, I
20% of I
ciency of 75%, a load regulation of 30 mV (70 mA to

is a 15V to 5V buck regulator with an output cur-

, of 350 mA. The circuit becomes discontinuous at

o

, has 10 mV of output voltage ripple, an effi-

o(max)

350 mA) and a line regulation of 10 mV (12

c

(ton)/(t

Figure 14

a

on

s

chops the

t

).

off

TL/H/8711– 5

s

V

in

18V).

Component values are selected as follows:

e

R1
R3
R3e0.15X

b

(V

1)cR2 where R2e10 kX

o

e

V/I

sw(max)

where:

V is the current limit sense voltage, 0.11V

is the maximum allowable current thru the out-

I

sw(max)

put transistor.

L1 is the inductor and may be found from the inductance
calculation chart (

Given V

in

I

o(max)

Discontinuous at 20% of I

Figure 16

e

15V V

e

350 mA f

) as follows:

e

5V

o

e

50 kHz

OSC

o(max)

.

Note that since the circuit will become discontinuous at
20% of I
below 70 mA.

, the load current must not be allowed to fall

o(max)

Step 1: Calculate the maximum DC current through the in­ductor, I
the top of the chart and show that I
buck configuration. Thus, I

Step 2: Calculate the inductor Volts-sec product, E-T
cording to the equations given from the chart. For the Buck:

E-T

op

e

(15b5) (5/15) (1000/50)

e

66V-ms.

with the oscillator frequency, f

. The necessary equations are indicated at

e

L(max)

b

(V

Vo)(Vo/Vin) (1000/f

in

L(max)

e

350 mA.

L(max)

osc

, expressed in kHz.

osc

)

e

I

o(max)

V

in

V

o

V

ripple

I

o

f

osc

R1
R2
R3
C1
C2
C3
L1
D1

e

e

e

e
e

e
e
e
e
e

e

for the

op

15V

5V

e

10 mV

350 mA

e

50 kHz
40 kX
10 kX

0.15X

1820 pF
220 mF
20 pF
470 mH

1N5818

TL/H/8711– 6

, ac-

FIGURE 15. Buck or Step-Down Regulator

Step 3: Using the graph with axis labeled ‘‘Discontinuous At

%I

’’ and ‘‘I

OUT

maximum inductor current, I

L(max, DC)

discontinuity percentage.

’’ find the point where the desired

L(max, DC)

intercepts the desired

In this example, the point of interest is where the 0.35A line
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 accom­plished by moving straight down the page to the point which
intercepts the desired E-T
66V-ms and the desired inductor value is 470 mH. Since this

. For this example, E-Topis

op

example was for 20% discontinuity, the bottom chart could
have been used directly, as noted in step 3 of the chart
instructions.

9

Typical Applications (Continued)

TL/H/8711– 31

FIGURE 16. DC/DC Inductance Calculator

10

Typical Applications (Continued)

For a full line of standard inductor values, contact Pulse
Engineering (San Diego, Calif.) regarding their PE526XX se­ries, or A. I. E. Magnetics (Nashville, Tenn.).

A more precise inductance value may be calculated for the
Buck, Boost and Inverting Regulators as follows:

BUCK

e

L

BOOST

e

L

INVERT

e

L

where DI
usually chosen based on the minimum load current expect­ed of the circuit. For the buck regulator, since the inductor
current I

DI

L

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.

b

Vo(V

Vin(V

V

e

Vo)/(DILVinf

in

b

Vin)/(DILf

o

V

/[DIL(V

l

l

in

o

is the current ripple through the inductor. DILis

L

equals the load current IO,

L

a

in

DI

L

oscVo

V

l

o

e

2#I

)

osc

)

]

)f

l

osc

O(min)

140 mA for this circuit. DILcan also be interpreted as

e

DI

2#(Discontinuity Factor)#I

L

L

The remainder of the components of

Figure 15

are chosen

as follows:

C1 is the timing capacitor found in

t

C2

where V

b

Vo(V

Vo)/(8f

in

is the peak-to-peak output voltage ripple.

ripple

osc

2

VinV

Figure 1

ripple

.

L1)

C3 is necessary for continuous operation and is general­ly in the 10 pF to 30 pF range.

D1 should be a Schottky type diode, such as the
1N5818 or 1N5819.

BUCK WITH BOOSTED OUTPUT CURRENT

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

s

s

V

18V).

in

Component values are selected as outlined for the buck
regulator with a discontinuity factor of 10%, with the addi­tion of R4 and R5:

e

R4

10V

BE1Bf/Ip

R5e(V

in

bVb

b

V

V

sat)Bf

/(I

L(max, DC)

BE1

a

IR4)

where:

V

is the VBEof transistor Q1.

BE1

V

is the saturation voltage of the LM1578A output

sat

transistor.

V is the current limit sense voltage.

B

is the forced current gain of transistor Q1 (B

f

for

Figure 17

e

I

R4

e

I

p

V

/R4

BE1

I

L(max, DC)

).

a

0.5DI

L

e

f

30

e

V

15V R4e200X

in

e

5V R5e330X

V

o

e

50 mV C1e1820 pF

V

ripple

e

1.5A C2e330 mF

I

o

e

50 kHz C3e20 pF

f

osc

e

40 kX L1e220 mH

R1

e

10 kX D1e1N5819

R2

e

0.05X Q1eD45

R3

TL/H/8711– 8

FIGURE 17. Buck Converter with Boosted Output Current

11

Typical Applications (Continued)

BOOST REGULATOR

The boost regulator converts a low input voltage into a high­er output voltage. The basic configuration is shown in

18

. Energy is stored in the inductor while the transistor is on
and then transferred with the input voltage to the output
capacitor for filtering when the transistor is off. Thus,

e

V

a

V

in

Vin(ton/t

o

).

off

Figure

t

Io(V

b

Vin)/(f

o

oscVoVripple

).

C2

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 configu-

ration and 20% discontinuity.

INVERTING REGULATOR

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 be­tween the magnitude of the input voltage and the output
voltage is V

c

e

V

(ton/t

o

in

).

off

FIGURE 18. Basic Boost Regulator

TL/H/8711– 9

The circuit of

Figure 19

converts a 5V supply into a 15V
supply 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

s

s

V

8.5V).

in

e

V

5V

in

e

15V

V

o

e

V

ripple

e

140 mA

I

o

e

50 kHz

f

osc

e

140 kX

R1

e

10 kX

R2

e

0.15X

R3

e

220 kX

R4

e

1820 pF

C1

e

470 mF

C2

e

20 pF

C3

e

0.0022 mF

C4

e

330 mH

L1

e

1N5818

D1

TL/H/8711– 11

10 mV

FIGURE 19. Boost or Step-Up Regulator

R1e(V

R3eV/(I

b

1) R2 where R2e10 kX.

o

L(max, DC)

a

0.5 DIL)

where:

e

DI

2(I

L

LOAD(min)

)(Vo/Vin)

DILis 200 mA in this example.

R4, C3 and C4 are necessary for continuous operation
and are typically 220 kX, 20 pF, and 0.0022 mF respec­tively.

C1 is the timing capacitor found in

Figure 1

.

FIGURE 21. Inverting Regulator

FIGURE 20. Basic Inverting Regulator

TL/H/8711– 10

Figure 21

shows an LM1578A configured as a 5V tob15V
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

R1

R4e10V

s

V

in

e

(lV

l

o

R3eV/(I

BE1Bf

s

8.5V).

a

1) R2 where R2e10 kX.

L(max, DC)

/(I

L (max, DC)

a

0.5 DIL).

a

0.5 DIL)

where:

V, V
with Boosted Output Current’’ section.

DI

L

, and Bfare defined in the ‘‘Buck Converter

BE1,Vsat

e

2(I

LOAD(min)

)(V

a

V

)/V

l

l

in

o

IN

R5 is defined in the ‘‘Buck with Boosted Output Cur­rent’’ section.

R6 serves the same purpose as R4 in the Boost Regula­tor circuit and is typically 220 kX.

C1, C3 and C4 are defined in the ‘‘Boost Regulator’’
section.

C2

t

I

V

/[f

l

l

o

o

osc

(lV

a

Vin)V

l

o

ripple

]

L1 is found as outlined in the section on buck convert­ers, using the inductance chart of

Figure 16

for the in-

vert configuration and 20% discontinuity.

e

V

5V

in

eb

15V

V

o

e

5mV

V

ripple

TL/H/8711– 12

e

300 mA, I

I

o

e

f

osc

e

160 kX R2e10 kX

R1

e

0.01 X R4e190X

R3

e

82X R6e220 kX

R5

e

1820 pF

C1

e

1000 mF

C2

e

20 pF

C3

e

0.0022 mF

C4

e

150 mH

L1

e

1N5818

D1

50 kHz

e

60 mA

min

12

Typical Applications (Continued)

BUCK-BOOST REGULATOR

The Buck-Boost Regulator, shown in
voltage up or down, depending upon whether or not the
desired 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.

e

R1

b

(V

1) R2 where R2e10 kX

o

R3eV/0. 75A

R4, C1, C3 and C4 are defined in the ‘‘Boost Regulator’’
section.

D1 and D2 are Schottky type diodes such as the 1N5818 or
1N5819.

(Io/V

C2

t

[

f

osc(Vin

ripple

a

V

o

where:

is the forward voltage drop of the diodes.

V

d

V

is the saturation voltage of the LM1578A output

sat

transistor.

V

is the saturation voltage of transistor Q1.

sat1

L1t(V

b

V

in

where:

(1/f

)(V

osc

e

t

on

a

(V

V

o

a

2Io(V

in

e

I

p

(V

o

a

b

2V

in

d

a

V

2V

o

b

V

in

sat

Figure 22

a

)(V

o

a

b

2V

d

b

V

sat

sat1

a

2Vd)

b

V

V

sat

b

V

d

sat

b

V

)

sat1

2Vd)

V

sat

)(ton/Ip)

sat1

b

V

b

)

sat1

V

)

, may step a

]

)

sat1

RS-232 LINE DRIVER POWER SUPPLY

The power supply, shown in

Figure 23

, operates from an
input voltage as low as 4.2V (5V nominal), and delivers an
output of
The circuit provides a load regulation of
10% to 100% of full load) and a line regulation of

g

12V atg40 mA with better than 70% efficiency.

g

150 mV (from

g

10 mV.
Other notable features include a cycle-by-cycle current limit
and an output voltage ripple of less than 40 mVp-p.

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.

The feedback resistors, R2 and R3, may be selected as
follows by assuming a value of 10 kX for R1;

e

R2

R3e(lV

b

(V

1V)/45.8 mAe240 kX

o

a

1V)/54.2 mAe240 kX

l

o

Actually, the currents used to program the values for the
feedback resistors may vary from 40 mAto60mA, as long
as their sum is equal to the 100 mA necessary to establish
the 1V threshold across R1. Ideally, these currents should
be equal (50 mA 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.

The current limit resistor, R4, is selected by dividing the cur­rent limit threshold voltage by the maximum peak current
level in the output switch. For our purposes R4
110 mV/750 mAe0.15X. A value of 0.1X was used.

e

9VsV

e

V

o

e

100 mA C2e220 mF

I

o

V

ripple

e

f

osc

e

R1

e

R2

e

R3

e

R4

TL/H/8711– 13

FIGURE 22. Buck-Boost Regulator

FIGURE 23. RS-232 Line Driver Power Supply

TL/H/8711– 14

13

s

15V R5

in

12V C1e1820 pF

e

50 mV C3e20 pF

50 kHz C4e0.0022 mF
110 k L1e220 mH
10 k D1, D2e1N5819

0.15 Q1eD44
220 k

e

V

in

e

V

o

e

g

I

o

e

f

osc

e

R1

e

R2

e

R3

e

R4

e

C1

e

C2

e

C3
D1, D2, D3

e

T1

5V

g

12V
40 mA
80 kHz

10 kX
240 kX
240 kX

0.15 X
820 pF
10 pF
220 mF

e

PE-64287

e

1N5819

270

Typical Applications (Continued)

Capacitor C1 sets the oscillator frequency and is selected
from

Figure 1

.

Capacitor C2 serves as a compensation capacitor for syn­chronous operation and a value of 10 to 50 pF should be
sufficient for most applications.

A minimum value for an ideal output capacitor C3, could be
calculated as C
is the transistor on time (typically 0.4/f
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.

c

e

I

t/DV where Iois the load current, t

o

), and DVisthe

osc

For good efficiency, the diodes must have a low forward
voltage drop and be fast switching. 1N5819 Schottky diodes
work well.

Transformer selection should be picked for an output tran­sistor ‘‘on’’ time of 0.4/f
enough to prevent the output transistor switch from ramping
higher than the transistor’s rating of 750 mA. Pulse Engi­neering (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
PE-64287.

, and a primary inductance high

osc

Figure 23

was a Pulse Engineering

14

Physical Dimensions inches (millimeters)

Order Number LM1578AH/883 or SMD

Metal Can Package (H)

NS Package Number H08C

Plastic Surface-Mount Package (M)

Order Number LM3578AM

NS Package Number M08A

Ý

5962-8958602

15

Physical Dimensions inches (millimeters) (Continued) Lit.

Molded Dual-In-Line Package (N)

Order Number LM2578AN or LM3578AN

NS Package Number N08E

Ý

107455

LM1578A/LM2578A/LM3578A Switching Regulator

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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL
SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or 2. A critical component is any component of a life
systems which, (a) are intended for surgical implant support device or system whose failure to perform can
into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life
failure to perform, when properly used in accordance support device or system, or to affect its safety or
with instructions for use provided in the labeling, can effectiveness.
be reasonably expected to result in a significant injury
to the user.

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Corporation Europe Hong Kong Ltd. Japan Ltd.

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Tel: [phone] Deutsch Tel: (
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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a

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