LM138.PDF

March 1995

LM138, LM338

5-Amp Adjustable Regulators

General Description

The LM138 series of adjustable 3-terminal positive voltage

regulators is capable of supplying in excess of 5A over a

1.2V to 32V output range. They are exceptionally easy to

use and require only 2 resistors to set the output voltage.

Careful circuit design has resulted in outstanding load and

line regulationÐcomparable to many commercial power

supplies. The LM138 family is supplied in a standard 3-lead

transistor package.

A unique feature of the LM138 family is time-dependent cur-

rent limiting. The current limit circuitry allows peak currents

of up to 12A to be drawn from the regulator for short periods

of time. This allows the LM138 to be used with heavy tran-

sient loads and speeds start-up under full-load conditions.

Under sustained loading conditions, the current limit de-

creases to a safe value protecting the regulator. Also includ-

ed on the chip are thermal overload protection and safe

area protection for the power transistor. Overload protection

remains functional even if the adjustment pin is accidentally

disconnected.

Normally, no capacitors are needed unless the device is

situated more than 6 inches from the input filter capacitors

in which case an input bypass is needed. An output capaci-

tor can be added to improve transient response, while by-

passing the adjustment pin will increase the regulator's rip-

ple rejection.

Besides replacing fixed regulators or discrete designs, the

LM138 is useful in a wide variety of other applications. Since

the regulator is ``floating'' and sees only the input-to-output

differential voltage, supplies of several hundred volts can be

regulated as long as the maximum input to output differen-

tial is not exceeded, i.e., do not short-circuit output to

ground. The part numbers in the LM138 series which have a

K suffix are packaged in a standard Steel TO-3 package,

while those with a T suffix are packaged in a TO-220 plastic

package. The LM138 is rated for b 55 § C s T J s a 150 § C,

and the LM338 is rated for 0 § C s T J s a 125 § C.

Features

Y Guaranteed 7A peak output current

Y Guaranteed 5A output current

Y Adjustable output down to 1.2V

Y Guaranteed thermal regulation

Y Current limit constant with temperature

Y P a Product Enhancement tested

Y Output is short-circuit protected

Applications

Y Adjustable power supplies

Y Constant current regulators

Y Battery chargers

Connection Diagrams (See Physical Dimension section for further information)

(TO-3 STEEL)

Metal Can Package

(TO-220)

Plastic Package

TL/H/9060±31

Front View

Order Number LM338T

See NS Package Number T03B

TL/H/9060±30

Bottom View

Order Number LM138K STEEL or LM338K STEEL

See NS Package Number K02A

C 1995 National Semiconductor Corporation

TL/H/9060

RRD-B30M115/Printed in U. S. A.

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

(Note 4)

Power Dissipation

Lead Temperature

Metal Package (Soldering, 10 seconds)

300 § C

Plastic Package (Soldering, 4 seconds)

260 § C

ESD Tolerance

TBD

Internally limited

Operating Temperature Range

LM138

Input/Output Voltage Differential

a 40V, b 0.3V

Storage Temperature

b 65 § Cto a 150 § C

b 55 § C s T J s a 150 § C

LM338

0 § C s T J s a 125 § C

Electrical Characteristics

Specifications with standard type face are for T J e 25 § C, and those with boldface type apply over full Operating Tempera-

ture Range. Unless otherwise specified, V IN b V OUT e 5V; and I OUT e 10 mA. (Note 2)

Symbol

Parameter

Conditions

LM138

Units

Min Typ Max

V REF

Reference Voltage

3V s (V IN b V OUT ) s 35V,

1.19 1.24 1.29 V

10 mA s I OUT s 5A, P s 50W

V RLINE

Line Regulation

3V s (V IN b V OUT ) s 35V (Note 3)

0.005 0.01 %/V

0.02 0.04 %/V

V RLOAD

Load Regulation

10 mA s I OUT s 5A (Note 3)

0.1 0.3 %

0.3 0.6 %

Thermal Regulation

20 ms Pulse

0.002 0.01 %/W

I ADJ

Adjustment Pin Current

45 100 m A

D I ADJ

Adjustment Pin Current Change 10 mA s I OUT s 5A,

3V s (V IN b V OUT ) s 35V

0.2 5 m A

D V R/T

Temperature Stability

T MIN s T J s T MAX

I LOAD (Min) Minimum Load Current

V IN b V OUT e 35V

3.5 5 mA

I CL

Current Limit

V IN b V OUT s 10V

5 8

0.5 ms Peak

7 12

V IN b V OUT e 30V

V N

RMS Output Noise, % of V OUT 10 Hz s f s 10 kHz

0.003

D V R

D V IN

Ripple Rejection Ratio

V OUT e 10V, f e 120 Hz, C ADJ e 0 m F

V OUT e 10V, f e 120 Hz, C ADJ e 10 m F 60 75

Long-Term Stability

T J e 125 § C, 1000 Hrs

0.3

1 %

i JC

Thermal Resistance,

K Package

1 § C/W

Junction to Case

i JA

Thermal Resistance, Junction to

K Package

§ C/W

Ambient (No Heat Sink)

Electrical Characteristics (Continued)

Specifications with standard type face are for T J e 25 § C, and those with boldface type apply over full Operating Tempera-

ture Range. Unless otherwise specified, V IN b V OUT e 5V; and I OUT e 10 mA. (Note 2)

Symbol

Parameter

Conditions

LM338

Units

Min Typ Max

V REF

Reference Voltage

3V s (V IN b V OUT ) s 35V,

1.19 1.24 1.29 V

10 mA s I OUT s 5A, P s 50W

V RLINE

Line Regulation

3V s (V IN b V OUT ) s 35V (Note 3)

0.005 0.03 %/V

0.02 0.06 %/V

V RLOAD

Load Regulation

10 mA s I OUT s 5A (Note 3)

0.1 0.5 %

0.3 1 %

Thermal Regulation

20 ms Pulse

0.002 0.02 %/W

I ADJ

Adjustment Pin Current

45 100 m A

D I ADJ

Adjustment Pin Current Change 10 mA s I OUT s 5A,

3V s (V IN b V OUT ) s 35V

0.2 5 m A

D V R/T

Temperature Stability

T MIN s T J s T MAX

I LOAD (Min) Minimum Load Current

V IN b V OUT e 35V

3.5 10 mA

I CL

Current Limit

V IN b V OUT s 10V

5 8

0.5 ms Peak

7 12

V IN b V OUT e 30V

V N

RMS Output Noise, % of V OUT 10 Hz s f s 10 kHz

0.003

D V R

D V IN Ripple Rejection Ratio V OUT e 10V, f e 120 Hz, C ADJ e 0 m F 60 dB

V OUT e 10V, f e 120 Hz, C ADJ e 10 m F 60 75 dB

Long-Term Stability T J e 125 § C, 1000 hrs 0.3 1 %

i JC Thermal Resistance K Package 1 § C/W

Junction to Case T Package 4 § C/W

i JA Thermal Resistance, Junction to K Package 35 § C/W

Ambient (No Heat Sink) T Package 50 § C/W

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: These specifications are applicable for power dissipations up to 50W for the TO-3 (K) package and 25W for the TO-220 (T) package. Power dissipation is

guaranteed at these values up to 15V input-output differential. Above 15V differential, power dissipation will be limited by internal protection circuitry. All limits (i.e.,

the numbers in the Min. and Max. columns) are guaranteed to National's AOQL (Average Outgoing Quality Level).

Note 3: Regulation is measured at a constant junction temperature, using pulse testing with a low duty cycle. Changes in output voltage due to heating effects are

covered under the specifications for thermal regulation.

Note 4: Refer to RETS138K drawing for military specifications of LM138K.

Typical Performance Characteristics

Current Limit

TL/H/9060±4

Typical Performance Characteristics (Continued)

Adjustment

Load Regulation

Dropout Voltage

Current

Minimum Operating

Temperature Stability

Output Impedance

Current

Ripple Rejection

Line Transient Response

Load Transient Response

TL/H/9060±5

Application Hints

In operation, the LM138 develops a nominal 1.25V refer-

ence voltage, V REF , between the output and adjustment ter-

minal. The reference voltage is impressed across program

resistor R1 and, since the voltage is constant, a constant

current I 1 then flows through the output set resistor R2, giv-

ing an output voltage of

Load Regulation

The LM138 is capable of providing extremely good load reg-

ulation but a few precautions are needed to obtain maxi-

mum performance. The current set resistor connected be-

tween the adjustment terminal and the output terminal (usu-

ally 240 X ) should be tied directly to the output of the regula-

tor (case) rather than near the load. This eliminates line

drops from appearing effectively in series with the reference

and degrading regulation. For example, a 15V regulator with

0.05 X resistance between the regulator and load will have a

load regulation due to line resistance of 0.05 X c I L .Ifthe

set resistor is connected near the load the effective line

resistance will be 0.05 X (1 a R2/R1) or in this case, 11.5

times worse.

Figure2shows the effect of resistance between the regula-

tor and 240 X set resistor.

V OUT e V REF

1 a R2

a I ADJ R2.

TL/H/9060±6

FIGURE 1

Since the 50 m A current from the adjustment terminal repre-

sents an error term, the LM138 was designed to minimize

I ADJ and make it very constant with line and load changes.

To do this, all quiescent operating current is returned to the

output establishing a minimum load current requirement. If

there is insufficient load on the output, the output will rise.

External Capacitors

An input bypass capacitor is recommended. A 0.1 m F disc

or 1 m F solid tantalum on the input is suitable input bypass-

ing for almost all applications. The device is more sensitive

to the absence of input bypassiing when adjustment or out-

put capacitors are used but the above values will eliminate

the possiblity of problems.

The adjustment terminal can be bypassed to ground on the

LM138 to improve ripple rejection. This bypass capacitor

prevents ripple from being amplified as the output voltage is

increased. With a 10 m F bypass capacitor 75 dB ripple re-

jection is obtainable at any output level. Increases over

20 m F do not appreciably improve the ripple rejection at

frequencies above 120 Hz. If the bypass capacitor is used, it

is sometimes necessary to include protection diodes to pre-

vent the capacitor from discharging through internal low cur-

rent paths and damaging the device.

In general, the best type of capacitors to use are solid tanta-

lum. Solid tantalum capacitors have low impedance even at

high frequencies. Depending upon capacitor construction, it

takes about 25 m F in aluminum electrolytic to equal 1 m F

solid tantalum at high frequencies. Ceramic capacitors are

also good at high frequencies; but some types have a large

decrease in capacitance at frequencies around 0.5 MHz.

For this reason, 0.01 m F disc may seem to work better than

a 0.1 m F disc as a bypass.

Although the LM138 is stable with no output capacitors, like

any feedback circuit, certain values of external capacitance

can cause excessive ringing. This occurs with values be-

tween 500 pF and 5000 pF. A 1 m F solid tantalum (or 25 m F

aluminum electrolytic) on the output swamps this effect and

insures stability.

TL/H/9060±7

FIGURE 2. Regulator with Line

Resistance in Output Lead

With the TO-3 package, it is easy to minimize the resistance

from the case to the set resistor, by using 2 separate leads

to the case. The ground of R2 can be returned near the

ground of the load to provide remote ground sensing and

improve load regulation.

Protection Diodes

When external capacitors are used withany IC regulator it is

sometimes necessary to add protection diodes to prevent

the capacitors from discharging through low current points

into the regulator. Most 20 m F capacitors have low enough

internal series resistance to deliver 20A spikes when short-

ed. Although the surge is short, there is enough energy to

damage parts of the IC.

When an output capacitor is connected to a regulator and

the input is shorted, the output capacitor will discharge into

the output of the regulator. The discharge current depends

on the value of the capacitor, the output voltage of the regu-

lator, and the rate of decrease of V IN . In the LM138 this

discharge path is through a large junction that is able to

sustain 25A surge with no problem. This is not true of other

types of positive regulators. For output capacitors of 100 m F

or less at output of 15V or less, there is no need to use

diodes.

The bypass capacitor on the adjustment terminal can dis-

charge through a low current junction. Discharge occurs

when either the input or output is shorted. Internal to the

LM138 is a 50 X resistor which limits the peak discharge

current. No protection is needed for output voltages of 25V

or less and 10 m F capacitance. Figure3 shows an LM138

with protection diodes included for use with outputs greater

than 25V and high values of output capacitance.

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