NSC LM338T Datasheet

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 regulation—comparable 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 cur­rent 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 tran­sient loads and speeds start-up under full-load conditions.
Under sustained loading conditions, the current limit de­creases to a safe value protecting the regulator. Also in­cluded 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 capacitor
can be added to improve transient response, while bypass­ing the adjustment pin will increase the regulator’s ripple re­jection.

May 1998

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 differential
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 −55˚C ≤ T
is rated for 0˚C ≤ T

≤ +125˚C.

J

≤ +150˚C, and the LM338

J

Features

n Guaranteed 7A peak output current
n Guaranteed 5A output current
n Adjustable output down to 1.2V
n Guaranteed thermal regulation
n Current limit constant with temperature

+

n P

Product Enhancement tested

n Output is short-circuit protected

Applications

n Adjustable power supplies
n Constant current regulators
n Battery chargers

LM138/LM338 5-Amp Adjustable Regulators

Connection Diagrams (See Physical Dimension section for further information)

(TO-3 STEEL)

Metal Can Package

DS009060-30

Bottom View

Order Number LM138K STEEL or LM338K STEEL

See NS Package Number K02A

© 1999 National Semiconductor Corporation DS009060 www.national.com

See NS Package Number T03B

(TO-220)

Plastic Package

DS009060-31

Front View

Order Number LM338T

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 Internally limited

Input/Output Voltage Differential +40V, −0.3V
Storage Temperature −65˚C to +150˚C

Lead Temperature

Metal Package (Soldering, 10 seconds)
Plastic Package (Soldering, 4 seconds)

300˚C
260˚C

ESD Tolerance TBD

Operating Temperature Range

LM138 −55˚C ≤ TJ≤ +150˚C
LM338 0˚C ≤ T

≤ +125˚C

J

Electrical Characteristics

Specifications with standard type face are for T

ture Range. Unless otherwise specified, V

Symbol Parameter Conditions LM138 Units

V

V

V

REF

RLINE

RLOAD

Reference Voltage 3V ≤ (VIN−V

Line Regulation 3V ≤ (VIN−V

Load Regulation 10 mA ≤ I

Thermal Regulation 20 ms Pulse 0.002 0.01

I

ADJ

∆I

ADJ

∆V

R/T

I

(Min) Minimum Load Current VIN−V

LOAD

I

CL

V

N

Adjustment Pin Current 45 100 µA
Adjustment Pin Current Change 10 mA ≤ I

Temperature Stability T

Current Limit VIN−V

RMS Output Noise,%of V
Ripple Rejection Ratio V

Long-Term Stability T

θ

JC

Thermal Resistance, K Package 1 ˚C/W
Junction to Case

θ

JA

Thermal Resistance, Junction to K Package 35 ˚C/W
Ambient (No Heat Sink)

IN−VOUT

OUT

=

25˚C, and those with boldface type apply over full Operating Tempera-

J

=

5V; and I

=

10 mA. (Note 2)

OUT

Min Typ Max

) ≤ 35V, 1.19 1.24 1.29 V

10 mA ≤ I

OUT

≤ 5A, P ≤ 50W

OUT

) ≤ 35V (Note 3) 0.005 0.01

OUT

0.02 0.04

≤ 5A (Note 3) 0.1 0.3

OUT

0.3 0.6

≤ 5A, 0.2 5 µA

MIN

OUT

IN−VOUT

≤ TJ≤ T

OUT
OUT

) ≤ 35V

MAX

=

35V 3.5 5 mA

≤ 10V

1

3V ≤ (V

DC 58 A

0.5 ms Peak 712 A

V

IN−VOUT

=

30V 1 1 A

10 Hz ≤ f ≤ 10 kHz 0.003

=

10V, f=120 Hz, C

OUT

=

V

10V, f=120 Hz, C

OUT

=

125˚C, 1000 Hrs 0.3 1

J

=

0µF 60 dB

ADJ

=

10 µF 60 75 dB

ADJ

%

/V

%

/V
%
%

%

/W

%

%

%

Electrical Characteristics

Symbol Parameter Conditions LM338 Units

Min Typ Max

V

REF

V

RLINE

V

RLOAD

I

ADJ

∆I

ADJ

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Reference Voltage 3V ≤ (VIN−V

10 mA ≤ I

Line Regulation 3V ≤ (VIN−V

) ≤ 35V, 1.19 1.24 1.29 V

OUT

≤ 5A, P ≤ 50W

OUT

) ≤ 35V (Note 3) 0.005 0.03

OUT

0.02 0.06

Load Regulation 10 mA ≤ I

≤ 5A (Note 3) 0.1 0.5

OUT

0.3 1

Thermal Regulation 20 ms Pulse 0.002 0.02
Adjustment Pin Current 45 100 µA
Adjustment Pin Current Change 10 mA ≤ I

3V ≤ (V

OUT

IN−VOUT

≤ 5A,

) ≤ 35V

0.2 5 µA

%

/V

%

/V
%
%

%

/W

Electrical Characteristics (Continued)

Symbol Parameter Conditions LM338 Units

Min Typ Max

∆V

R/T

I

(Min) Minimum Load Current VIN−V

LOAD

I

CL

Temperature Stability T

MIN

≤ TJ≤ T

Current Limit VIN−V

MAX

=

35V 3.5 10 mA

OUT

≤ 10V

OUT

1

DC 58 A

0.5 ms Peak 712 A

V

V

N

RMS Output Noise,%of V

OUT

Ripple Rejection Ratio V

Long-Term Stability T

θ

JC

Thermal Resistance K Package 1 ˚C/W

IN−VOUT

10 Hz ≤ f ≤ 10 kHz 0.003

OUT

V

OUT

J

=

30V 1 A

=

10V, f=120 Hz, C

=

10V, f=120 Hz, C

=

125˚C, 1000 hrs 0.3 1

=

0µF 60 dB

ADJ

=

10 µF 60 75 dB

ADJ

Junction to Case T Package 4 ˚C/W

θ

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

Load Regulation

DS009060-32

DS009060-35

Current Limit

Dropout Voltage

DS009060-33

DS009060-36

Current Limit

DS009060-34

Adjustment
Current

DS009060-37

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Typical Performance Characteristics (Continued)

Temperature Stability

Ripple Rejection

Line Transient Response

DS009060-38

DS009060-41

Output Impedance

Ripple Rejection

DS009060-39

DS009060-42

Load Transient Response

Minimum Operating
Current

DS009060-40

Ripple Rejection

DS009060-43

DS009060-44

Application Hints

In operation, the LM138 develops a nominal 1.25V reference
voltage, V
The reference voltage is impressed across program resistor
R1 and, since the voltage is constant, a constant current I
then flows through the output set resistor R2, giving an out­put voltage of

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, between the output and adjustment terminal.

REF

1

DS009060-45

Application Hints (Continued)

DS009060-6

FIGURE 1.

Since the 50 µA current from the adjustment terminal repre­sents an error term, the LM138 was designed to minimize

and make it very constant with line and load changes.

I

ADJ

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 µF disc or
1 µF solid tantalum on the input is suitable input bypassing
for almost all applications. The device is more sensitive to
the absence of input bypassiing when adjustment or output
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 µF bypass capacitor 75 dB ripple rejec­tion is obtainable at any output level. Increases over 20 µF
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 prevent
the capacitor from discharging through internal low current
paths and damaging the device.

In general, the best type of capacitors to use are solid tanta­lum. Solid tantalum capacitors have low impedance even at
high frequencies. Depending upon capacitor construction, it
takes about 25 µF in aluminum electrolytic to equal 1 µF
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 µF disc may seem to work better than a

0.1 µF 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 be­tween 500 pF and 5000 pF. A 1 µF solid tantalum (or 25 µF
aluminum electrolytic) on the output swamps this effect and
insures stability.

Load Regulation

The LM138 is capable of providing extremely good load
regulation but a few precautions are needed to obtain maxi­mum performance. The current set resistor connected be­tween the adjustment terminal and the output terminal (usu­ally 240Ω) should be tied directly to the output of the
regulator (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 0.05Ω resistance between the regulator and load will
have a load regulation due to line resistance of 0.05Ω xI
the set resistor is connected near the load the effective line

L

resistance will be 0.05Ω (1 + R2/R1) or in this case, 11.5
times worse.

Figure 2

shows the effect of resistance between the regula-

tor and 240Ω set resistor.

DS009060-7

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

any

When external capacitors are used with

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 µF capacitors have low enough
internal series resistance to deliver 20A spikes when
shorted. 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 regu­lator, and the rate of decrease of V
charge path is through a large junction that is able to sustain

. In the LM138 this dis-

IN

25A surge with no problem. This is not true of other types of
positive regulators. For output capacitors of 100 µF or less at
output of 15V or less, there is no need to use diodes.

The bypass capacitor on the adjustment terminal can dis­charge through a low current junction. Discharge occurs
when

either

the input or output is shorted. Internal to the
LM138 is a 50Ω resistor which limits the peak discharge cur­rent. No protection is needed for output voltages of 25V or
less and 10 µF capacitance.

Figure 3

shows an LM138 with
protection diodes included for use with outputs greater than
25V and high values of output capacitance.

.If

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