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 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 in-
cluded 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 situ-
ated 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 bypass-
ing the adjustment pin will increase the regulator’s ripple re-
jection.
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
J
≤+150˚C, and the LM338
is rated for 0˚C ≤T
J
≤+125˚C.
Features
nGuaranteed 7A peak output current
nGuaranteed 5A output current
nAdjustable output down to 1.2V
nGuaranteed thermal regulation
nCurrent limit constant with temperature
nP
+
Product Enhancement tested
nOutput is short-circuit protected
Applications
nAdjustable power supplies
nConstant current regulators
nBattery chargers
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
(TO-220)
Plastic Package
DS009060-31
Front View
Order Number LM338T
See NS Package Number T03B
May 1998
LM138/LM338 5-Amp Adjustable Regulators
© 1999 National Semiconductor Corporation DS009060 www.national.com
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 ≤T
J
≤+150˚C
LM338 0˚C ≤T
J
≤+125˚C
Electrical Characteristics
Specifications with standard type face are for T
J
=25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
−V
OUT
=5V; and I
OUT
=10 mA. (Note 2)
Symbol Parameter Conditions LM138 Units
Min Typ Max
V
REF
Reference Voltage 3V ≤(V
IN
−V
OUT
)≤35V, 1.19 1.24 1.29 V
10 mA ≤I
OUT
≤5A, P ≤50W
V
RLINE
Line Regulation 3V ≤(V
IN
−V
OUT
)≤35V (Note 3) 0.005 0.01 %/V
0.02 0.04 %/V
V
RLOAD
Load Regulation 10 mA ≤I
OUT
≤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 µA
∆I
ADJ
Adjustment Pin Current Change 10 mA ≤I
OUT
≤5A, 0.2 5 µA
3V ≤(V
IN
−V
OUT
)≤35V
∆V
R/T
Temperature Stability T
MIN
≤T
J
≤T
MAX
1%
I
LOAD
(Min) Minimum Load Current V
IN
−V
OUT
=35V 3.5 5 mA
I
CL
Current Limit V
IN
−V
OUT
≤10V
DC 58 A
0.5 ms Peak 712 A
V
IN
−V
OUT
=30V 1 1 A
V
N
RMS Output Noise, %of V
OUT
10 Hz ≤f≤10 kHz 0.003 %
Ripple Rejection Ratio V
OUT
=10V, f =120 Hz, C
ADJ
=0µF 60 dB
V
OUT
=10V, f =120 Hz, C
ADJ
=10 µF 60 75 dB
Long-Term Stability T
J
=125˚C, 1000 Hrs 0.3 1 %
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)
Electrical Characteristics
Symbol Parameter Conditions LM338 Units
Min Typ Max
V
REF
Reference Voltage 3V ≤(V
IN
−V
OUT
)≤35V, 1.19 1.24 1.29 V
10 mA ≤I
OUT
≤5A, P ≤50W
V
RLINE
Line Regulation 3V ≤(V
IN
−V
OUT
)≤35V (Note 3) 0.005 0.03 %/V
0.02 0.06 %/V
V
RLOAD
Load Regulation 10 mA ≤I
OUT
≤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 µA
∆I
ADJ
Adjustment Pin Current Change 10 mA ≤I
OUT
≤5A,
3V ≤(V
IN
−V
OUT
)≤35V 0.2 5 µA
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Electrical Characteristics (Continued)
Symbol Parameter Conditions LM338 Units
Min Typ Max
∆V
R/T
Temperature Stability T
MIN
≤T
J
≤T
MAX
1%
I
LOAD
(Min) Minimum Load Current V
IN
−V
OUT
=35V 3.5 10 mA
I
CL
Current Limit V
IN
−V
OUT
≤10V
DC 58 A
0.5 ms Peak 712 A
V
IN
−V
OUT
=30V 1 A
V
N
RMS Output Noise, %of V
OUT
10 Hz ≤f≤10 kHz 0.003 %
Ripple Rejection Ratio V
OUT
=10V, f =120 Hz, C
ADJ
=0µF 60 dB
V
OUT
=10V, f =120 Hz, C
ADJ
=10 µF 60 75 dB
Long-Term Stability T
J
=125˚C, 1000 hrs 0.3 1 %
θ
JC
Thermal Resistance K Package 1 ˚C/W
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 in-
tended 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
DS009060-32
Current Limit
DS009060-33
Current Limit
DS009060-34
Load Regulation
DS009060-35
Dropout Voltage
DS009060-36
Adjustment
Current
DS009060-37
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Typical Performance Characteristics (Continued)
Application Hints
In operation, the LM138 develops a nominal 1.25V reference
voltage, V
REF
, between the output and adjustment terminal.
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, giving an out-
put voltage of
Temperature Stability
DS009060-38
Output Impedance
DS009060-39
Minimum Operating
Current
DS009060-40
Ripple Rejection
DS009060-41
Ripple Rejection
DS009060-42
Ripple Rejection
DS009060-43
Line Transient Response
DS009060-44
Load Transient Response
DS009060-45
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Application Hints (Continued)
Since the 50 µA 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 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 rejec-
tion is obtainable at any output level. Increases over 20 µF
do not appreciably improve the ripple rejection at frequen-
cies 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 tanta-
lum. 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 be-
tween 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 maxi-
mum performance. The current set resistor connected be-
tween the adjustment terminal and the output terminal (usu-
ally 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 refer-
ence and degrading regulation. For example, a 15V regula-
tor with 0.05Ωresistance between the regulator and load will
have a load regulation due to line resistance of 0.05ΩxI
L
.If
the set resistor is connected near the load the effective line
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 240Ωset resistor.
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 with
any
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 regu-
lator, and the rate of decrease of V
IN
. In the LM138 this dis-
charge 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 µF 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 50Ωresistor which limits the peak discharge cur-
rent. 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.
DS009060-6
FIGURE 1.
DS009060-7
FIGURE 2. Regulator with Line
Resistance in Output Lead
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Application Hints (Continued)
DS009060-8
D1 protects against C1
D2 protects against C2
FIGURE 3. Regulator with Protection Diodes
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Typical Applications
Regulator and Voltage Reference
DS009060-3
1.2V–25V Adjustable Regulator
DS009060-1
Temperature Controller
DS009060-10
Full output current not available
at high input-output voltages
†Optional— improves transient response. Output capacitors in the range of
1 µF to 1000 µF of aluminum or tantalum electrolytic are commonly used
to provide improved output impedance and rejection of transients.
*Needed if device is more than 6 inches from filter capacitors.
**R1 =240Ωfor LM138. R1, R2 as an assembly can be ordered from
Bourns:
MIL part no. 7105A-AT2-502
COMM part no. 7105A-AT7-502
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Schematic Diagram
DS009060-9
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Typical Applications
Precision Power Regulator with Low Temperature Coefficient
DS009060-12
* Adjust for 3.75 across R1
Slow Turn-On 15V Regulator
DS009060-13
Adjustable Regulator with Improved Ripple Rejection
DS009060-14
†Solid tantalum
*Discharges C1 if output is shorted to ground
**R1 =240Ωfor LM138
High Stability 10V Regulator
DS009060-15
Digitally Selected Outputs
DS009060-16
*Sets maximum VOUT
**R1 =240Ωfor LM138
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Typical Applications (Continued)
15A Regulator
DS009060-17
* Minimum load— 100 mA
5V Logic Regulator with Electronic Shutdown**
DS009060-18
** Minimum output ≈1.2V
Light Controller
DS009060-11
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Typical Applications (Continued)
0 to 22V Regulator
DS009060-19
*R1=240Ω,R2=5k for LM138
Full output current not available
at high input-output voltages
12V Battery Charger
DS009060-20
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Typical Applications (Continued)
Adjustable Current Regulator
DS009060-21
Precision Current Limiter
DS009060-22
5A Current Regulator
DS009060-23
Tracking Preregulator
DS009060-24
Adjusting Multiple On-Card Regulators with Single Control*
DS009060-25
†Minimum load— 10 mA
*All outputs within ±100 mV
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Typical Applications (Continued)
Power Amplifier
DS009060-27
AV=1, RF=10k, CF=100 pF
AV=10, RF=100k, CF=10 pF
Bandwidth ≥100 kHz
Distortion ≤0.1%
Simple 12V Battery Charger
DS009060-28
Use of RSallows low charging rates with fully charged battery.
**The 1000 µF is recommended to filter out input transients
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Typical Applications (Continued)
Adjustable 15A Regulator
DS009060-26
Current Limited 6V Charger
DS009060-29
* Set max charge current to 3A
** THE 1000 µF is recommended to filter out input transients.
10A Regulator
DS009060-2
* Minimum load— 100 mA
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Physical Dimensions inches (millimeters) unless otherwise noted
2 Lead TO-3 Metal Can Package (K)
Order Number LM138K or LM338K STEEL
NS Package Number K02A
3 Lead Molded TO-220 (T)
Order Number LM338T
NS Package Number T03B
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be reasonably expected to result in a significant injury
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LM138/LM338 5-Amp Adjustable Regulators
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.
