April 2008 Rev 2 1/23
23
LM138 - LM238 - LM338
Three-terminal 5 A adjustable voltage regulators
Features
■Guaranteed 7 A peak output current
■Guaranteed 5 A output current
■Adjustable output down to 1.2 V
■Line regulation typically 0.005 %/V
■Load regulation typically 0.1 %
■Guaranteed thermal regulation
■Current limit constant with temperature
■Standard 3-lead transistor package
Description
The LM138, LM238, LM338 are adjustable 3-
terminal positive voltage regulators capable of
supplying in excess of 5 A over a 1.2 V to 32 V
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-de-
pendent current limiting. The current limit circuitry
allows peak currents of up to 12 A 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 far from the input filter
capacitors in which case an input bypass is
needed. An optional output capacitor can be
added to improve transient response. The
adjustment terminal can be bypassed to
achieve.very high ripple rejection ratios which are
difficult to achieve with standard 3-terminal
regulators.
Besides replacing fixed regulators or discrete
designs, the LM238 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 input
differential is not exceeded.
The LM138, LM238, LM338 are packaged in
standard steel TO-3 transistor package. The
LM138 is rated for operation from - 55 °C to
150 °C, the LM238 from - 25 °C to 150 °C and the
LM338 from 0 °C to 125 °C.
TO-3
Table 1. Device summary
Part numbers Order codes Temperature range
LM138 LM138K -55 °C to 150 °C
LM238 LM238K -25 °C to 150 °C
LM338 LM338K 0 °C to 125 °C
www.st.com
Contents LM138 - LM238 - LM338
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Contents
1 Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5 Typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6 Typical application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7 Application hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1 External capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2 Load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.3 Protection diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
LM138 - LM238 - LM338 Diagram
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1 Diagram
Figure 1. Schematic diagram
Pin configuration LM138 - LM238 - LM338
4/23
2 Pin configuration
Figure 2. Pin connections (top view)
TO-3
LM138 - LM238 - LM338 Maximum ratings
5/23
3 Maximum ratings
Note: Absolute maximum ratings are those values beyond which damage to the device may occur.
Functional operation under these condition is not implied.
Table 2. Absolute maximum ratings
Symbol Parameter Value Unit
VI - VOInput-output voltage differential 35 V
PDPower dissipation Internally limited
TSTG Storage temperature range -65 to 150 °C
TLEAD Lead temperature (Soldering, 10 seconds) 300 °C
TOP Operating junction temperature range
LM138 -55 to 150
°CLM238 -25 to 125
LM338 0 to 125
Table 3. Thermal data
Symbol Parameter Value Unit
RthJC Thermal resistance junction-case 1.4 °C/W
RthJA Thermal resistance junction-ambient 35 °C/W
Electrical characteristics LM138 - LM238 - LM338
6/23
4 Electrical characteristics
Table 4. Electrical characteristics for LM138/LM238 (1)
1. (TJ = -55 to 150 °C for LM138, TJ = -25 to 150 °C for LM238, VI - VO = 5 V, IO = 2.5 A. Although power dissipation is
internally limited, these specifications apply to power dissipation up to 50 W, unless otherwise specified)
Symbol Parameter Test conditions Min. Typ. Max. Unit
KVI Line regulation (2)
2. Regulation is measured at constant junction temperature. Changes in output voltage due to heating effects are taken into
account separately by thermal rejection.
TA = 25°C, VI - VO = 3 to 35 V 0.005 0.01 %/V
KVO Load regulation (2) TA = 25°C
IO = 10 mA to 5 A
VO ≤ 5 V 5 15 mV
VO ≥ 5 V 0.1 0.3 %
Thermal regulation Pulse = 20 ms 0.002 0.01 %/W
IADJ Adjustment pin current 45 100 µA
ΔIADJ
Adjustment pin current
change IL = 10 mA to 5 A, VI - VO = 3 to 35 V 0.2 5 µA
VREF Reference voltage VI - VO = 3 to 35 V, IO = 10 mA to 5 A
P ≤ 50 W 1.19 1.24 1.29 V
KVI Line regulation (2) VI - VO = 3 to 35 V 0.02 0.04 %/V
KVO Load regulation (2) IO = 10 mA to 5 A VO ≤ 5 V 20 30 mV
VO ≥ 5 V 0.3 0.6 %
KVT Temperature stability TJ = TMIN to TMAX 1%
IO(MIN) Minimum load current VI - VO ≤ 35 V 3.5 5 mA
IO(MAX) Current limit VI - VO ≤ 10 V
DC 5 8
A0.5 ms Peak 7 12
VI - VO = 30 V 1
VNO
RMS output noise (% of
VO)Ta = 25°C, f = 10 Hz to 10 kHz 0.003 %
RVF Ripple rejection ratio VO = 10 V, f = 120 Hz 60 dB
CADJ = 10 µF 60 75
KVH Long term stability TA = 125°C 0.3 1 %
LM138 - LM238 - LM338 Electrical characteristics
7/23
Table 5. Electrical characteristics for LM338 (1)
Symbol Parameter Test conditions Min. Typ. Max. Unit
KVI Line regulation (2) TA = 25°C, VI - VO = 3 to 35 V 0.005 0.03 %/V
KVO Load regulation (2) TA = 25°C
IO = 10 mA to 5 A
VO ≤ 5 V 5 25 mV
VO ≥ 5 V 0.1 0.5 %
Thermal regulation Pulse = 20 ms 0.002 0.02 %/W
IADJ Adjustment pin current 45 100 µA
ΔIADJ
Adjustment pin current
change IL = 10 mA to 5 A, VI - VO = 3 to 35 V 0.2 5 µA
VREF Reference voltage VI - VO = 3 to 35 V, IO = 10 mA to 5 A
P ≤ 50 W 1.19 1.24 1.29 V
KVI Line regulation (2) VI - VO = 3 to 35 V 0.02 0.06 %/V
KVO Load regulation (2) IO = 10 mA to 5 A VO ≤ 5 V 20 50 mV
VO ≥ 5 V 0.3 1 %
KVT Temperature stability TJ = TMIN to TMAX 1%
IO(MIN) Minimum load current VI - VO ≤ 35 V 3.5 10 mA
IO(MAX) Current limit VI - VO ≤ 10 V
DC 5 8
A0.5 ms Peak 7 12
VI - VO = 30 V 1
VNO
RMS output noise (% of
VO)Ta = 25°C, f = 10 Hz to 10 kHz 0.003 %
RVF Ripple rejection ratio VO = 10 V, f = 120 Hz 60 dB
CADJ = 10 µF 60 75
KVH Long term stability TA = 125°C 0.3 1 %
1. (TJ = 0 to150 °C, VI - VO = 5 V, IO = 2.5 A. Although power dissipation is internally limited, these specifications apply to
power dissipation up to 50 W, unless otherwise specified)
2. Regulation is measured at constant junction temperature. Changes in output voltage due to heating effects are taken into
account separately by thermal rejection.
Typical characteristics LM138 - LM238 - LM338
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5 Typical characteristics
Figure 3. Current limit Figure 4. Current limit
Figure 5. Current limit Figure 6. Load regulation
LM138 - LM238 - LM338 Typical characteristics
9/23
Figure 7. Dropout voltage Figure 8. Adjustment current
Figure 9. Temperature stability Figure 10. Output impedance
Typical characteristics LM138 - LM238 - LM338
10/23
Figure 11. Minimum operating current Figure 12. Ripple rejection
Figure 13. Ripple rejection Figure 14. Ripple rejection
LM138 - LM238 - LM338 Typical characteristics
11/23
Figure 15. Line transient response Figure 16. Load transient response
Typical application LM138 - LM238 - LM338
12/23
6 Typical application
Needed if device is far from filter capacitors.
* Optional-improves transient response. Output capacitors in the range of 1mF to 100mF of aluminium or
tantalum electrolytic are commonly used to provide improved output impedance and rejection of transients
** VO = 1.25 V (1 + R2/R1)
*** R1 = 240 Ω for LM138 and LM238
Figure 17. 1.2 V to 25 V adjustable regulator
LM138 - LM238 - LM338 Application hints
13/23
7 Application hints
In operation, the LM338 develops a nominal 1.25 V 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 I1 then flows through the output set
resistor R2, giving an output voltage of
VO = V(REF) (1+ R2/R1) + IADJR2
Since the 50 µA current from the adjustment terminal represents an error term, the LM338
was designed to minimize IADJ 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.
7.1 External capacitors
An input bypass capacitor is recommended. A 0.1 µF disc or 1 µF solid tantalum on the input
is suitable input by passing for almost all applications. The device is more sensitive to the
absence of input bypassing when adjustment or output capacitors are used by the above
values will eliminate the possibility of problems. The adjustment terminal can be bypassed
to ground on the LM338 to improve ripple rejection. This bypass capacitor prevents ripple
form 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
Figure 18. Application circuit
Application hints LM138 - LM238 - LM338
14/23
0.1 µF disc as a bypass. Although the LM338 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 mF solid tantalum (or 25 µF aluminium
electrolytic) on the output swamps this effect and insures stability.
7.2 Load regulation
The LM338 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 240 Ω) should be tied directly to the
output of the regulator rather than near the load. This eliminates line drops from appearing
effectively in series with the reference and degrading regulation. For example, a 15 V
regulator with 0.05 Ω resistance between the regulator and load will have a load regulation
due to line resistance of 0.05 Ω x IL. If the set resistor is connected near the load the
effective line resistance will be 0.05 Ω (1 + R2/R1) or in this case, 11.5 times worse. Figure 4
on page 8 shows the effect of resistance between the regulator and 140 Ω set 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.
7.3 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
20 A 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 VI. In the LM338 this discharge path is through a large junction that is
able to sustain 25 A 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 15 V 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
LM338 is a 50 Ω resistor which limits the peak discharge current. No protection is needed
for output voltages of 25 V or less and 10 µF capacitance. Figure 5 on page 8 shows an
LM338 with protection diodes included for use with outputs greater than 25 V and high
values of output capacitance output capacitance an LM338 with protection diodes included
for use with outputs greater than 25 V and high values of output capacitance.
LM138 - LM238 - LM338 Application hints
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Figure 19. Regulator with line resistance in output lead
Figure 20. Regulator with protection diodes
Application hints LM138 - LM238 - LM338
16/23
* Minimum load - 100 mA
VI ≥ 10 V
VO ≥ 3 V
VI - VO ≥ 3.5 V
* Minimum load - 100 mA
VI ≥ 10 V
VO ≥ 3 V
Figure 21. 10 A regulator
Figure 22. 5 A current regulator
LM138 - LM238 - LM338 Application hints
17/23
* Minimum load - 100mA
VI ≥ 10 V
VO ≥ 3 V
VI - VO ≥ 4V
Figure 23. 15 A regulator
Application hints LM138 - LM238 - LM338
18/23
* R1 = 240 Ω for LM138 or LM238
* R2 = 720 Ω for LM138 or LM238
** Minimum load - 100 mA
* R1 = 240 Ω for LM138 or LM238
* R2 = 720 Ω for LM138 or LM238
* * Minimum output = 1.2 V
Figure 24. 5 V logic regulator with electronic shutdown
Figure 25. Tracking pre-regulator
LM138 - LM238 - LM338 Application hints
19/23
* R1 = 240 Ω for LM138 or LM238
* R2 = 2.7 kΩ for LM138 or LM238
Figure 26. Slow turn-on 15 V regulator
Package mechanical data LM138 - LM238 - LM338
20/23
8 Package mechanical data
In order to meet environmental requirements, ST offers these devices in ECOPACK®
packages. These packages have a lead-free second level interconnect. The category of
second Level Interconnect is marked on the package and on the inner box label, in
compliance with JEDEC Standard JESD97. The maximum ratings related to soldering
conditions are also marked on the inner box label. ECOPACK is an ST trademark.
ECOPACK specifications are available at: www.st.com.
LM138 - LM238 - LM338 Package mechanical data
21/23
Dim. mm. inch.
Min. Typ. Max. Min. Typ. Max.
A 11.85 0.466
B0.96 1.05 1.10 0.037 0.041 0.043
C 1.70 0.066
D8.7 0.342
E 20.0 0.787
G 10.90.429
N 16.90.665
P 26.2 1.031
R3.88 4.090.152 0.161
U39.5 1.555
V30.10 1.185
TO-3 mechanical data
P003C/C
E
B
R
C
D
A
P
G
N
V
U
O
LM138 - LM238 - LM338
23/23
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