LM138/238
LM338
March1993
ADJUSTABLE VOLTAGE REGULATORS
THREE-TERMINAL 5-A
.GUARANTEED 7A PEAK OUTPUTCURRENT
.GUARANTEED 5A OUTPUTCURRENT
.ADJUSTABLE OUTPUT DOWN TO 1.2V
.LINE REGULATION TYPICALLY 0.005% /V
.LOAD REGULATION TYPICALLY 0.1%
.GUARANTEED THERMAL REGULATION
.CURRENT LIMIT CONSTANT WITH TEM-
PERATURE
.STANDARD3-LEAD TRANSISTOR PACKAGE
DESCRIPTION
The LM138/LM238/LM338 are adjustable 3-terminal
positivevoltageregulators capable of supplyinginex-
cess of 5A over a 1.2V to 32V output range. They are
exceptionally easytouse and require only 2 resistors
tosettheoutput voltage. Careful circuitdesignhasre-
sulted in outstanding load and line regulation com-
parable to many commercial power supplies. The
LM138 family is supplied in a standard 3-lead transis-
torpackage.
A unique featureof the LM138 family is time-de-
pendent current limiting. The current limit circuitry
allowspeakcurrentsofupto12Atobedrawn fromthe
regulator for short periods of time. This allows the
LM138 to be used with heavy transient loads and
speedsstart-up under full-load conditions. Under sus-
tained loading conditions, the current limit decreases
toa safe value protecting the regulator. Alsoincluded
on thechip are thermal overload protection andsafe
areaprotection forthepowertransistor. Overloadpro-
tection remains functional even if the adjustment pin
is accidentally disconnected.
Normally,nocapacitors are needed unlessthedevice
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 res-
ponse. The adjustment terminal can be bypassed to
achieve very highripple rejection ratioswhich aredif-
ficulttoachieve with standard 3-terminal regulators.
Besidesreplacing fixedregulatorsordiscretedesigns,
the LM238 is useful in awide variety of other applica-
tions. Since the regulator is ”floating” and sees only
theinput-to-output differential voltage,suppliesofsev-
eral hundred volts can be regulated as long as the
maximum input to input differential is not exceeded.
The LM138/LM238/LM338 arepackaged instandard
steel TO-3 transistor packages. The LM138 is rated
foroperation from -55 oCto150o
C, the LM238 from
–25°C to + 150°C and the LM338 from 0°Cto+
125°C.
TO3
K SUFFIX
(SteelCan)
ORDER CODE
PART
NUMBER TEMPERATURE
RANGE PACKAGE
K
LM138
LM238
LM338
-55 oC to + 150 oC
-25 oC to + 150 oC
0oC to + 125 oC
•
•
•
EXAMPLE: LM138K
PIN CONNECTION
(bottom view)
Caseis output
1/12
ABSOLUTE MAXIMUM RATING
Symbol Parameter Value Unit
Ptot Power Dissipation Internally Limited W
VI-V
OInput-Output Voltage DIfferential 35 V
Toper Operating Junction Temperature Range LM138
LM238
LM338
-55 to 150
-25 to 150
0 to 125
oC
Tstg Storage Temperature Range -65 to 150 oC
Tlead Lead Temperature (Soldering, 10 seconds) 300 oC
THERMAL CHARACTERISTICS
Symbol Parameter Value Unit
Rth(j-c) Typical Junction-Case Thermal Resistance 1.4 oC/W
Rth(j-a) Max Junction-Ambient Thermal Resistance 35 oC/W
SCHEMATIC DIAGRAM
LM138-LM238-LM338
2/12
ELECTRICAL CHARACTERISTICS
LM138: -55 ≤Tj≤150 oC, VI-V
O= 5V, IO=2.5A
LM238: -25 ≤Tj≤150 oC, VI-V
O= 5V, IO=2.5A
LM338: 0≤Tj≤150 oC, VI-V
O= 5V, IO= 2.5A
Although power dissipation is internally limited, these specifications apply to power dissipation up to
50W (unless otherwise specified).
Symbol Parameter LM138-LM238 LM338 Unit
Min. Typ. Max. Min. Typ. Max.
KVI Line Regulation - (note 1)
Tamb =25o
C, 3 V ≤(VI-V
O
)≤35 V 0.005 0.01 0.005 0.03 %/V
KVO Load Regulation Tamb =25o
C, 10 mA ≤IO≤5A
V
O≤5V - (note 1)
VO≥5V - (note 1) 5
0.1 15
0.3 5
0.1 25
0.5 mV
%
Thermal Regulation (pulse = 20 ms) 0.002 0.01 0.002 0.02 %/W
Iadj Adjustment Pin Current 45 100 45 100 µA
∆Iadj Adjustment Pin Current Change
10 mA ≤IL≤5A,3V≤(VI-V
O
)≤35 V 0.2 5 0.2 5 µA
v(ref) Reference Voltage
3V ≤(VI-V
O
)≤35 V, 10 mA ≤IO≤5A, P ≤50W 1.19 1.24 1.29 1.19 1.24 1.29 V
KVI Line Regulation - (note 1)
3V≤(VI-V
O
)≤35 V 0.02 0.04 0.02 0.06 %/V
KVO Load Regulation 10 mA ≤IO≤5A
V
O≤5V - (note 1)
VO≥5V - (note 1) 20
0.3 30
0.6 20
0.3 50
1mV
%
KVT Temperature Stability (Tmin ≤Tj≤Tmax)11%
I
O(min) Minimum Load Current (VI-V
O≤35 V) 3.5 5 3.5 10 mA
IO(max) Current Limit (VI-V
O≤10 V)
DC
0.5 ms Peak
VI-V
O=30V
5
78
12
1
5
78
12
1
A
RMS Output Noise, % of VO
(Tamb =25o
C, 10 Hz ≤f≤10 KHz) 0.003 0.003 %
Rvf Ripple Rejection Ratio
VO= 10 V, f = 120 Hz
Cadi =10µF60
60
75 60 60
75
dB
KVH Long Term Stability (Tamb = 125 oC) 0.3 1 0.3 1 %
Note 1 : Regulation is measured at constant junction temperature. Changes in output voltage due to heating effects are taken
into account separately by thermal rejection.
LM138-LM238-LM338
3/12
LM138-LM238-LM338
4/12
LM138-LM238-LM338
5/12
TYPICAL APPLICATIONS
+ 1.2V to + 25V ADJUSTABLE REGULATOR
APPLICATION HINTS
In operation, theLM338 develops anominal 1.25V
reference voltage, V(ref), between the output and
adjustment terminal. The reference voltage is im-
pressed across program resistor R1 and, since the
voltage is constant,a constant currentI1then flows
through theoutput setresistor R2,givingan output
voltage of
VO=V
(ref) (1+
R
2
R
1)+I
adjR2
Figure 1.
Since the 50µA current from the adjustment termi-
nal represents an error term, the LM338 was de-
signed to minimize Iadj and make it very constant
with lineand loadchanges.Todothis, allquiescent
operating current is returned to the output establi-
shing a minimumload current requirement.Ifthere
is insufficientloadon theoutput, theoutputwill rise.
EXTERNAL CAPACITORS
An input bypass capacitor is recommended. A
0.1µF discor1µF solid tantalumonthe inputis suit-
able input by passing for almost all applications.
Thedeviceismore sensitivetothe absenceofinput
bypassing when adjustment or output capacitors
are used byt 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
a10µF bypass capacitor 75dB ripple rejection is
obtainableatanyoutputlevel.Increasesover20µF
do not appreciably improve the ripple rejection at
frequencies above 120Hz. If the bypass capacitor
is used, it is sometimes necessary to include pro-
tection diodes to prevent the capacitor from dis-
charging 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 25µFin
aluminumelectrolytictoequal1µFsolidtantalumat
high frequencies. Ceramic capacitors are also
good at high frequencies, but some types have a
large decrease in capacitance at frequencies
around 0.5MHz. For this reason, 0.01µF disc may
seem to work better than a 0.1µF discas abypass.
Although the LM338 is stable with no output capa-
citors, like any feedback circuit, certain values of
external capacitance can cause excessiveringing.
This occurs with values between 500pF and
5000pF. A 1µF solid tantalum (or 25µF aluminium
electrolytic) on the output swamps this effect and
insures stability.
LOADREGULATION
The LM338is capable ofproviding extremelygood
load regulation but a few precautions are needed
to obtain maximum performance. The current set
resistor connected between the adjustment termi-
nal and the output terminal (usually 240Ω) should
be tied directly tothe 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 15V
regulator with 0.05Ωresistance between the regu-
lator and loadwillhave aload regulation due toline
resistance of 0.05ΩxI
L
. If the set resistor is con-
nected near the load the effective line resistance
∆Needed if device is far from filter capacitors.
*Optional-improves transient response. Output capacitors in the
range of 1µF to 100µF of aluminium or tantalum electrolytic are
commonly used toprovide improved output impedance and rejec-
tion of tran-sients. R2
** V
O=1.25V(1+ )
R1
*** R1=240Ωfor LM138 and LM238
LM138-LM238-LM338
6/12
will be 0.05Ω(1 +R2/R1) or in this case, 11.5 times
worse.
Figure 2shows the effect of resistancebetween the
regulator and 140Ωset resistor.
Withthe TO-3package, itiseasy tominimize there-
sistance from the case to the set resistor, by using
2 separate leads to the case. The ground of R2can
be returned near the ground of the load to provide
remote ground sensing and improve load regula-
tion.
PROTECTION DIODES
WhenexternalcapacitorsareusedwithanyICregu-
lator it is sometimes necessary to add protection
diodes to prevent the capacitors from discharging
through low current points into the regulator. Most
Figure 2 : Regulator with Line Resistance inOut-
put Lead.
20µF capacitors have low enough internal seriesre-
sistance to deliver 20A spikes when shorted. Al-
thoughthe surge is short, there is enough energy to
damage parts of the IC.
When anoutput capacitor is connected toa regula-
tor and the input isshorted, the output capacitor will
discharge into the output of the regulator. The dis-
chargecurrent depends on the value of thecapaci-
tor, the output voltage of the regulator, and therate
of decrease of VI. In the LM338 this discharge path
is through alarge junction that isable tosustain 25A
surgewithno problem. Thisis not true ofother types
of positive regulators. For output capacitors of
100µF 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. Dis-
charge occurs when either the input or output is
shorted. Internal to the LM338 is a 50Ωresistor
which limits the peak discharge current. No protec-
tionis neededfor outputvoltages of25V or lessand
10µF capacitance. Figure 3 shows an LM338 with
protection diodes included for use with outputs
greaterthan 25V and high values ofoutput capacit-
ance.
Figure 3 : Regulator with Protection Diodes.
LM138-LM238-LM338
7/12
10A REGULATOR
*Minimum load– 100mA
VI≥10V
V0≥3V
VI–V
0≥3.5V
5A CURRENT REGULATOR
*Minimum load– 100mA
VI≥10V
V0≥3V
VI–V
0≥3.5V
LM138-LM238-LM338
8/12
15A REGULATOR
*Minimum load– 100mA
VI≥+10V
V
0≥+3V
V
I–V
0≥+4V
5V LOGIC REGULATOR WITH ELECTRONIC SHUTDOWN**
*R1 = 240Ωfor LM138or LM238
*R2 = 720Ωfor LM138or LM238
**
Minimumoutput ≈+1.2V
LM138-LM238-LM338
9/12
*R1 = 240Ωfor LM138or LM238
*R2 = 720Ωfor LM138or LM238
**
Minimumoutput ≈+1.2V
SLOW TURN-ON 15V REGULATOR
*R1 = 240Ω}for LM138 and LM238
*R2 = 2.7kΩ
TRACKING PREREGULATOR
LM138-LM238-LM338
10/12
DIM. mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A 11.00 13.10 0.433 0.516
B 0.97 1.15 0.038 0.045
C 1.50 1.65 0.059 0.065
D 8.32 8.92 0.327 0.351
E 19.00 20.00 0.748 0.787
G 10.70 11.10 0.421 0.437
N 16.50 17.20 0.649 0.677
P 25.00 26.00 0.984 1.023
R 4.00 4.09 0.157 0.161
U 38.50 39.30 1.515 1.547
V 30.00 30.30 1.187 1.193
C
D
N
B
V
U
R
A
P
E
G
O
P003F
TO-3 MECHANICAL DATA
LM138-LM238-LM338
11/12
Information furnished is believed to be accurate and reliable.However, SGS-THOMSON Microelectronics assumes no responsability forthe
consequences of useof such information nor for any infringementof patents orother rights of third partieswhich may results from its use. No
license is granted by implication or otherwiseunderany patent or patent rights of SGS-THOMSON Microelectronics. Specificationsmentioned
in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied.
SGS-THOMSON Microelectronicsproductsare notauthorizedforuse ascritical componentsin lifesupportdevices orsystemswithout express
written approval of SGS-THOMSON Microelectonics.
1994 SGS-THOMSON Microelectronics- All Rights Reserved
SGS-THOMSON Microelectronics GROUP OF COMPANIES
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LM138-LM238-LM338
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