Page 1
ADJUSTABLE VOLTAGE REGULATORS
.GUARANTEED 7A PEAK OUTPUTCURRENT
.GUARANTEED 5A OUTPUTCURRENT
.ADJUSTABLE OUTPUT DOWN TO 1.2V
.LINE REGULATIONTYPICALLY 0.005% /V
.LOAD REGULATION TYPICALLY0.1%
.GUARANTEED THERMAL REGULATION
.CURRENT LIMIT CONSTANT WITH TEM-
PERATURE
.STANDARD3-LEAD TRANSISTOR PACKAGE
DESCRI PTIO N
TheLM138/LM238/LM338 are adjustable 3-terminal
positivevoltageregulatorscapable ofsupplyinginexcess of 5A over a 1.2V to 32V output range. They are
exceptionally easytouseandrequireonly 2 resistors
tosettheoutput voltage. Careful circuitdesignhasresulted in outstanding load and line regulation comparable to many commercial power supplies. The
LM138familyis supplied in astandard3-leadtransistorpackage.
A unique featureof the LM138 family is time-dependent current limiting. The current limit circuitry
allowspeakcurrents ofupto12Atobedrawnfromthe
regulator for short periods of time. This allows the
LM138 to be used with heavy transient loads and
speedsstart-up underfull-load conditions. Under sustainedloading conditions, the currentlimitdecreases
toa safevalueprotectingthe regulator. Also included
on thechip are thermal overload protection and safe
areaprotectionforthepowertransistor.Overloadprotection remains functional evenif theadjustment pin
is accidentally disconnected.
Normally,nocapacitorsareneeded unlessthedevice
is situatedfar fromthe input filter capacitors inwhich
case an input bypass is needed. An optional output
capacitor can be added to improve transient response. The adjustmentterminal can be bypassed to
achieve very highripple rejection ratioswhich aredifficulttoachieve withstandard 3-terminal regulators.
Besidesreplacing fixedregulatorsordiscretedesigns,
theLM238is usefulin awide varietyof other applications. Since the regulator is ”floating” and sees only
theinput-to-outputdifferential voltage,suppliesofseveral hundred volts can be regulated as long as the
maximuminputto inputdifferential isnotexceeded.
TheLM138/LM238/LM338 arepackaged in standard
steelTO-3 transistor packages. The LM138 is rated
foroperation from -55oCto150oC, the LM238from
–25°C to + 150°C and the LM338 from 0°Cto+
125°C.
THREE-TERMINAL 5-A
ORDER CODE
PART
NUMBER
LM138
LM238
LM338
EXAMPLE: LM138K
PIN CONNE C TI ON
LM13 8/2 38
TO3
K SUFFIX
(SteelCan)
TEMPERATURE
RANGE
o
-55
C to + 150oC
o
-25
C to + 150oC
0oC to + 125oC
(bot tom view)
Caseis output
LM338
PACKAGE
K
•
•
•
March1993
1/12
Page 2
LM138-LM238-LM338
ABSOLUTE MAXIMUM RATING
Symbol Parameter Value Unit
P
Power Dissipation Internally Limited W
tot
V
T
T
THERMAL CHARACTERISTICS
Input-Output Voltage DIfferential 35 V
I-VO
Operating Junction Temperature Range LM138
oper
LM238
LM338
Storage Temperature Range -65 to 150
T
stg
Lead Temperature (Soldering, 10 seconds) 300
lead
-55 to 150
-25 to 150
0 to 125
o
C
o
C
o
C
Symbol Parameter Value Unit
R
R
Typical Junction-Case Thermal Resistance 1.4
th(j-c)
Max Junction-Ambient Thermal Resistance 35
th(j-a)
o
C/W
o
C/W
SCHEM ATIC DIAG RA M
2/12
Page 3
LM138-LM238-LM338
ELECTRICAL CHARACTERISTICS
LM138: -55 ≤ Tj≤ 150oC, VI-VO= 5V, IO=2.5A
LM238: -25 ≤ Tj≤ 150oC, VI-VO= 5V, IO=2.5A
LM338: 0 ≤ Tj≤ 150oC, VI-VO= 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.
Line Regulation - (note 1)
K
VI
T
=25oC, 3 V ≤ (VI-VO)≤35 V
amb
Load Regulation T
K
VO
=25oC, 10 mA ≤ IO≤ 5A
amb
VO≤5V - (note 1)
VO≥ 5V - (note 1)
Thermal Regulation (pulse = 20 ms) 0.002 0.01 0.002 0.02 %/W
Adjustment Pin Current 45 100 45 100 µA
I
adj
∆I
Adjustment Pin Current Change
adj
10 mA ≤ IL≤ 5A,3V≤(VI-VO)≤35 V
v
Reference Voltage
(ref)
3V ≤ (VI-VO)≤35 V, 10 mA ≤ IO≤ 5A, P ≤ 50W
Line Regulation - (note 1)
K
VI
3V≤(VI-VO)≤35 V
Load Regulation 10 mA ≤ IO≤ 5A
K
VO
VO≤5V - (note 1)
VO≥ 5V - (note 1)
K
Temperature Stability (T
VT
I
O(min)
I
O(max)
Minimum Load Current (VI-VO≤35 V) 3.5 5 3.5 10 mA
Current Limit (VI-VO≤10 V)
min
≤ Tj≤ T
)11%
max
DC
0.5 ms Peak
V
=30V
I-VO
RMS Output Noise, % of V
(T
=25oC, 10 Hz ≤ f ≤ 10 KHz)
amb
Ripple Rejection Ratio
R
vf
O
VO= 10 V, f = 120 Hz
C
=10µF60
adi
Long Term Stability (T
K
VH
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.
= 125oC) 0.3 1 0.3 1 %
amb
0.005 0.01 0.005 0.03 %/V
5
0.1150.3
5
0.1250.5
mV
0.2 5 0.2 5 µA
1.19 1.24 1.29 1.19 1.24 1.29 V
0.02 0.04 0.02 0.06 %/V
5
7
20
0.3300.6
8
12
1
20
0.3
5
7
12
50
mV
1
8
1
0.003 0.003 %
60
75 60
60
75
%
%
A
dB
3/12
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LM138-LM238-LM338
4/12
Page 5
LM138-LM238-LM338
5/12
Page 6
LM138-LM238-LM338
TYPICAL APPLICATI O NS
+ 1.2V to + 25V ADJUSTABLE REGULATOR
∆ 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
commonlyused toprovide improvedoutput impedance and rejec-
tion of tran-sients.
** VO=1.25V(1+ )
*** R1=240Ωfor LM138 and LM238
R2
R1
APPLICATION HINTS
In operation,the LM338 develops a nominal 1.25V
reference voltage, V
, between the output and
(ref)
adjustment terminal. The reference voltage is impressed across program resistor R1and, since the
voltage is constant, aconstant current I1then flows
through theoutput setresistor R2,giving an output
voltage of
R
VO=V
(ref)
(1+
R
2
1
)+I
adjR2
Figure 1.
Since the 50µAcurrent from the adjustmentterminal represents anerror term,the LM338 was designed to minimize I
and make it very constant
adj
withlineand load changes. To do this, allquiescent
operating current is returned to the output establishing a minimum load current requirement. Ifthere
is insufficientloadonthe output, the outputwill rise.
EXTERNAL CAPACITORS
An input bypass capacitor is recommended. A
0.1µFdisc or1µFsolid tantalum on the inputis suitable 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.Increases over20µF
do not appreciably improve the ripple rejection at
frequencies above 120Hz. 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 capacitorconstruction, it takesabout 25µFin
aluminumelectrolyticto equal1µFsolid tantalum at
high frequencies. Ceramic capacitors are also
good at high frequencies, but some types have a
large decrease in capacitance at frequencies
around 0.5MHz. For this reason, 0.01µF disc may
seem towork better than a0.1µF disc as a bypass.
Although the LM338 is stable with no output capacitors, like any feedback circuit, certain values of
external capacitancecan cause excessive ringing.
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.
6/12
LOADREGULATION
The LM338is 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 tothe output of theregulator 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 theregulator and loadwill have a load regulation duetoline
resistance of 0.05Ω xIL. If the set resistor is connected near the load the effective line resistance
Page 7
LM138-LM238-LM338
will be 0.05Ω(1 + R2/R1)or in this case, 11.5times
worse.
Figure 2showsthe effectof resistancebetween the
regulator and 140Ω set resistor.
Withthe TO-3package,it is easytominimizetheresistance from the case to the set resistor, by using
2 separate leadsto the case. The ground of R2can
be returned near the ground of the load to provide
remote ground sensing and improve load regulation.
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 enoughinternal seriesresistance to deliver 20A spikes when shorted. Althoughthe surgeis short,there is enoughenergy to
damage parts of the IC.
When an output capacitor is connected toa regulator and theinput is shorted, the outputcapacitor will
discharge into the output of the regulator. The dischargecurrentdepends on the value of thecapacitor, theoutputvoltage ofthe regulator, and therate
of decrease of VI. Inthe LM338 this discharge path
is through alarge junctionthatisable to sustain25A
surgewithno problem. Thisis nottrue ofothertypes
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. Discharge occurs when either the input or output is
shorted. Internal to the LM338 is a 50Ω resistor
which limits the peak dischargecurrent. No protectionis neededforoutputvoltages of25V or less and
10µF capacitance. Figure 3 shows an LM338 with
protection diodes included for use with outputs
greaterthan 25V and high values of output capacitance.
Figure 3 : Regulator with Protection Diodes.
7/12
Page 8
LM138-LM238-LM338
10A REGULATOR
*
Minimum load – 100mA
VI≥ 10V
V0≥ 3V
VI–V0≥3.5V
5A CURR ENT REG UL AT O R
*
Minimum load – 100mA
VI≥ 10V
V0≥ 3V
VI–V0≥3.5V
8/12
Page 9
15A REGULATOR
LM138-LM238-LM338
*
Minimum load – 100mA
VI≥ +10V
V0≥+3V
VI–V0≥+4V
5V LO GI C REGULATO R WIT H ELECTRONIC SHUT DOW N **
*
R1 = 240Ωfor LM138or LM238
*
R2 = 720Ωfor LM138or LM238
**
Minimumoutput ≈+1.2V
9/12
Page 10
LM138-LM238-LM338
TRA CKI NG PR EREGUL AT O R
*
R1 = 240Ωfor LM138or LM238
*
R2 = 720Ωfor LM138or LM238
**
Minimumoutput ≈+1.2V
SLOW TURN-ON 15V REGULATOR
*
R1 = 240Ω
*
R2 = 2.7kΩ
} f or LM138 a nd LM238
10/12
Page 11
TO-3 MECHANICAL DATA
LM138-LM238-LM338
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
P
G
U
V
N
O
R
B
A
D
C
E
P003F
11/12
Page 12
LM138-LM238-LM338
Information furnished is believed to be accurate and reliable.However, SGS-THOMSON Microelectronics assumes no responsability for the
consequences of useof suchinformation nor for any infringement of patents or other rightsof third parties which mayresults from its use. No
license isgranted by implication or otherwise underany patent or patent rights of SGS-THOMSON Microelectronics. Specificationsmentioned
in this publication are subject to changewithout notice.This publication supersedes andreplaces all information previously supplied.
SGS-THOMSON Microelectronicsproductsare not authorized foruse ascritical componentsin life support devices or systemswithout express
written approval of SGS-THOMSON Microelectonics.
1994 SGS-THOMSON Microelectronics- All RightsReserved
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