1
LT138A/LT338A
LM138/LM338
5A Positive Adjustable
Voltage Regulator
s
Guaranteed 1% Initial Tolerance
s
Guaranteed 0.3% Load Regulation
s
Guaranteed 5A Output Current
s
100% Thermal Limit Burn-In
s
12A Transient Output Current
The LT
138A series of adjustable regulators provide 5A
output current over an output voltage range of 1.2V to 32V.
The internal voltage reference is trimmed to less than 1%,
enabling a very tight output voltage. In addition to excel-
lent line and load regulation, with full overload protection,
the LT138A incorporates new current limiting circuitry
allowing large transient load currents to be handled for
short periods. Transient load currents of up to 12A can be
supplied without limiting, eliminating the need for a large
output capacitor.
The LT138A is an improved version of the popular LM138
with improved circuit design and advanced process tech-
niques to provide superior performance and reliability.
The graph below shows the significant improvement in
output voltage tolerance achieved by using the LT138A or
LT338A.
s
High Power Linear Regulator
s
Battery Chargers
s
Power Driver
s
Constant-Current Regulator
, LTC and LT are registered trademarks of Linear Technology Corporation.
Parallel Regulators for Higher Current*
LT338A
0.01
**
V
IN
V
IN
THIS CIRCUIT WILL NOT WORK WITH LM VERSION DEVICES
CURRENT SHARING RESISTORS DEGRADE REGULATION TO 1%
*
**
V
OUT
ADJ
LT350A
V
IN
138/338 TA01
121
1%
5V
8A
V
OUT
ADJ
0.016
**
365
1%
Output Voltage Error
OUTPUT VOLTAGE (V)
1
0
OUTPUT VOLTAGE ERROR (%)
3
5
7
10
10
100
138A/338A TA02
1
4
6
9
12
11
8
2
2% RESISTORS
2% RESISTORS
1% RESISTORS
1% RESISTORS
LM338
LM338
LT338A
LT338A
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
2
LT138A/LT338A
LM138/LM338
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
Power Dissipation .............................. Internally Limited
Input-to-Output Voltage Differential ........................ 35V
Operating Junction Temperature Range
LT138A/LM138 ............................... 55
C to 150
C
LT338A/LM338 ................................... 0
C to 125
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec)................. 300
C
PRECO
N
DITIO
N
I G
U
U
U
100% Thermal Limit Burn-In
(Note 1)
LT138A
LM138
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
MIN
TYP
MAX
UNITS
V
REF
Reference Voltage
I
OUT
= 10mA, T
J
= 25
C
1.238
1.250
1.262
V
3V
(V
IN
V
OUT
)
35V,
q
1.225
1.250
1.270
1.19
1.24
1.29
V
10mA
I
OUT
5A, P
50W
V
OUT
Line Regulation
3V
(V
IN
V
OUT
)
35V, (Note 3)
0.005
0.01
0.005
0.01
%/V
V
IN
q
0.02
0.04
0.02
0.04
%/V
V
OUT
Load Regulation
10mA
I
OUT
5A, (Note 3)
I
OUT
V
OUT
5V
5
15
5
15
mV
V
OUT
5V
0.1
0.3
0.1
0.3
%
V
OUT
5V
q
20
30
20
30
mV
V
OUT
5V
q
0.3
0.6
0.3
0.6
%
Thermal Regulation
20ms Pulse
0.002
0.01
0.002
0.01
%/W
Ripple Rejection
V
OUT
= 10V, f = 120Hz
C
ADJ
= 0
F
q
60
60
dB
C
ADJ
= 10
F
q
60
75
60
75
dB
I
ADJ
Adjust Pin Current
q
45
100
45
100
A
I
ADJ
Adjust Pin Current Change
10mA
I
OUT
5A,
q
0.2
5
0.2
5
A
3V
(V
IN
V
OUT
)
35V
Minimum Load Current
(V
IN
V
OUT
) = 35V
q
3.5
5
3.5
5
mA
I
SC
Current Limit
(V
IN
V
OUT
)
10V
DC
q
5
8
5
8
A
0.5ms Peak
q
6
12
6
12
A
(V
IN
V
OUT
) = 30V, T
J
= 25
C
1
2
1
A
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. (Note 2)
W
U
U
PACKAGE/ORDER I FOR ATIO
ORDER
PART NUMBER
LT138AK
LT338AK
LM138K
LM338K
T
JMAX
= 150
C,
JA
= 35
C/ W,
JC
= 1
C/ W (LT138A/LT138)
T
JMAX
= 125
C,
JA
= 35
C/ W,
JC
= 1
C/ W (LT338A/LT338)
Consult factory for Industrial grade parts.
ORDER
PART NUMBER
LM338P
V
OUT
V
IN
ADJ
3
2
1
FRONT VIEW
P PACKAGE
3-LEAD PLASTIC TO-3P
T
JMAX
= 125
C,
JA
= 45
C/ W
2
1
V
IN
CASE
IS OUTPUT
ADJ
K PACKAGE
2-LEAD TO-3 METAL CAN
BOTTOM VIEW
3
LT138A/LT338A
LM138/LM338
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. (Note 2)
LT138A
LM138
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
MIN
TYP
MAX
UNITS
V
OUT
Temperature Stability
q
1
2
1
%
Temp
V
OUT
Long-Term Stability
T
A
= 125
C, 1000 Hours
0.3
1
0.3
1
%
Time
e
n
RMS Output Noise (% of V
OUT
) 10Hz
f
10kHz
0.001
0.003
%
JC
Thermal Resistance
K Package
1
1
C/W
Junction-to-Case
The
q
denotes specifications which apply over the full operating temperature range,
otherwise specifications are at T
A
= 25
C. (Note 2)
LT338A
LM338
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
MIN
TYP
MAX
UNITS
V
REF
Reference Voltage
I
OUT
= 10mA
1.238
1.250
1.262
V
3V
(V
IN
V
OUT
)
35V,
q
1.225
1.250
1.270
1.19
1.24
1.29
V
10mA
I
OUT
5A, P
50W
V
OUT
Line Regulation
3V
(V
IN
V
OUT
)
35V, (Note 3)
0.005
0.01
0.005
0.03
%/V
V
IN
q
0.02
0.04
0.02
0.06
%/V
V
OUT
Load Regulation
10mA
I
OUT
5A, (Note 3)
I
OUT
V
OUT
5V
5
15
5
25
mV
V
OUT
5V
0.1
0.3
0.1
0.5
%
V
OUT
5V
q
20
30
20
50
mV
V
OUT
5V
q
0.3
0.6
0.3
1
%
Thermal Regulation
20ms Pulse
0.002
0.02
0.002
0.02
%/W
Ripple Rejection
V
OUT
= 10V, f = 120Hz
C
ADJ
= 0
F
q
60
60
dB
C
ADJ
= 10
F
q
60
75
60
75
dB
I
ADJ
Adjust Pin Current
q
45
100
45
100
A
I
ADJ
Adjust Pin Current Change
10mA
I
OUT
5A,
q
0.2
5
0.2
5
A
3V
(V
IN
V
OUT
)
35V
Minimum Load Current
(V
IN
V
OUT
) = 35V
q
3.5
10
3.5
10
mA
I
SC
Current Limit
(V
IN
V
OUT
)
10V
DC
q
5
8
5
8
A
0.5ms Peak
q
6
12
6
12
A
(V
IN
V
OUT
) = 30V, T
J
= 25
C
1
2
1
A
V
OUT
Temperature Stability
q
1
2
1
%
Temp
V
OUT
Long-Term Stability
T
A
= 125
C, 1000 Hours
0.3
1
0.3
1
%
Time
e
n
RMS Output Noise (% of V
OUT
)
10Hz
f
10kHz
0.001
0.003
%
JC
Thermal Resistance
K Package
1
1
C/W
Junction-to-Case
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: Unless otherwise specified, these specifications apply:
V
IN
V
OUT
= 5V and I
OUT
= 2.5A. These specifications are applicable for
power dissipations up to 50W.
Note 3: See thermal regulation specifications for changes in output voltage
due to heating effects. Load and line regulation are measured at a constant
junction temperature by low duty cycle pulse testing.
4
LT138A/LT338A
LM138/LM338
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
TEMPERATURE (
C)
75
0.4
OUTPUT VOLTAGE DEVIATION (%) 0.3
0.1
0
0.1
25
25
50
150
138/338 G01
0.2
50
0
75 100 125
0.2
I
OUT
= 5A
I
OUT
= 3A
V
IN
= 15V
V
OUT
= 10V
PRELOAD = 50mA
Load Regulation
Load Regulation
TEMPERATURE (
C)
50
1.230
REFERENCE VOLTAGE (V) 1.240
1.250
1.260
1.270
25
0
25
50
138A/338A G04
75
100 125 150
Temperature Stability
INPUT-OUTPUT DIFFERENTIAL (V)
0
QUIESCENT CURRENT (mA)
3
4
5
15
25
40
138A/338A G06
2
1
0
5
10
20
30
35
T
J
= 25
C
T
J
= 150
C
T
J
= 55
C
TEMPERATURE (
C)
75
ADJUSTMENT CURRENT (
A)
35
40
45
75
65
138A/338A G03
30
25
25
125
50
55
60
Adjustment Current
TEMPERATURE (
C)
75
1
INPUT-OUTPUT DIFFERENTIAL (V)
3
25
25
50
150
138/338 G02
2
50
0
75 100 125
4
I
OUT
= 5A
I
OUT
= 3A
I
OUT
= 1A
V
OUT
= 100mV
Dropout Voltage
FREQUENCY (Hz)
10
OUTPUT IMPEDANCE (
)
0.1
1
10
100k
138/338 G05
0.01
0.001
0.0001
100
1k
10k
1M
V
IN
= 15V
V
OUT
= 10V
I
OUT
= 500mA
C
ADJ
= C
OUT
= 10
F
C
ADJ
= C
OUT
= 0
F
Output Impedance
Minimum Operating Current
Ripple Rejection
Ripple Rejection
Ripple Rejection
OUTPUT VOLTAGE (V)
0
5
0
RIPPLE REJECTION (dB)
40
100
10
20
25
138A/338A G07
20
80
60
15
30
35
C
ADJ
= 10
F
C
ADJ
= 0
F
V
IN
V
OUT
= 5V
I
OUT
= 500mA
f = 120Hz
T
J
= 25
C
FREQUENCY (Hz)
10
RIPPLE REJECTION (dB)
60
80
100
100k
138A/338A G08
40
20
0
100
1k
10k
1M
C
ADJ
= 10
F
C
ADJ
= 0
F
V
IN
= 15V
V
OUT
= 10V
I
OUT
= 0.2A
OUTPUT CURRENT (A)
0.1
40
RIPPLE REJECTION (dB)
50
60
80
1
10
138/338 G09
70
C
ADJ
= 10
F
C
ADJ
= 0
F
V
IN
= 15V
V
OUT
= 10V
f = 120Hz
T
CASE
= 25
C
5
LT138A/LT338A
LM138/LM338
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Current Limit
Current Limit
Current Limit
INPUT-OUTPUT DIFFERENTIAL (V)
0
OUTPUT CURRENT (A)
8
12
40
138A/338A G10
4
0
10
20
30
16
PEAK CURRENT LIMIT
DC CURRENT LIMIT
T
CASE
= 25
C
PRELOAD = 0A
PRELOAD = 1A
PRELOAD = 5A
TIME (ms)
0.1
8
OUTPUT CURRENT (A)
10
12
14
1
10
100
138A/338A G11
6
4
2
0
PRELOAD = 0A
PRELOAD = 5A
V
IN
= 10V
V
OUT
= 5V
T
CASE
= 25
C
TIME (ms)
0.1
8
OUTPUT CURRENT (A)
10
12
14
1
10
100
138A/338A G12
6
4
2
0
PRELOAD CURRENT = 0
T
CASE
= 25
C
V
IN
V
OUT
= 10V
V
IN
V
OUT
= 15V
V
IN
V
OUT
= 20V
V
IN
V
OUT
= 30V
Line Transient Response
TIME (
s)
0
0
OUTPUT VOLTAGE
DEVIATION (V)
INPUT VOLTAGE
CHANGE (V)
0.5
1.5
1.0
0.5
20
30
1.5
138A/338A G13
1.0
10
40
0
0.5
1.0
C
L
= 1
F
C
L
= 10
F
C
OUT
= 0
C
ADJ
= 0
V
OUT
= 10V
I
OUT
= 50mA
T
J
= 25
C
Load Transient Response
TIME (
s)
0
OUTPUT VOLTAGE
DEVIATION (V)
LOAD CURRENT (A)
2
6
3
2
3
0
20
40
138A/338A G14
4
1
2
1
0
10
30
V
IN
= 15V
V
OUT
= 10V
T
CASE
= 25
C
PRELOAD = 100mA
C
L
= 1
F
C
ADJ
= 10
F
C
L
= 0
C
ADJ
= 0
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
General
The LT138A develops a 1.25V reference voltage between
the output and the adjustable terminal (see Figure 1). By
placing a resistor, R1, between these two terminals, a con-
stant current is caused to flow through R1 and down through
R2 to set the overall output voltage. Normally this current
is the specified minimum load current of 5mA or 10mA.
Because I
ADJ
is very small and constant when compared
with the current through R1, it represents a small error and
can usually be ignored. It is easily seen from the output
voltage equation, that even if the resistors were of exact
value, the accuracy of the output is limited by the accuracy
of V
REF
. Earlier adjustable regulators had a reference
tolerance of
4% which is dangerously close to the
5%
supply tolerance required in many logic and analog sys-
tems. Further, even 1% resistors can drift 0.01%/
C, add-
ing additional error to the output voltage tolerance.
Figure 1. Basic Adjustable Regulator
LT338A
V
IN
R1
138A/338A F01
V
OUT
I
ADJ
50
A
ADJ
V
REF
V
OUT
V
IN
R2
V
OUT
= V
REF
R2
R1
1 +
+ I
ADJ
R2
( )
+
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