LT138A/LT338A
LM138/LM338
5A Positive Adjustable
Voltage Regulator
FEATURES
s
s
s
s
s
DESCRIPTIO
Guaranteed
1% Initial Tolerance
Guaranteed
0.3% Load Regulation
Guaranteed
5A Output Current
100% Thermal Limit Burn-In
12A Transient Output Current
APPLICATIO S
s
s
s
s
High Power Linear Regulator
Battery Chargers
Power Driver
Constant-Current Regulator
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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
Parallel Regulators for Higher Current*
12
LT338A
V
IN
V
IN
ADJ
V
OUT
0.01Ω**
OUTPUT VOLTAGE ERROR (%)
5V
8A
11
10
9
8
7
6
5
4
3
2
1
0
1% RESISTORS
2% RESISTORS
LT338A
2% RESISTORS
1% RESISTORS
LM338
LM338
LT338A
LT350A
V
IN
ADJ
V
OUT
0.016Ω**
121Ω
1%
365Ω
1%
138/338 TA01
1
*THIS CIRCUIT WILL NOT WORK WITH LM VERSION DEVICES
**CURRENT SHARING RESISTORS DEGRADE REGULATION TO 1%
U
Output Voltage Error
10
OUTPUT VOLTAGE (V)
100
138A/338A TA02
U
U
1
LT138A/LT338A
LM138/LM338
ABSOLUTE
AXI U
RATI GS
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
PRECONDITIONI G
100% Thermal Limit Burn-In
PACKAGE/ORDER I FOR ATIO
BOTTOM VIEW
V
IN
2
CASE
IS OUTPUT
1
ADJ
K PACKAGE
2-LEAD TO-3 METAL CAN
T
JMAX
= 150°C,
θ
JA
= 35°C/ W,
θ
JC
= 1°C/ W (LT138A/LT138)
T
JMAX
= 125°C,
θ
JA
= 35°C/ W,
θ
JC
= 1°C/ W (LT338A/LT338)
ORDER
PART NUMBER
LT138AK
LT338AK
LM138K
LM338K
FRONT VIEW
3
2
1
P PACKAGE
3-LEAD PLASTIC TO-3P
T
JMAX
= 125°C,
θ
JA
= 45°C/ W
V
OUT
V
IN
ADJ
Consult factory for Industrial grade parts.
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 2)
SYMBOL
V
REF
PARAMETER
Reference Voltage
CONDITIONS
I
OUT
= 10mA, T
J
= 25°C
3V
≤
(V
IN
– V
OUT
)
≤
35V,
10mA
≤
I
OUT
≤
5A, P
≤
50W
∆V
OUT
∆V
IN
∆V
OUT
∆I
OUT
Line Regulation
Load Regulation
3V
≤
(V
IN
– V
OUT
)
≤
35V, (Note 3)
q
q
ELECTRICAL CHARACTERISTICS
MIN
1.238
1.225
LT138A
TYP
MAX
1.250
1.250
0.005
0.02
5
0.1
1.262
1.270
0.01
0.04
15
0.3
30
0.6
0.01
10mA
≤
I
OUT
≤
5A, (Note 3)
V
OUT
≤
5V
V
OUT
≥
5V
V
OUT
≤
5V
V
OUT
≥
5V
q
q
20
0.3
0.002
Thermal Regulation
Ripple Rejection
20ms Pulse
V
OUT
= 10V, f = 120Hz
C
ADJ
= 0µF
C
ADJ
= 10µF
10mA
≤
I
OUT
≤
5A,
3V
≤
(V
IN
– V
OUT
)
≤
35V
(V
IN
– V
OUT
) = 35V
(V
IN
– V
OUT
)
≤
10V
DC
0.5ms Peak
(V
IN
– V
OUT
) = 30V, T
J
= 25°C
q
q
q
q
q
q
q
60
60
75
45
0.2
3.5
100
5
5
I
ADJ
∆I
ADJ
Adjust Pin Current
Adjust Pin Current Change
Minimum Load Current
I
SC
Current Limit
5
6
8
12
1
2
2
UU
U
U
U
W
W W
U
W
(Note 1)
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER
PART NUMBER
LM338P
MIN
1.19
LM138
TYP
1.24
0.005
0.02
5
0.1
20
0.3
0.002
60
75
45
0.2
3.5
MAX
1.29
0.01
0.04
15
0.3
30
0.6
0.01
UNITS
V
V
%/V
%/V
mV
%
mV
%
%/W
dB
dB
60
100
5
5
µA
µA
mA
A
A
A
5
6
8
12
1
LT138A/LT338A
LM138/LM338
ELECTRICAL CHARACTERISTICS
SYMBOL
∆V
OUT
∆Temp
∆V
OUT
∆Time
e
n
θ
JC
PARAMETER
Temperature Stability
Long-Term Stability
T
A
= 125°C, 1000 Hours
CONDITIONS
q
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 2)
MIN
LT138A
TYP
MAX
1
0.3
0.001
1
2
1
MIN
LM138
TYP
1
0.3
0.003
1
1
MAX
UNITS
%
%
%
°C/W
RMS Output Noise (% of V
OUT
) 10Hz
≤
f
≤
10kHz
Thermal Resistance
Junction-to-Case
K Package
The
q
denotes specifications which apply over the full operating temperature range,
otherwise specifications are at T
A
= 25°C. (Note 2)
SYMBOL
V
REF
PARAMETER
Reference Voltage
CONDITIONS
I
OUT
= 10mA
3V
≤
(V
IN
– V
OUT
)
≤
35V,
10mA
≤
I
OUT
≤
5A, P
≤
50W
∆V
OUT
∆V
IN
∆V
OUT
∆I
OUT
Line Regulation
Load Regulation
3V
≤
(V
IN
– V
OUT
)
≤
35V, (Note 3)
q
q
MIN
1.238
1.225
LT338A
TYP
MAX
1.250
1.250
0.005
0.02
5
0.1
1.262
1.270
0.01
0.04
15
0.3
30
0.6
0.02
MIN
1.19
LM338
TYP
1.24
0.005
0.02
5
0.1
20
0.3
0.002
60
75
45
0.2
3.5
MAX
1.29
0.03
0.06
25
0.5
50
1
0.02
UNITS
V
V
%/V
%/V
mV
%
mV
%
%/W
dB
dB
10mA
≤
I
OUT
≤
5A, (Note 3)
V
OUT
≤
5V
V
OUT
≥
5V
V
OUT
≤
5V
V
OUT
≥
5V
q
q
20
0.3
0.002
Thermal Regulation
Ripple Rejection
20ms Pulse
V
OUT
= 10V, f = 120Hz
C
ADJ
= 0µF
C
ADJ
= 10µF
10mA
≤
I
OUT
≤
5A,
3V
≤
(V
IN
– V
OUT
)
≤
35V
(V
IN
– V
OUT
) = 35V
(V
IN
– V
OUT
)
≤
10V
DC
0.5ms Peak
(V
IN
– V
OUT
) = 30V, T
J
= 25°C
q
q
q
q
q
q
q
q
60
60
75
45
0.2
3.5
100
5
10
60
I
ADJ
∆I
ADJ
Adjust Pin Current
Adjust Pin Current Change
Minimum Load Current
100
5
10
µA
µA
mA
A
A
A
%
I
SC
Current Limit
5
6
8
12
1
1
0.3
0.001
1
2
2
1
5
6
8
12
1
1
0.3
0.003
1
1
∆V
OUT
∆Temp
∆V
OUT
∆Time
e
n
θ
JC
Temperature Stability
Long-Term Stability
RMS Output Noise (% of V
OUT
)
Thermal Resistance
Junction-to-Case
T
A
= 125°C, 1000 Hours
10Hz
≤
f
≤
10kHz
K Package
%
%
°C/W
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.
3
LT138A/LT338A
LM138/LM338
TYPICAL PERFOR A CE CHARACTERISTICS
Load Regulation
0.2
V
IN
= 15V
V
OUT
= 10V
PRELOAD = 50mA
I
OUT
= 3A
I
OUT
= 5A
4
OUTPUT VOLTAGE DEVIATION (%)
ADJUSTMENT CURRENT (µA)
0.1
0
– 0.1
– 0.2
– 0.3
INPUT-OUTPUT DIFFERENTIAL (V)
– 0.4
–75 – 50 – 25
0 25 50 75 100 125 150
TEMPERATURE (°C)
138/338 G01
Temperature Stability
1.270
10
OUTPUT IMPEDANCE (Ω)
1.260
QUIESCENT CURRENT (mA)
REFERENCE VOLTAGE (V)
1.250
1.240
1.230
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
138A/338A G04
Ripple Rejection
100
C
ADJ
= 10µF
80
RIPPLE REJECTION (dB)
RIPPLE REJECTION (dB)
C
ADJ
= 0µF
60
60
C
ADJ
= 0µF
40
RIPPLE REJECTION (dB)
40
V
IN
– V
OUT
= 5V
I
OUT
= 500mA
f = 120Hz
T
J
= 25°C
0
5
20
15
10
25
OUTPUT VOLTAGE (V)
30
35
20
0
4
U W
138A/338A G07
Dropout Voltage
∆V
OUT
= 100mV
65
60
55
50
45
40
35
1
–75 – 50 – 25
Adjustment Current
3
I
OUT
= 5A
I
OUT
= 3A
2
I
OUT
= 1A
0 25 50 75 100 125 150
TEMPERATURE (°C)
138/338 G02
30
–75
–25
25
75
TEMPERATURE (°C)
125
138A/338A G03
Output Impedance
V
IN
= 15V
V
OUT
= 10V
I
OUT
= 500mA
C
ADJ
= C
OUT
= 0µF
5
Minimum Operating Current
1
4
T
J
= 150°C
T
J
= 25°C
3
T
J
= – 55°C
0.1
0.01
C
ADJ
= C
OUT
= 10µF
2
0.001
1
0.0001
0
10
100
10k
1k
FREQUENCY (Hz)
100k
1M
0
25 30 35
5
10 15 20
INPUT-OUTPUT DIFFERENTIAL (V)
40
138/338 G05
138A/338A G06
Ripple Rejection
100
80
Ripple Rejection
C
ADJ
= 10µF
80
C
ADJ
= 10µF
70
C
ADJ
= 0µF
60
50
20
0
V
IN
= 15V
V
OUT
= 10V
I
OUT
= 0.2A
10
100
10k
1k
FREQUENCY (Hz)
100k
1M
40
V
IN
= 15V
V
OUT
= 10V
f = 120Hz
T
CASE
= 25°C
0.1
1
OUTPUT CURRENT (A)
10
138/338 G09
138A/338A G08
LT138A/LT338A
LM138/LM338
TYPICAL PERFOR A CE CHARACTERISTICS
Current Limit
16
PEAK CURRENT LIMIT
DC CURRENT LIMIT
T
CASE
= 25°C
OUTPUT CURRENT (A)
14
12
PRELOAD = 0A
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
12
PRELOAD = 0A
PRELOAD = 5A
8
PRELOAD = 1A
4
0
0
20
10
30
INPUT-OUTPUT DIFFERENTIAL (V)
Line Transient Response
1.5
OUTPUT VOLTAGE
DEVIATION (V)
OUTPUT VOLTAGE
DEVIATION (V)
1.0
0.5
0
– 0.5
–1.0
–1.5
V
OUT
= 10V
I
OUT
= 50mA
T
J
= 25°C
LOAD CURRENT (A)
INPUT VOLTAGE
CHANGE (V)
1.0
0.5
0
0
10
20
TIME (µs)
30
40
APPLICATIONS INFORMATION
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.
LT338A
V
IN
V
IN
ADJ
V
OUT
V
REF
R1
V
OUT
U
W
U W
138A/338A G10
Current Limit
14
12
10
Current Limit
PRELOAD CURRENT = 0
T
CASE
= 25°C
10
PRELOAD = 5A
8
6
4
2
V
IN
= 10V
V
OUT
= 5V
T
CASE
= 25°C
1
TIME (ms)
138A/338A G11
V
IN
– V
OUT
= 10V
8
6
4
2
0
0.1
V
IN
– V
OUT
= 15V
V
IN
– V
OUT
= 20V
V
IN
– V
OUT
= 30V
40
0
0.1
10
100
1
TIME (ms)
10
100
138A/338A G12
Load Transient Response
3
2
1
0
–1
–2
–3
6
4
2
0
0
10
20
TIME (µs)
30
40
V
IN
= 15V
V
OUT
= 10V
T
CASE
= 25°C
PRELOAD = 100mA
C
L
= 1µF
C
ADJ
= 10µF
C
L
= 0
C
ADJ
= 0
C
L
= 1µF
C
L
= 10µF
C
OUT
= 0
C
ADJ
= 0
138A/338A G13
138A/338A G14
U
U
+
I
ADJ
50µA
R2
V
OUT
= V
REF
1 + R2 + I
ADJ
• R2
R1
( )
138A/338A F01
Figure 1. Basic Adjustable Regulator
5
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