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
LINER [ LINEAR TECHNOLOGY ]
