5

LT138A/LT338A

LM138/LM338

TYPICAL PERFOR A CE CHARACTERISTICS

Current Limit

Current Limit

Current Limit

INPUT-OUTPUT DIFFERENTIAL (V)

0

8

12

40

138A/338A G10

4

0

10

20

30

16

PEAK CURRENT LIMIT

DC CURRENT LIMIT

PRELOAD = 0A

PRELOAD = 1A

PRELOAD = 5A

TIME (ms)

0.1

8

10

12

14

1

10

100

138A/338A G11

6

4

2

0

PRELOAD = 0A

PRELOAD = 5A

TIME (ms)

0.1

8

10

12

14

1

10

100

138A/338A G12

6

4

2

0

PRELOAD CURRENT = 0

Line Transient Response

TIME (

µs)

0

0

0.5

–1.5

–1.0

– 0.5

20

30

1.5

138A/338A G13

1.0

10

40

0

0.5

1.0

Load Transient Response

TIME (

µs)

0

2

6

–3

–2

3

0

20

40

138A/338A G14

4

1

2

–1

010

30

PRELOAD = 100mA

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.

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

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

R1

138A/338A F01

50

µA

ADJ

R2

R2

R1

1 +

()

+