LT138A/LT339A

LM138/LM338

6

138afd

For more information www.linear.com/LT138A

Typical perForMance characTerisTics

Line Transient Response

Load Transient Response

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

constant 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

comparedwiththecurrentthroughR1,itrepresentsasmall

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 ac-

tolerance of ± 4% which is dangerously close to the

±5% supply tolerance required in many logic and analog

systems. Further, even 1% resistors can drift 0.01%/°C,

adding additional error to the output voltage tolerance.

For example, using 2% resistors and ±4% tolerance for

approximately ±7%. If the same example were used for a

15V regulator, the expected tolerance would be ±8%. With

these results most applications required some method of

trimming,usuallyatrimpot.Thissolutionisbothexpensive

and not conductive to volume production.

One of the enhancements of Linear Technology’s adjust-

able regulators over existing devices is the tightened initial

or 2% film resistors to be used for R1 and R2 to set the

output voltage within an acceptable tolerance.

With a guaranteed 1% reference, a 5V power supply

design, using ±2% resistors, would have a worst-case

manufacturing tolerance of ±4%. If 1% resistors are used,

the tolerance will drop to ±2.5%. A plot of the worst-case

output voltage tolerance as a function of resistor tolerance

is shown on the front page of this data sheet.

For convenience, a table of standard 1% resistor values

is shown in Table 1.

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

TIME (μs)

0

2

6

–3

–2

3

0

20

40

138A/338A G14

4

1

2

–1

0

10

30

PRELOAD = 100mA

Figure 1. Basic Adjustable Regulator

LT338A

R1

138A/338A F01

50μA

ADJ

R2

R2

R1

1 +

( )

+