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LT138AK

5A Positive Adjustable Voltage Regulator

Linear Technology 

APPLICATIONS INFORMATION

negative side of the load. Although it may not be immedi-

ately obvious, best load regulation is obtained when the

top of the resistor divider, R1, is connected directly to the

case not to the load. This is illustrated in Figure 3. If R1

were connected to the load, the effective resistance be-

tween the regulator and the load would be:

RP





R2 + R1

R1 

,

RP

=

Parasitic

Line

Re sistance

Connected as shown, RP is not multiplied by the divider

ratio. RP is about 0.004Ω per foot using 16 gauge wire.

This translates to 4mV/ft at 1A load current, so it is

important to keep the positive lead between regulator and

load as short as possible, and use large wire or PC board

traces.

LT138A/LT338A

LM138/LM338

LT338A

RP

PARASITIC

LINE RESISTANCE

VIN

VIN

VOUT

ADJ

CONNECT

R1 TO CASE

R1

RL

R2

CONNECT

R2 TO LOAD

138A/338A F03

Figure 3. Connections for Best Load Regulation

TYPICAL APPLICATIONS

Improving Ripple Rejection

LT338A

VIN

+

VIN

1µF

VOUT

ADJ

R1

121Ω

1%

R2 +

365Ω

1%

5V

CL*

10µF

*C1 IMPROVES RIPPLE REJECTION, XC

SHOULD BE SMALL COMPARED TO R2

138A/338A TA03

1.2V to 25V Adjustable Regulator

LT338A

VIN

VIN

VOUT

ADJ

+

C1*

R2

1µF

5k

R1

240Ω

+

VOUT†

C2**

1µF

138A/338A TA04

*NEEDED IF DEVICE IS FAR FROM FILTER CAPACITORS

**OPTIONAL, IMPROVES TRANSIENT RESPONSE

( ) †VOUT = 1.25V

1 + R2

R1

138afb

7

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