LT1038

5

APPLICATIO S I FOR ATIO

General

The LT1038 develops a 1.25V reference voltage between

the output and the adjustment 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 10mA or

compared with the current through R1, it represents a

small error and can usually be ignored.

Bypass Capacitors

Input bypassing using a 1

µF tantalum or 25µF electrolytic

is recommended when the input filter capacitors are more

than 5 inches from the device. Improved ripple rejection

(80dB) can be accomplished by adding a 10

µF capacitor

from the ADJ pin to ground. Increasing the size of the

capacitor to 20

µF will help ripple rejection at low output

voltage since the reactance of this capacitor should be

small compared to the voltage setting resistor, R2. For

improved AC transient response and to prevent the possi-

bility of oscillation due to unknown reactive load, a 1

µF

capacitor is also recommended at the output. Because of

their low impedance at high frequencies, the best type of

capacitor to use is solid tantalum.

Protection Diodes

The LT1038 does not require a protection diode from the

adjustment terminal to the output (see Figure 2). Im-

proved internal circuitry eliminates the need for this diode

when the adjustment pin is bypassed with a capacitor to

improve ripple rejection.

If a very large output capacitor is used, such as a 100

µF

shown in Figure 2, the regulator could be damaged or

destroyed if the input is accidentally shorted to ground or

crowbarred, due to the output capacitor discharging into

the output terminal of the regulator. To prevent this, a

diode, D1 as shown, is recommended to safely discharge

the capacitor.

Load Regulation

Because the LT1038 is a three-terminal device, it is not

possible to provide true remote load sensing. Load regu-

lation will be limited by the resistance of the wire connect-

ing the regulator to the load. The data sheet specification

for load regulation is measured at the bottom of the

package. Negative side sensing can be a true Kelvin

connection if the bottom of resistor R2 is returned to the

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:

R

RR

R

21

1

+









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

important to keep the lead between the regulator and the

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

traces.

Figure 1. Basic Adjustable Regulator

Figure 2

Figure 3. Connections for

Best Load Regulation

LT1038

ADJ

R1

R2

50

µA

R2

R1

1038 F01

LT1038

ADJ

+

+

R1

R2

1038 F02

10

µF

100

µF

*NOT NEEDED

*

D1

1N4002

R1

R2

LT1038

ADJ

1038 F03

CONNECT R2

TO LOAD

CONNECT

R1 TO CASE

PARASITIC

LINE RESISTANCE