6

LT1117/LT1117-2.85

LT1117-3.3/LT1117-5

The adjust pin can be driven, on a transient basis,

±25V

with respect to the output without any device degradation.

Diodes between input and output are not usually needed.

The internal diode between the output and input pins of the

device can withstand microsecond surge currents of 10A

to 20A. Normal power supply cycling can not generate

currents of this magnitude. Only with extremely large

output capacitors, such as 1000

µFandlarger,andwiththe

input pin instantaneously shorted to ground can damage

occur. A crowbar circuit at the input of the LT1117 in

combination with a large output capacitor could generate

currents large enough to cause damage. In this case a

diode from output to input is recommended, as shown in

Figure 1.

to set the overall output voltage. Normally this current is

chosen to be the specified minimum load current of 10mA.

to the current through R1, it represents a small error and

can usually be ignored. For fixed voltage devices R1 and

R2 are included in the device.

Load Regulation

Because the LT1117 is a 3-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 output pin of the

device. Negative side sensing is a true Kelvin connection,

with the bottom of the output divider 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 returned directly to the

output pin of the device, not to the load. This is illustrated

the divider ratio. If R1 were connected to the load, the

effective resistance between the regulator and the load

would be:

R

RR

R

R

PP

×

+

=

21

1

,

Parasitic Line Resistance

Figure 2. Basic Adjustable Regulator

Output Voltage

The LT1117 develops a 1.25V reference voltage between

the output and the adjust terminal (see Figure 2). By

placing a resistor between these two terminals, a constant

current is caused to flow through R1 and down through R2

Figure 1

LT1117 • TA02

+

+

D1

1N4002

(OPTIONAL)

10

µF

R2

R1

150

µF

LT1117

IN

ADJ

OUT

LT1117 • TA03

R2

R1

50

µA

LT1117

IN

ADJ

OUT

R2

—

R1

()

+

Figure 3. Connections for Best Load Regulation

LT1117 • TA04

R2

R1

PARASITIC

LINE RESISTANCE

CONNECT

R1 TO CASE

CONNECT

R2 TO LOAD

LT1117

IN

ADJ

OUT

For fixed voltage devices the top of R1 is internally Kelvin

connected, and the ground pin can be used for negative

side sensing.

APPLICATIO HI TS