LT1117/LT1117-2.85

LT1117-3.3/LT1117-5

7

1117fd

APPLICATIONS INFORMATION

The LT1117 family of 3-terminal regulators are easy to

use. They are protected against short circuit and thermal

overloads. Thermal protection circuitry will shut down the

regulator should the junction temperature exceed 165°C

at the sense point. These regulators are pin compatible

with older 3-terminal adjustable regulators, offer lower

dropout voltage and more precise reference tolerance.

Reference stability over temperature is improved over

older types of regulators.

Stability

The LT1117 family of regulators requires an output

capacitor as part of the device frequency compensation.

A minimum of 10μF of tantalum or 50μF of aluminum

electrolytic is required. The ESR of the output capacitor

should be less than 0.5Ω. Surface mount tantalum

capacitors, which have very low ESR, are available from

several manufacturers.

When using the LT1117 adjustable device, the adjust

terminal can be bypassed to improve ripple rejection.

When the adjust terminal is bypassed, the required value

of the output capacitor increases. The device will require

an output capacitor of 22μF tantalum or 150μF aluminum

electrolytic when the adjust pin is bypassed.

Normally, capacitor values on the order of 100μF are

used in the output of many regulators to ensure good

load transient response with large load current changes.

Output capacitance can be increased without limit and

larger values of output capacitance further improve stability

and transient response.

Protection Diodes

In normal operation, the LT1117 family does not need any

protection diodes. Older adjustable regulators required

protection diodes between the adjust pin and the output

and between the output and input to prevent over stressing

the die. The internal current paths on the LT1117 adjust

pin are limited by internal resistors. Therefore, even with

capacitors on the adjust pin, no protection diode is needed

to ensure device safety under short-circuit conditions.

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 1000μF and larger, and with the 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.

1117 F01

+

+

D1

1N4002

(OPTIONAL)

10μF

R2

R1

150μF

LT1117

IN

ADJ

OUT

Figure 1