8

LT1083/LT1084/LT1085

108345fe

input pin instantaneously shorted to ground, can damage

occur. A crowbar circuit at the input of the LT1083 can

generate those kinds of currents, and a diode from output

to input is then recommended. Normal power supply

cycling or even plugging and unplugging in the system will

not generate current large enough to do any damage.

The adjustment pin can be driven on a transient basis

±25V, with respect to the output without any device

degradation. Of course, as with any IC regulator, exceed-

ing the maximum input to output voltage differential

causes the internal transistors to break down and none of

the protection circuitry is functional.

Normally, capacitor values on the order of 100

µF are used

in the output of many regulators to ensure good transient

response with heavy load current changes. Output capaci-

tance can be increased without limit and larger values of

output capacitor further improve stability and transient

response of the LT1083 regulators.

Another possible stability problem that can occur in mono-

lithic IC regulators is current limit oscillations. These can

occur because, in current limit, the safe area protection

exhibits a negative impedance. The safe area protection

decreases the current limit as the input-to-output voltage

increases. That is the equivalent of having a negative

resistance since increasing voltage causes current to

decrease. Negative resistance during current limit is not

unique to the LT1083 series and has been present on all

power IC regulators. The value of the negative resistance

is a function of how fast the current limit is folded back as

input-to-output voltage increases. This negative resis-

tance can react with capacitors or inductors on the input

to cause oscillation during current limiting. Depending on

the value of series resistance, the overall circuitry may end

up unstable. Since this is a system problem, it is not

necessarily easy to solve; however, it does not cause any

problems with the IC regulator and can usually be ignored.

Protection Diodes

In normal operation, the LT1083 family does not need any

protection diodes. Older adjustable regulators required

protection diodes between the adjustment pin and the

output and from the output to the input to prevent over-

stressing the die. The internal current paths on the LT1083

adjustment pin are limited by internal resistors. Therefore,

even with capacitors on the adjustment pin, no protection

diode is needed to ensure device safety under short-circuit

conditions.

Diodes between input and output are usually not needed.

The internal diode between the input and the output pins

of the LT1083 family can handle microsecond surge

currents of 50A to 100A. Even with large output capaci-

tances, it is very difficult to get those values of surge

currents in normal operations. Only with a high value of

output capacitors, such as 1000

µF to 5000µF and with the

R1

R2

IN

OUT

ADJ

LT1083

D1

1N4002

(OPTIONAL)

1083/4/5 ADJ F00

150

µF

+

10

µF

+

Overload Recovery

Like any of the IC power regulators, the LT1083 has safe

area protection. The safe area protection decreases the

current limit as input-to-output voltage increases and

keeps the power transistor inside a safe operating region

for all values of input-to-output voltage. The LT1083

protection is designed to provide some output current at

all values of input-to-output voltage up to the device

breakdown.

When power is first turned on, as the input voltage rises,

the output follows the input, allowing the regulator to start

up into very heavy loads. During the start-up, as the input

voltage is rising, the input-to-output voltage differential

remains small, allowing the regulator to supply large

output currents. With high input voltage, a problem can

occur wherein removal of an output short will not allow the

output voltage to recover. Older regulators, such as the

7800 series, also exhibited this phenomenon, so it is not

unique to the LT1083.