8

LT1083/LT1084/LT1085 Fixed

1083ffd

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.

The problem occurs with a heavy output load when the

input voltage is high and the output voltage is low, such as

immediately after a removal of a short. The load line for

such a load may intersect the output current curve at two

points. If this happens there are two stable output operat-

ing points for the regulator. With this double intersection

the power supply may need to be cycled down to zero and

brought up again to make the output recover.

Ripple Rejection

In applications that require improved ripple rejection the

LT1083 series adjustable regulators should be used. With

LT1083 series adjustable regulators the addition of a

bypass capacitor from the adjust pin to ground will reduce

series adjustable regulator data sheet.

Load Regulation

Because the LT1083 is a three-terminal device, it is not

possible to provide true remote load sensing. Load regu-

lation will not be limited by the resistance of the wire

connecting the regulator to the load. The data sheet

specification for the load regulation is measured at the

bottom of the package. Negative side sensing is a true

Kelvin connection, with the ground pin of the device

returned to the negative side of the load.

Thermal Considerations

The LT1083 series of regulators have internal power and

thermal limiting circuitry designed to protect the device

under overload conditions. For continuous normal load

conditions however, maximum junction temperature rat-

ings must not be exceeded. It is important to give careful

consideration to all sources of thermal resistance from

junction to ambient. This includes junction-to-case,

folded back as input-to-output voltage increases. This

negative resistance can react with capacitors or inductors

on the input to cause oscillations 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, 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 capacitances it is very difficult to get those

values of surge current in normal operation. Only with high

value output capacitors, such as 1000

μF to 5000μF and

with the 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 sup-

ply cycling or even plugging and unplugging in the system

will not generate currents large enough to do any damage.

IN

OUT

ADJ

LT1083

D1

1N4002

(OPTIONAL)

1083/4/5 AI01

150

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

APPLICATIO S I FOR ATIO