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LT1083/LT1084/LT1085 Fixed

1083ffe

For more information www.linear.com/LT1083

applicaTions inForMaTion

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 negative resistance is

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

input-to-outputvoltageincreases.Thisnegativeresistance

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 1000µF 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 supply cycling or even plugging and unplugging

in the system will not generate currents large enough to

do any damage.

OUT

ADJ

VOUT

LT1083

1N4002

(OPTIONAL)

VIN

1083/4/5 AI01

COUT

150µF

Overload Recovery

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

area protection. The safe area protection decreases the

currentlimitasinput-to-outputvoltageincreasesandkeeps

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.

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 by-

pass capacitor from the adjust pin to ground will reduce

outputripplebytheratioofVOUT/1.25V.SeeLT1083series

adjustable regulator data sheet.

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