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LT1083/84/85 Fixed

The LT1083 family of three-terminal regulators are easy to

use and have all the protection features that are expected

in high performance voltage regulators. They are short

circuit protected, have safe area protection as well as

thermal shutdown to turn off the regulator should the

temperature exceed about 165

°C.

These regulators offer lower dropout voltage and more

precise reference tolerance. Further, the reference stabil-

ity with temperature is improved over older types of

regulators. The only circuit difference between using the

LT1083 family and older regulators is that they require an

output capacitor for stability.

Stability

The circuit design used in the LT1083 family requires the

use of an output capacitor as part of the device frequency

compensation. For all operating conditions, the addition of

150

µF aluminum electrolytic or a 22µF solid tantalum on

the output will ensure stability. Normally capacitors much

smaller than this can be used with the LT1083. Many

different types of capacitors with widely varying charac-

teristics are available. These capacitors differ in capacitor

tolerance (sometimes ranging up to

±100%), equivalent

series resistance, and capacitance temperature coeffi-

cient. The 150

µF or 22µF values given will ensure stability.

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 capacitance 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 negitive resis-

tance since increasing voltage causes current to decrease.

Negitive 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-

THERMAL

LIMIT

GND

LT1083/4/5 BD

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