LT1083/LT1084/LT1085 Fixed
BLOCK DIAGRA
APPLICATIO S I FOR ATIO
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
150μF 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-
U
W
W
THERMAL
LIMIT
V
IN
+
–
V
OUT
GND
LT1083/4/5 BD
U U
teristics are available. These capacitors differ in capacitor
tolerance (sometimes ranging up to
±100%),
equivalent
series resistance, and capacitance temperature coeffi-
cient. The 150μF or 22μF values given will ensure stability.
Normally, capacitor values on the order of 100μF 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
monolithic 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 resistance 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
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LINEAR [ LINEAR INTEGRATED SYSTEMS ]
