LT1083/84/85 Fixed
BLOCK DIAGRAM
APPLICATIONS INFORMATION
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-
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 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-
U
W
W
V
IN
+
–
THERMAL
LIMIT
GND
LT1083/4/5 BD
U
U
7
LINER [ LINEAR TECHNOLOGY ]
