LT1083/LT1084/LT1085
APPLICATIONS INFORMATION
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 capacitor 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 negative
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 the negative resistance
is a function of how fast the current limit is folded back as
input-to-output voltage increases. This negative resis-
tance can react with capacitors or inductors on the input
to cause oscillation 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. Older adjustable regulators required
protection diodes between the adjustment pin and the
output and from the output to the input to prevent over-
stressing the die. The internal current paths on the LT1083
adjustment pin are limited by internal resistors. Therefore,
even with capacitors on the adjustment pin, no protection
diode is needed to ensure device safety under short-circuit
conditions.
Diodes between input and output are usually not needed.
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 capaci-
tances, it is very difficult to get those values of surge
currents in normal operations. Only with a high value of
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 current large enough to do any damage.
The adjustment pin can be driven on a transient basis
±25V,
with respect to the output without any device
degradation. Of course, as with any IC regulator, exceed-
ing the maximum input to output voltage differential
causes the internal transistors to break down and none of
the protection circuitry is functional.
D1
1N4002
(OPTIONAL)
8
U
W
U
U
V
IN
IN
LT1083
ADJ
OUT
R1
+
V
OUT
C
OUT
150µF
+
C
ADJ
10µF
R2
1083/4/5 ADJ F00
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
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.
108345fe
LINER [ LINEAR TECHNOLOGY ]
