LT1084_15 Datasheet, PDF (11/22 Pages) Linear Technology – 3A, 5A, 7.5A Low Dropout Positive Fixed Regulators

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 volt-
age 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 re-
quired protection diodes between the adjustment pin and
the output and from the output to the input to prevent
overstressing 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 cur-
rents of 50A to 100A. Even with large output capacitances,
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 deg-
radation. Of course, as with any IC regulator, exceeding
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)
V_IN
IN LT1083 OUT
V_OUT
ADJ
R1 ^{+ C}OUT
150µF
+ _CADJ
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.
For more information www.linear.com/LT1083
108345fh
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