LT1083/LT1084/LT1085 Fixed
10
1083ffe
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applicaTions inForMaTion
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 negative resistance is
a function of how fast the current limit is folded back as
input-to-
output voltage increases. This negative resistance
can react with capacitors or inductors on the input to
cause oscillations 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, 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 capacitances it is very difficult to get those
values of surge current in normal operation. Only with
high value 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 currents large enough to
do any damage.
IN OUT
ADJ
V
OUT
LT1083
D1
1N4002
(OPTIONAL)
V
IN
1083/4/5 AI01
C
OUT
150µF
+
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.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low, such
as immediately after a removal of a short. The load line for
such a load may intersect the output current curve at two
points. If this happens there are two stable output operat
-
ing points for the regulator. With this double intersection
the power supply may need to be cycled down to zero and
brought up again to make the output recover.
Ripple Rejection
In applications that require improved ripple rejection the
LT
1083 series adjustable regulators should be used. With
LT1083
series adjustable regulators the addition of a by
-
pass capacitor from the adjust pin to ground will reduce
output ripple by the ratio of V
OUT
/1.25V. See LT1083 series
adjustable regulator data sheet.
