LT1963A Series
applicaTions inForMaTion
The output of the LT1963A can be pulled below ground
withoutdamagingthedevice.Iftheinputisleftopencircuit
or grounded, the output can be pulled below ground by
20V. For fixed voltage versions, the output will act like a
large resistor, typically 5k or higher, limiting current flow
to typically less than 600µA. For adjustable versions, the
output will act like an open circuit; no current will flow out
of the pin. If the input is powered by a voltage source, the
output will source the short-circuit current of the device
and will protect itself by thermal limiting. In this case,
grounding the SHDN pin will turn off the device and stop
the output from sourcing the short-circuit current.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled
to ground, pulled to some intermediate voltage, or is left
open circuit. Current flow back into the output will follow
the curve shown in Figure 12.
When the IN pin of the LT1963A is forced below the OUT
pin or the OUT pin is pulled above the IN pin, input cur-
rent will typically drop to less than 2µA. This can happen
if the input of the device is connected to a discharged
(low voltage) battery and the output is held up by either
a backup battery or a second regulator circuit. The state
of the SHDN pin will have no effect on the reverse output
current when the output is pulled above the input.
The ADJ pin of the adjustable device can be pulled above
or below ground by as much as 7V without damaging the
device. Iftheinputisleftopencircuitorgrounded, theADJ
pin will act like an open circuit when pulled below ground
and like a large resistor (typically 5k) in series with a diode
when pulled above ground.
5.0
LT1963A
V
= V
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
OUT
ADJ
LT1963A-1.5
= V
V
OUT
FB
In situations where the ADJ pin is connected to a resistor
dividerthatwouldpulltheADJpinaboveits7Vclampvolt-
age if the output is pulled high, the ADJ pin input current
must be limited to less than 5mA. For example, a resistor
divider is used to provide a regulated 1.5V output from the
1.21V reference when the output is forced to 20V. The top
resistor of the resistor divider must be chosen to limit the
current into the ADJ pin to less than 5mA when the ADJ
pin is at 7V. The 13V difference between OUT and ADJ
pins divided by the 5mA maximum current into the ADJ
pin yields a minimum top resistor value of 2.6k.
LT1963A-1.8
= V
V
OUT
FB
LT1963A-2.5
= V
V
OUT
FB
LT1963A-3.3
= V
V
OUT
FB
T = 25°C
J
IN
V
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
0
1
2
3
4
5
6
7
8
9
10
OUTPUT VOLTAGE (V)
1963A F12
Figure 12. Reverse Output Current
1963aff
20
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