Datasheet

LT1761 Series
19
1761sff
OUTPUT VOLTAGE (V)
100
90
80
70
60
50
40
30
20
10
0
REVERSE OUTPUT CURRENT (μA)
1761 F06
0123
4
5
678910
T
J
= 25°C
V
IN
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
V
OUT
= V
ADJ
(LT1761-BYP, -SD)
LT1761-BYP
LT1761-SD
LT1761-2
LT1761-3.3
LT1761-5
LT1761-1.8
LT1761-1.5
LT1761-2.5
LT1761-2.8
LT1761-3
LT1761-1.2
Figure 6. Reverse Output Current
APPLICATIONS INFORMATION
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp volt-
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
1.22V 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 output and ADJ
pin divided by the 5mA maximum current into the ADJ pin
yields a minimum top resistor value of 2.6k.
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 fl ow back into the output will follow
the curve shown in Figure 6.
When the IN pin of the LT1761-X 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.