Datasheet

12
LT3012B
3012bf
APPLICATIO S I FOR ATIO
WUUU
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp
voltage 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 1.24V reference when the output is forced to 60V.
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 53V difference between the OUT and
ADJ pins divided by the 5mA maximum current into the
ADJ pin yields a minimum top resistor value of 10.6k.
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
Figure 4. Reverse Output Current
OUTPUT VOLTAGE (V)
0
REVERSE OUTPUT CURRENT (µA)
120
160
200
8
3012B F04
80
40
100
140
180
60
20
0
21
43
67 9
5
10
ADJ
PIN CLAMP
(SEE ABOVE)
T
J
= 25°C
V
IN
= 0V
V
OUT
= V
ADJ
CURRENT FLOWS
INTO OUTPUT PIN
ground, pulled to some intermediate voltage, or is left
open circuit. Current flow back into the output will follow
the curve shown in Figure 4. The rise in reverse output
current above 7V occurs from the breakdown of the 7V
clamp on the ADJ pin. With a resistor divider on the
regulator output, this current will be reduced depending
on the size of the resistor divider.
When the IN pin of the LT3012B is forced below the OUT
pin or the OUT pin is pulled above the IN pin, input current
will typically drop to less than 2µA. This can happen if the
input of the LT3012B is connected to a discharged (low
voltage) battery and the output is held up by either a
backup battery or a second regulator circuit.