LT3011
13
3011f
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C
(LT3011E/LT3011I) or 150°C (LT3011H).
The input of the device will withstand reverse voltages
of 80V. Current fl ow into the device will be limited to less
than 6mA (typically less than 100μA) and no negative
voltage will appear at the output. The device will protect
both itself and the load. This provides protection against
batteries which can be plugged in backwards.
The ADJ pin of the adjustable device can be pulled above
or below ground by as much as 7V without damaging the
device. If the input is left open-circuit or grounded, the ADJ
pin will act like an open-circuit when pulled below ground,
and like a large resistor (typically 100k) in series with a
diode when pulled above ground. If the input is powered by
a voltage source, pulling the ADJ pin below the reference
voltage will cause the device to try and force the current
limit out of the output. This will cause the output to go to
an unregulated high voltage. Pulling the ADJ pin above the
reference voltage will turn off all output current.
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
1.24V 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
pin is 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 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 5. 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 regula-
tor output, this current will be reduced depending on the
size of the resistor divider.
When the IN pin of the LT3011 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 LT3011 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.
OUTPUT VOLTAGE (V)
07
24
19
35
8
610
3011 F05
T
J
= 25°C
V
IN
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
V
OUT
= V
ADJ
REVERSE OUTPUT CURRENT (μA)
100
120
140
80
60
20
0
40
160
ADJ PIN CLAMP
(SEE ABOVE)
Figure 5. Reverse Output Current
APPLICATIONS INFORMATION
