LT1965
APPLICATIONS INFORMATION
Protection Features
TheLT1965incursnodamageiftheADJpinispulledabove
or below ground by 9V. If the input is left open circuit or
grounded, the ADJ pin performs like an open circuit when
pulled below ground and like a large resistor (typically 5k
up to 3V on the ADJ pin and then 1.5k up to 9V) in series
with a diode when pulled above ground.
The LT1965 incorporates several protection features
that make it ideal for use in battery-powered circuits.
In addition to the normal protection features associated
with monolithic regulators, such as current limiting and
thermal limiting, the device also protects against reverse
input voltages, reverse output voltages and reverse out-
put-to-input voltages.
In situations where the ADJ pin connects to a resistor
dividerthatwouldpulltheADJpinaboveits9Vclampvolt-
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
1.20V 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 9V. The 11V difference between the OUT and ADJ
pins divided by the 5mA maximum current into the ADJ
pin yields a minimum top resistor value of 2.2k.
Current limit protection and thermal overload protection
protect the device against current overload conditions
at its output. For normal operation, do not exceed the
maximum rated junction temperature of 125°C.
Theinputofthedevicewithstandsreversevoltagesof22V.
The LT1965 limits current flow to less than 1mA (typically
less than 200μA) and no negative voltage appears at the
output.Thedeviceprotectsbothitselfandtheloadagainst
batteries that are plugged in backwards.
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 follows the
curve shown in Figure 5.
The LT1965 incurs no damage if its output is pulled below
ground. If the input is left open circuit or grounded, the
output can be pulled below ground by 22V. For the adjust-
able version, the output acts like an open circuit and no
current flows from the output. However, current flows in
(but is limited by) the resistor divider that sets the output
voltage. If the input is powered by a voltage source, the
output sources current equal to its current limit capability
and the LT1965 protects itself by thermal limiting. In this
case, grounding the SHDN pin turns off the device and
stops the output from sourcing current.
If the LT1965’s IN pin is forced below the OUT pin or the
OUT pin is pulled above the IN pin, input current typically
drops to less than 2μA. This occurs if the LT1965 input is
connected to a discharged (low voltage) battery and either
abackupbatteryorasecondregulatorholdsuptheoutput.
The state of the SHDN pin has no effect on the reverse
output current if the output is pulled above the input.
6
T
= 25°C
IN
J
V
= 0V
5
CURRENT FLOWS INTO
OUTPUT PIN
V
= V
OUT
ADJ
4
3
2
1
0
0
2
4
6
8
10
OUTPUT VOLTAGE (V)
1965 F05
Figure 5. Reverse Output Current
1965f
12
Linear [ Linear ]
