LT3009 Series
16
3009fd
OUTPUT AND ADJ VOLTAGE (V)
0
60
80
100
8
40
20
50
70
90
30
10
0
21
43
67 9
5
10
3009 F05
REVERSE CURRENT (μA)
OUT CURRENT
ADJ CURRENT
Figure 5. Reverse Output Current
Protection Features
The LT3009 incorporates several protection features that
make it ideal for use in battery-powered circuits. In ad-
dition 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 output-to-
input voltages.
Current limit protection and thermal overload protection
protect the device against current overload conditions at
the output of the device. For normal operation, do not
exceed a junction temperature of 125°C.
The LT3009 IN pin withstands reverse voltages of 22V. The
device limits current fl ow to less than 1mA (typically less
than 220μA) and no negative voltage appears at OUT. The
device protects both itself and the load against batteries
that are plugged in backwards.
The SHDN pin cannot be driven below GND unless tied to
the IN pin. If the SHDN pin is driven below GND while IN
is powered, the output will turn on. SHDN pin logic cannot
be referenced to a negative rail.
The LT3009 incurs no damage if OUT is pulled below
ground. If IN is left open circuit or grounded, OUT can be
pulled below ground by 22V. No current fl ows from the
pass transistor connected to OUT. However, current fl ows
in (but is limited by) the resistor divider that sets output
voltage. Current fl ows from the bottom resistor in the
divider and from the ADJ pin’s internal clamp through the
top resistor in the divider to the external circuitry pulling
OUT below ground. If IN is powered by a voltage source,
OUT sources current equal to its current limit capability
and the LT3009 protects itself by thermal limiting if neces-
sary. In this case, grounding the SHDN pin turns off the
LT3009 and stops OUT from sourcing current.
The LT3009 incurs no damage if the ADJ pin is pulled
above or below ground by 22V. If IN is left open circuit or
grounded, ADJ acts like a 100k resistor in series with a
diode when pulled above or below ground.
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 follows the
curve shown in Figure 5.
If the LT3009 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 1μA. This occurs if the LT3009 input is
connected to a discharged (low voltage) battery and either
a backup battery or a second regulator circuit holds up
the output. The state of the SHDN pin has no effect in the
reverse current if OUT is pulled above IN.
APPLICATIONS INFORMATION
