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LT3009 Series

3009fd

OUTPUT AND ADJ VOLTAGE (V)

100

3009 F05

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 ﬂow 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 ﬂows from the

pass transistor connected to OUT. However, current ﬂows

in (but is limited by) the resistor divider that sets output

voltage. Current ﬂows 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 ﬂow 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

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