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LT3008 Series

3008fc

APPLICATIONS INFORMATION

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 LT3008 incurs no damage if OUT is pulled below

ground. If IN is left open circuit or grounded, OUT can be

pulled below ground by 50V. 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 LT3008 protects itself by thermal limiting if necessary.

In this case, grounding the SHDN pin turns off the LT3008

and stops OUT from sourcing current.

The LT3008 incurs no damage if the ADJ pin is pulled

above or below ground by 50V. 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

OUTPUT AND ADJ VOLTAGE (V)

100

3008 F06

OUT CURRENT

ADJ CURRENT

Figure 6. Reverse-Output Current

to ground, pulled to some intermediate voltage or is left

open circuit. Current ﬂow back into the output follows the

curve shown in Figure 6.

If the LT3008 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 LT3008 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.

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