13

LT1764 Series

1764fa

device will protect both itself and the load. This provides

protection against batteries which can be plugged in

backward.

The output of the LT1764-X can be pulled below ground

without damaging the device. If the input is left open circuit

or grounded, the output can be pulled below ground by

20V. For fixed voltage versions, the output will act like a

large resistor, typically 5k or higher, limiting current flow

to typically less than 600

µA. For adjustable versions, the

output will act like an open circuit; no current will flow out

of the pin. If the input is powered by a voltage source, the

output will source the short-circuit current of the device

and will protect itself by thermal limiting. In this case,

grounding the SHDN pin will turn off the device and stop

the output from sourcing the short-circuit current.

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 5k) in series with a diode

when pulled above ground.

In situations where the ADJ pin is connected to a resistor

divider that would pull the ADJ pin above its 7V clamp

voltage 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 1.21V 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 7V. The 13V difference between OUT and ADJ

pins divided by the 5mA maximum current into the ADJ pin

yields a minimum top resistor value of 2.6k.

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 flow back into the output will follow

the curve shown in Figure 5.

When the IN pin of the LT1764-X is forced below the OUT

pin or the OUT pin is pulled above the IN pin, input current

will typically drop to less than 2

µA. This can happen if the

input of the device 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.

Figure 5. Reverse Output Current

OUTPUT VOLTAGE (V)

0123456789

10

1764 F05

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

LT1764-2.5

LT1764-3.3

LT1764-1.8

LT1764

CURRENT FLOWS INTO

OUTPUT PIN

LT1764-2.5, LT1764-3.3)