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LT1965T-1.8

1.1A, Low Noise, Low Dropout Linear Regulator

Linear Technology 

LT1965 Series

Applications Information

The input of the device withstands reverse voltages of 22V.

The LT1965 limits current flow to less than 1mA (typically

less than 300µA) and no negative voltage appears at the

output. The device protects both itself and the load against

batteries that are plugged in backwards.

The LT1965 incurs no damage if its output is pulled be-

low ground. If the input is left open-circuit or grounded,

the output can be pulled below ground by 22V. For fixed

voltage versions, the output will act like a large resistor,

typically 5k or higher, limiting current flow to typically

less than 300µA. For the adjustable version, the output

acts like an open circuit and no current flows from the

output. However, current flows out of (but is limited by)

the resistor divider that sets the output voltage. If the input

is powered by a voltage source, the LT1965 protects itself

by shutting off the output.

The LT1965 adjustable version incurs no damage if the

ADJ pin is pulled above or below ground by 9V. If the input

is left open-circuit or grounded, the ADJ pin performs like

a diode in series with typically 1.5k of resistance 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.

In situations where the ADJ pin connects to a resistor

divider that would pull the ADJ pin above its 9V 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 1.20V 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.

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 6.

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 a backup battery or a second regulator holds up

the output. The state of the SHDN pin has no effect on

the reverse-output current if the output is pulled above

the input.

6 TJ = 25°C

VIN = 0V

5 VOUT = VADJ (LT1965)

VOUT = VSENSE

(LT195-1.5/-1.8/-2.5/-3.3)

4 CURRENT FLOWS INTO

OUTPUT PIN

3

LT1965-1.5

2

LT1965-1.8

LT1965

1

LT1965-2.5

LT1965-3.3

0

0123456789

OUTPUT VOLTAGE (V)

1965 F06

Figure 6. Reverse-Output Current

16

For more information www.linear.com/LT1965

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