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LT1965

1965f

OUTPUT VOLTAGE (V)

246

1965 F05

TJ = 25°C

VIN = 0V

CURRENT FLOWS INTO

OUTPUT PIN

VOUT = VADJ

Protection Features

The LT1965 incorporates several protection features

that make it ideal for use in battery-powered circuits.

In addition 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 out-

put-to-input voltages.

Current limit protection and thermal overload protection

protect the device against current overload conditions

at its output. For normal operation, do not exceed the

maximum rated junction temperature of 125°C.

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

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

less than 200μA) 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 below

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

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

able version, the output acts like an open circuit and no

current ﬂows from the output. However, current ﬂows in

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

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

output sources current equal to its current limit capability

and the LT1965 protects itself by thermal limiting. In this

case, grounding the ⎯S⎯H⎯D⎯N pin turns off the device and

stops the output from sourcing current.

The LT1965 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 an open circuit 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 volt-

age 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 5.

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 ⎯S⎯H⎯D⎯N pin has no effect on the reverse

output current if the output is pulled above the input.

Figure 5. Reverse Output Current

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