13

LT1963 Series

1963fa

APPLICATIO S I FOR ATIO

Figure 5. Reverse Output Current

OUTPUT VOLTAGE (V)

0

3.0

4.0

5.0

8

1963 F05

2.0

1.0

2.5

3.5

4.5

1.5

0.5

0

2

1

3

4

6

9

7

5

10

LT1963

LT1963-1.8

LT1963-2.5

CURRENT FLOWS

INTO OUTPUT PIN

LT1963-3.3

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

Protection Features

The LT1963 regulators incorporate several protection

features which make them 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 devices are protected

against reverse input voltages, reverse output voltages

and reverse voltages from output to input.

Current limit protection and thermal overload protection

are intended to protect the device against current overload

conditions at the output of the device. For normal opera-

tion, the junction temperature should not exceed 125

°C.

The input of the device will withstand reverse voltages of

20V. Current flow into the device will be limited to less than

1mA (typically less than 100

µA) and no negative voltage

will appear at the output. The device will protect both itself

and the load. This provides protection against batteries

that can be plugged in backward.

The output of the LT1963 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 LT1963 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.