1963A Просмотр технического описания (PDF) - Linear Technology

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1963A

1.5A, Low Noise, Fast Transient Response LDO Regulators

Linear Technology 

LT1963A Series

APPLICATIONS INFORMATION

The output of the LT1963A 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 600μA. 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 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

1.21V 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 12.

When the IN pin of the LT1963A is forced below the OUT

pin or the OUT pin is pulled above the IN pin, input cur-

rent will typically drop to less than 2μA. 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.

5.0

LT1963A

4.5

VOUT = VADJ

4.0 LT1963A-1.5

3.5 VOUT = VFB

LT1963A-1.8

3.0 VOUT = VFB

2.5 LT1963A-2.5

VOUT = VFB

2.0

LT1963A-3.3

1.5

VOUT = VFB

1.0

TJ = 25°C

VIN = 0V

0.5

CURRENT FLOWS

INTO OUTPUT PIN

0

0 1 2 3 4 5 6 7 8 9 10

OUTPUT VOLTAGE (V)

1963A F12

Figure 12. Reverse Output Current

1963afd

20

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