LT1521 datasheet(12/18 Pages) AD

12 / 18 page

LT1521/LT1521-3

LT1521-3.3/LT1521-5

Rev. C

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APPLICATIONS INFORMATION

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 temperatures 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 100µA) 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.

For fixed voltage versions of the device, the output 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. The output will act like an

open circuit, no current will flow out of the pin. If the input

is powered by voltage source, the output will source the

short-circuit current of the device and will protect itself by

thermal limiting. For the adjustable version of the device,

the output pin is internally clamped at one diode drop

below ground. Reverse current for the adjustable device

must be limited to 5mA.

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 vary

depending on the conditions. Many battery-powered

circuits incorporate some form of power management.

The following information will help optimize battery life.

Table 4 summarizes the following information.

The reverse output current will follow the curve in Fig-

ure 3 when the input is pulled to ground. This current

flows through the output pin to ground. The state of the

shutdown pin will have no effect on output current when

the input pin is pulled to ground.

In some applications it may be necessary to leave the

input on the LT1521 unconnected when the output is

held high. This can happen when the LT1521 is powered

from a rectified AC source. If the AC source is removed,

then the input of the LT1521 is effectively left floating. The

reverse output current also follows the curve in Figure 3

if the input pin is left open. The state of the shutdown pin

will have no effect on the reverse output current when the

input pin is floating.

When the input of the LT1521 is forced to a voltage below

its nominal output voltage and its output is held high, the

outputcurrentwillfollowthecurveshowninFigure 3.This

can happen if the input of the LT1521 is connected to a

discharged (low voltage) battery and the output is held up

by either a backup battery or by second regulator circuit.

When the input pin is forced below the output pin or the

output pin is pulled above the input pin, the input current

will typically drop to less than 2µA (see Figure 4). The

state of the shutdown pin will have no effect on the reverse

output current when the output is pulled above the input.

OUTPUT VOLTAGE (V)

LT1521 • F03

TJ = 25°C

VIN = 0V

CURRENT FLOWS

INTO OUTPUT PIN

VOUT = VSENSE

(LT1521-3/LT1521-3.3

LT1521-5)

VOUT = VADJ (LT1521)

LT1521

LT1521-3.3

LT1521-3

LT1521-5

INPUT VOLTAGE (V)

LT1521 • F04

VOUT = 3V (LT1521-3)

VOUT = 3.3V (LT1521-3.3)

VOUT = 5V (LT1521-5)

LT1521-3

LT1521-3.3

LT1521-5

Figure 3. Reverse Output Current

Figure 4. Input Current

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