LT1129/LT1129-3.3/LT1129-5

12

112935ff

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 operation,

the junction temperature should not exceed 125°C.

The input of the device will withstand reverse voltages

of 30V. 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 backwards.

For fixed voltage versions of the device, the sense pin

is internally clamped to one diode drop below ground.

For the adjustable version of the device, the output pin

is internally clamped at one diode drop below ground. If

the output pin of an adjustable device, or the sense pin

of a fixed voltage device, is pulled below ground, with the

input open or grounded, current must be limited to less

than 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 Figure 3

when the input pin 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 to the LT1129 unconnected when the output is held

high. This can happen when the LT1129 is powered from

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

the input of the LT1129 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 LT1129 is forced to a voltage below

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

reverse output current will still follow the curve shown

in Figure 3. This can happen if the input of the LT1129 is

connected to a discharged (low voltage) battery and the

output is held up by either a backup battery or by a 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)

0

100

90

80

70

60

50

40

30

20

10

0

8

112935 F03

2

4

6

10

13

57

9

CURRENT FLOWS

INTO DEVICE

LT1129-3.3

LT1129

LT1129-5

Figure 3. Reverse Output Current

INPUT VOLTAGE (V)

0

5

4

3

2

1

0

4

112935 F04

1

2

3

5

Figure 4. Input Current