10

LT1121/LT1121-3.3/LT1121-5

S

APPLICATI

I FOR ATIO

that the ESR of the capacitor is low (ceramic) the sug-

gested series resistor is shown in Table 5. The LT1121 is

a micropower device and output transient response will be

a function of output capacitance. See the Transient Re-

sponse curves in the Typical Performance Characteristics.

Larger values of output capacitance will decrease the peak

deviations and provide improved output transient re-

sponse. Bypass capacitors, used to decouple individual

components powered by the LT1121, will increase the

effective value of the output capacitor.

Table 5.

SUGGESTED SERIES

OUTPUT CAPACITANCE

RESISTOR

0.33

µF2Ω

0.47

µF1Ω

0.68

µF1Ω

>1

µF

None Needed

Protection Features

The LT1121 incorporates several protection features which

make it ideal for use in battery-powered circuits. In addi-

tion to the normal protection features associated with

monolithic regulators, such as current limiting and ther-

mal limiting, the device is 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

30V. 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 backwards.

For fixed voltage versions of the device, the output can be

pulled below ground without damaging the device. If the

input is 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 a 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 cir-

cuits incorporate some form of power management. The

following information will help optimize battery life. Table

6 summarizes the following information.

The reverse output current will follow the curve in Figure

2 when the input pin is pulled to ground. This current flows

through the output pin to ground. The state of the shut-

down 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 LT1121 unconnected when the output is held high.

This can happen when the LT1121 is powered from a

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

input of the LT1121 is effectively left floating. The reverse

output current also follows the curve in Figure 2 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.

OUTPUT VOLTAGE (V)

0

100

90

80

70

60

50

40

30

20

10

0

8

1121• F02

2

4

6

10

13

57

9

CURRENT FLOWS

INTO OUTPUT PIN

TO GROUND

LT1121-3.3

LT1121

LT1121-5

Figure 2. Reverse Output Current