7

LT1121/LT1121-3.3/LT1121-5

CC

HARA TERISTICS

A

TYPICALPERFOR

CE

PI FU CTIO S

Input Pin: Power is supplied to the device through the

input pin. The input pin should be bypassed to ground if

the device is more than 6 inches away from the main input

filter capacitor. In general the output impedance of a

battery rises with frequency so it is usually adviseable to

include a bypass capacitor in battery-powered circuits. A

bypass capacitor in the range of 0.1

µF to 1µF is sufficient.

The LT1121 is designed to withstand reverse voltages on

the input pin with respect to both ground and the output

pin. In the case of a reversed input, which can happen if a

battery is plugged in backwards, the LT1121 will act as if

there is a diode in series with its input. There will be no

reverse current flow into the LT1121 and no reverse

voltage will appear at the load. The device will protect both

itself and the load.

Output Pin: The output pin supplies power to the load. An

output capacitor is required to prevent oscillations. See

the Applications Information section for recommended

value of output capacitance and information on reverse

output characteristics.

Shutdown Pin: This pin is used to put the device into

shutdown. In shutdown the output of the device is turned

Load Regulation

LT1121-5

Load Transient Response

LT1121-5

Load Transient Response

off. This pin is active low. The device will be shut down if

the shutdown pin is pulled low. The shutdown pin current

with the pin pulled to ground will be 6

µA. The shutdown

allows the shutdown pin to be driven directly by 5V logic

or by open collector logic with a pull-up resistor. The pull-

up resistor is only required to supply the leakage current

of the open collector gate, normally several microam-

peres. Pull-up current must be limited to a maximum of

20mA. A curve of shutdown pin input current as a function

of voltage appears in the Typical Performance Character-

istics. If the shutdown pin is not used it can be left open

circuit. The device will be active, output on, if the shutdown

pin is not connected.

Adjust Pin: For the adjustable LT1121, the adjust pin is the

input to the error amplifier. This pin is internally clamped

which flows into the pin. See Bias Current curve in the

Typical Performance Characteristics. The adjust pin refer-

ence voltage is 3.75V referenced to ground. The output

voltage range that can be produced by this device is 3.75V

to 30V.

TIME (ms)

0

0.2

0.1

0

–0.1

–0.2

0.8

1121 G30

0.1

0.4

0.6

1.0

150

100

0.2 0.3

0.5

0.7

0.9

TIME (ms)

0

0.2

0.1

0

–0.1

–0.2

0.8

1121 G31

0.1

0.4

0.6

1.0

150

100

50

0.2 0.3

0.5

0.7

0.9

TEMPERATURE (°C)

–50

0

–5

–10

–15

–20

–25

–30

–35

–40

0

50

75

1121 G21

–25

25

100

125

LT1121*

LT1121-5

LT1121-3.3

* ADJ PIN TIED TO

OUTPUT PIN