12

LT1761 Series

function if the SHDN pin is not connected. For the

BYP (Pins 3/4, Fixed/-BYP Devices): Bypass. The BYP

pin is used to bypass the reference of the LT1761 regula-

tors to achieve low noise performance from the regulator.

The BYP pin is clamped internally to

ground. A small capacitor from the output to this pin will

bypass the reference to lower the output voltage noise. A

maximum value of 0.01

µF can be used for reducing output

voltage noise to a typical 20

bandwidth. If not used, this pin must be left unconnected.

ADJ (Pin 4, Adjustable Devices Only): Adjust Pin. For the

adjustable LT1761, this is the input to the error amplifier.

This pin is internally clamped to

±7V. It has a bias current

of 30nA which flows into the pin (see curve of ADJ Pin Bias

Current vs Temperature in the Typical Performance Char-

acteristics section). The ADJ pin voltage is 1.22V referenced

to ground and the output voltage range is 1.22V to 20V.

OUT (Pin 5): Output. The output supplies power to the

load. A minimum output capacitor of 1

µF is required to

prevent oscillations. Larger output capacitors will be

required for applications with large transient loads to limit

peak voltage transients. See the Applications Information

section for more information on output capacitance and

reverse output characteristics.

IN (Pin 1): Input. Power is supplied to the device through

the IN pin. A bypass capacitor is required on this pin if the

device is more than six inches away from the main input

filter capacitor. In general, the output impedance of a

battery rises with frequency, so it is advisable to include a

bypass capacitor in battery-powered circuits. A bypass

capacitor in the range of 1

µF to 10µF is sufficient. The

LT1761 regulators are designed to withstand reverse

voltages on the IN pin with respect to ground and the OUT

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

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

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

reverse current flow into the regulator and no reverse

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

itself and the load.

GND (Pin 2): Ground.

SHDN (Pin 3, Fixed/-SD Devices): Shutdown. The SHDN

pin is used to put the LT1761 regulators into a low power

shutdown state. The output will be off when the SHDN pin

is pulled low. The SHDN pin can be driven either by 5V

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

pull-up resistor is required to supply the pull-up current of

the open-collector gate, normally several microamperes,

and the SHDN pin current, typically 1

µA. If unused, the

PI FU CTIO S

APPLICATIO S I FOR ATIO

The LT1761 series are 100mA low dropout regulators with

micropower quiescent current and shutdown. The devices

are capable of supplying 100mA at a dropout voltage of

300mV. Output voltage noise can be lowered to 20

over a 10Hz to 100kHz bandwidth with the addition of a

0.01

µF reference bypass capacitor. Additionally, the refer-

ence bypass capacitor will improve transient response of

the regulator, lowering the settling time for transient load

conditions. The low operating quiescent current (20

µA)

drops to less than 1

µA in shutdown. In addition to the low

quiescent current, the LT1761 regulators incorporate sev-

eral protection features which make them ideal for use in

battery-powered systems. The devices are protected

against both reverse input and reverse output voltages. In

battery backup applications where the output can be held

up by a backup battery when the input is pulled to ground,

the LT1761-X acts like it has a diode in series with its

output and prevents reverse current flow. Additionally, in

dual supply applications where the regulator load is

returned to a negative supply, the output can be pulled

below ground by as much as 20V and still allow the device

to start and operate.

Adjustable Operation

The adjustable version of the LT1761 has an output

voltage range of 1.22V to 20V. The output voltage is set by

the ratio of two external resistors as shown in Figure 1. The

device servos the output to maintain the ADJ pin voltage

at 1.22V referenced to ground. The current in R1 is then