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LT1763
(Rev.:1999)

500mA, Low Noise, LDO Micropower Regulators

Linear Technology 

LT1763 Series

PIN FUNCTIONS

Note that the voltage drop across the external PC traces

will add to the dropout voltage of the regulator. The SENSE

pin bias current is 10µA at the nominal rated output

voltage. The SENSE pin can be pulled below ground (as in

a dual supply system where the regulator load is returned

to a negative supply) and still allow the device to start and

operate.

ADJ (Pin 2): Adjust. For the adjustable LT1763, 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 Characteristics section). The ADJ

pin voltage is 1.22V referenced to ground and the output

voltage range is 1.22V to 20V.

BYP (Pin 4): Bypass. The BYP pin is used to bypass the

reference of the LT1763 regulators to achieve low noise

performance from the regulator. The BYP pin is clamped

internally to ±0.6V (one VBE). A small capacitor from the

output to this pin will bypass the reference to lower the

output voltage noise. A maximum value of 0.01µF can be

used for reducing output voltage noise to a typical 20µVRMS

over a 10Hz to 100kHz bandwidth. If not used, this pin

must be left unconnected.

GND (Pins 3, 6, 7): Ground.

SHDN (Pin 5): Shutdown. The SHDN pin is used to put the

LT1763 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 1µA. If unused, the SHDN pin

must be connected to VIN. The device will be in the low

power shutdown state if the SHDN pin is not connected.

IN (Pin 8): 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 1µF to 10µF is sufficient. The

LT1763 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.

APPLICATIONS INFORMATION

The LT1763 series are 500mA low dropout regulators with

micropower quiescent current and shutdown. The devices

are capable of supplying 500mA at a dropout voltage of

300mV. Output voltage noise can be lowered to 20µVRMS

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

0.01µF 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 (30µA)

drops to less than 1µA in shutdown. In addition to the low

quiescent current, the LT1763 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 LT1763-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 re-

turned 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 LT1763 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 2. The

device servos the output to maintain the ADJ pin voltage

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

equal to 1.22V/R1 and the current in R2 is the current in R1

1763fa

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