HIGH = ON

LOW = OFF

Overcurrent and

Thermal Protection

VREFERENCE

1.23 V

Fast Turnon

Circuit

R2

IN

EN

BYPASS

OUT

OUT-SENSE

GND

LP3981

SNVS159H – OCTOBER 2001 – REVISED JULY 2015

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8 Detailed Description

8.1 Overview

The LP3981 family of fixed-output, ultra-low-dropout, and low-noise regulators offers exceptional performance for

battery-powered applications. Available for voltages from 2.5-V to 3.3-V, the family is capable of delivering

300-mA continuous load current.

The LP3981 contains several features to facilitate battery-powered designs:

•

Low dropout voltage , typical dropout of 132-mV at 300-mA load current.

•

Low quiescent current and low ground current. Ground current is typically 170 µA at 150-mA load, and 70 µA

at 1-mA load.

•

A shutdown feature is available , allowing the regulator to consume only 0.003 µA typically when the EN pin

is pulled low.

•

Power supply rejection is 60-dB at 1 kHZ.

•

Low noise; BYPASS pin allows for low-noise operation, with typically 35-µVRMS output noise over 10 Hz to

100 kHz.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 On/Off Input Operation

The LP3981 is turned off by pulling the EN pin low, and turned on by pulling it high. If this feature is not used, the

EN pin must be tied to VIN to keep the regulator output on at all time. To assure proper operation, the signal

source used to drive the EN input must be able to swing above and below the specified turnon and turnoff

voltage thresholds listed in Electrical Characteristics under VIL and VIH.

8.3.2 Fast On-Time

The LP3981 utilizes a speed up circuitry to ramp up the internal VREF voltage to its final value to achieve a fast

output turn on time.

10

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