R2

GND

Fast Turnon

Circuit

Vreference

1.23V

Overcurrent &

Thermal Protection

IN

EN

BYPASS

OUT

R1

1.4V

ON

J

0.4V

OFF@

LP3985

www.ti.com

SNVS087AE – OCTOBER 2000 – REVISED MAY 2015

7 Detailed Description

7.1 Overview

The LP3985 family of fixed-output, ultra-low-dropout and low noise regulators offers exceptional, cost-effective

performance for battery powered applications. Available in output voltages from 2.5 V to 5 V, the family is

capable of delivering 150-mA continuous load current. Standard regulator features, such as overcurrent and

overtemperature protection, are also included.

The LP3985 contains several features to facilitate battery powered designs:

• Multiple voltage options

• Low dropout voltage, typical dropout of 60 mV at 150-mA load current

• Low quiescent current and low ground current, typically 140

μA at 150-mA load, and 85-μA at 0-mA load

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

pulled low

• Overtemperature protection and overcurrent protection circuitry is designed to safeguard the device during

unexpected conditions

• Enhanced stability: The LP3985 is stable with output capacitor, which allows the use of ceramic capacitors on

the output

• Power supply rejection is better than 50 dB at low frequencies and starts to roll off at 1 kHz.

• Low noise: A BYPASS pin allows for low-noise operation, with a typical output noise of 30 µVRMS, with the use

of a 10-nF bypass capacitor.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 No-Load Stability

The LP3985 will remain stable and in regulation with no external load. This is specially important in CMOS RAM

keep-alive applications.

7.3.2 On/Off Input Operation

The LP3985 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 should 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/turnoff voltage

thresholds listed in Electrical Characteristics under VIL and VIH.

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