OFF

THERMAL

PROTECTION

1.25-V

BANDGAP

FAST

START-UP

CIRCUIT

CURRENT

LIMIT

FAULT

COMPARATORS

R1

R2

IN

OUT

SET

CC

SHDN

FAULT

GND

ERROR

AMP

+

-

LP3982

www.ti.com

SNVS185E – FEBRUARY 2002 – REVISED OCTOBER 2015

7 Detailed Description

7.1 Overview

The LP3982 is package, pin, and performance compatible with Maxim's MAX8860, excluding reverse battery

protection and dual-mode function (fixed and adjustable combined).

A 1.25-V bandgap reference, an error amplifier, and a PMOS pass transistor perform voltage regulation while

being supported by shutdown, fault, and the usual temperature and current protection circuitry (see Functional

Block Diagram).

The regulator topology is the classic type with negative feedback from the output to one of the inputs of the error

amplifier. Feedback resistors R1 and R2 are either internal or external to the device, depending on whether it is

the fixed-voltage version or the adjustable version. The negative feedback and high open loop gain of the error

amplifier cause the two inputs of the error amplifier to be virtually equal in voltage. If the output voltage changes

due to load changes, the error amplifier provides the appropriate drive to the pass transistor to maintain the error

amplifier's inputs as virtually equal. In short, the error amplifier keeps the output voltage constant in order to keep

its inputs equal.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 No-Load Stability

The LP3982 remains stable during no-load conditions, a necessary feature for CMOS RAM keep-alive

applications.

7.3.2 Fast Start-Up

The LP3982 provides fast start-up time for better system efficiency. The start-up speed is maintained when using

the optional noise bypass capacitor. An internal 500-

μA current source charges the capacitor until it reaches

about 90% of its final value.

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