S2

S4

S1

S3

1.2-V

Ref.

2-MHz

Osc.

OCL

OCL =

Current Limit

C1-

C1+

EN

LM2775

GND

PFM EN

PFM

OUTDIS

LM2775

www.ti.com

SNVSA57 – MAY 2015

7 Detailed Description

7.1 Overview

The LM2775 is a regulated switched capacitor doubler that, by combining the principles of switched-capacitor

voltage boost and linear regulation, generates a regulated output from an extended Li-Ion input voltage range. A

two-phase non-overlapping clock generated internally controls the operation of the doubler. During the charge

phase (

φ1), the flying capacitor (C1) is connected between the input and ground through internal pass transistor

switches and is charged to the input voltage. In the pump phase that follows (

φ2), the flying capacitor is

connected between the input and output through similar switches. Stacked atop the input, the charge of the flying

capacitor boosts the output voltage and supplies the load current.

A traditional switched capacitor doubler operating in this manner uses switches with very low on-resistance to

generate an output voltage that is 2× the input voltage. Regulation is achieved by modulating the current of the

two switches connected to the VIN pin (one switch in each phase).

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Pre-Regulation

The very low input current ripple of the LM2775, resulting from internal pre-regulation, adds minimal noise to the

input line. The core of the LM2775 is very similar to that of a basic switched capacitor doubler: it is composed of

four switches and a flying capacitor (external). Regulation is achieved by controlling the current through the two

switches connected to the VIN pin (one switch in each phase). The regulation is done before the voltage

doubling, giving rise to the term "pre-regulation". It is pre-regulation that eliminates most of the input current

ripple that is a typical and undesirable characteristic of a many switched capacitor converters.

7.3.2 Input Current Limit

The LM2775 contains current limit circuitry that protects the device in the event of excessive input current and/or

output shorts to ground. The input current is limited to 600 mA (typical) when the output is shorted directly to

ground. When the LM2775 is current limiting, power dissipation in the device is likely to be quite high. In this

event, thermal cycling should be expected.

Copyright © 2015, Texas Instruments Incorporated

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