LM2772

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SNVS486A – DECEMBER 2006 – REVISED MARCH 2007

OPERATION DESCRIPTION

Overview

The LM2772 is a switched capacitor converter that produces a regulated, low voltage output. The core of the part

is a highly efficient charge pump that utilizes fixed frequency pre-regulation and Pulse Frequency Modulation to

minimize ripple and power losses over wide input voltage and output current ranges. A description of the

principal operational characteristics of the LM2772 is detailed in the Circuit Description, and Efficiency

Performance sections. These sections refer to details in the Block Diagram.

Circuit Description

The core of the LM2772 is a two-phase charge pump controlled by an internally generated non-overlapping

input to the output. At input voltages below 3.5V (typ.) the LM2772 operates in a 1/2x Gain, with the input current

being equal to 1/2 of the load current. At input voltages between 3.5V to 4.6V(typ.) the part utilizes a gain of 2/5x,

resulting in an input current equal to 2/5 times the load current. At input voltages above 4.6V (typ.), the part is in

a gain of 1/3, with the input current being 1/3 of the load current.

The two phases of the switched capacitor switching cycle will be referred to as the "charge phase" and the

"discharge phase". During the charge phase, the flying capacitor is charged by the input supply. After half of the

that was stored on the flying capacitors in the charge phase is transferred to the output.

The LM2772 uses fixed frequency pre-regulation to regulate the output voltage to 1.2V during moderate to high

load currents. The input and output connections of the flying capacitors are made with internal MOS switches.

Pre-regulation limits the gate drive of the MOS switch connected between the voltage input and the flying

capacitors. Controlling the on resistance of this switch limits the amount of charge transferred into and out of

each flying capacitor during the charge and discharge phases, and in turn helps to keep the output ripple very

low.

When output currents are low (<40mA typ.), the LM2772 automatically switches to a low-ripple Pulse Frequency

Modulation (PFM) form of regulation. In PFM mode, the flying capacitors stay in the discharge phase until the

output voltage drops below a predetermined trip point. When this occurs, the flying capacitors switch back to the

charge phase. After being charged, the flying capacitors repeat the process of staying in the discharge phase

and switching to the charge phase when necessary.

Efficiency Performance

Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are

neglected for simplicity):

(1)

Refer to the efficiency graph in the Typical Performance Characteristics section for detailed efficiency data. The

transition between gains of 1/2, 2/5, and 1/3 are clearly distinguished by the sharp discontinuity in the efficiency

curve.

Shutdown

The LM2772 is in shutdown mode when the voltage on the enable pin (EN) is logic-low. In shutdown, the

LM2772 draws virtually no supply current. When in shutdown, the output of the LM2772 is completely

disconnected from the input. Internal feedback resistors pull the output voltage down to 0V during shutdown.

Soft Start

The LM2772 employs soft start circuitry to prevent excessive input inrush currents during startup. At startup, the

output voltage gradually rises from 0V to the nominal output voltage. This occurs in 150µs (typ.). Soft-start is

and EN pins.

Copyright © 2006–2007, Texas Instruments Incorporated

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