SD6251D6G-R datasheet(6/9 Pages) SHOUDING

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SD6251

The SD6251 is designed for high efficiency over

85T

Application Information

Controller Circuit

The device is based on a current-mode control

topology

and

uses

constant

frequency

pulse-width modulator to regulate the output

voltage.

The controller limits the current through

the power switch on a pulse by pulse basis.

The

current sensing circuit is integrated in the device;

therefore, no additional components are required.

Due to the nature of the boost converter topology

used here, the peak switch current is the same as

the peak inductor current, which will be limited by

the integrated current limiting circuits under normal

operating conditions.

Synchronous Rectifier

The device integrates an N-channel and a P-

channel

MOSFET

transistor

realize

synchronous rectifier.

There is no additional

Schottky diode required.

Because the device

uses a integrated low RDS(ON) PMOS switch for

rectification,

the

power

conversion

efficiency

reaches 93%.

A special circuit is applied to disconnect the load

from the input during shutdown of the converter.

In conventional synchronous rectifier circuits, the

backgate diode of the high-side PMOS is forward

biased in shutdown and allows current flowing from

the battery to the output.

This device, however,

uses a special circuit to disconnect the backgate

diode of the high-side PMOS and so, disconnects

the output circuitry from the source when the

regulator is not enabled (EN=low).

PSM Mode

wide output current range.

Even at light load, the

efficiency stays high because the switching losses

of the converter are minimized by effectively

reducing the switching frequency.

The controller

will enter a power saving mode if certain conditions

are met.

In this mode, the controller only switches

on the transistor if the output voltage trips below a

set threshold voltage.

It ramps up the output

voltage with one or several pulses, and goes again

into PSM mode once the output voltage exceeds a

set threshold voltage.

Device Enable

The device will be shut down when EN is set to

GND.

In this mode, the regulator stops switching,

all internal control circuitry including the low-battery

comparator will be switched off, and the load will be

disconnected from the input (as described in above

synchronous rectifier section).

This also means

that the output voltage may drop below the input

voltage during shutdown.

The device is put into operation when EN is set

high.

During start-up of the converter, the duty

cycle is limited in order to avoid high peak currents

drawn from the battery.

The limit is set internally

by the current limit circuit.

Anti-Ringing Switch

The device integrates a circuit which removes

the ringing that typically appears on the SW node

when the converter enters the discontinuous

current mode.

In this case, the current through

the inductor ramps to zero and the integrated

PMOS switch turns off to prevent a reverse

current from the output capacitors back to the

battery.

Due to remaining energy that is stored

in parasitic components of the semiconductors

and the inductor, a ringing on the SW pin is

induced.

The integrated anti-ringing switch

clamps this voltage internally to VIN; therefore,

dampens this ringing.

Adjustable Output Voltage

The accuracy of the output voltage is determined by

the accuracy of the internal voltage reference, the

controller topology, and the accuracy of the external

resistor.

The reference voltage has an accuracy of

± 2%.

The controller switches between fixed

frequency and PSM mode, depending on load

current.

The tolerance of the resistors in the

feedback divider determines the total system

accuracy.

Design Procedure

The

boost converter family is intended for

systems that are powered by a single-cell Ion

battery with a typical terminal voltage between 3V

to 4.2V.

SD6251

http://www.sdw-tw.com

Ver 1.0 Oct 2012

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