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 2004 Semtech Corp.

www.semtech.com

SC2672

POWER MANAGEMENT

Main Loop(s)

The SC2672 is a dual, voltage mode synchronous Buck

controller, the two separate channels are identical and

share only IC supply pins (Vcc and GND), output driver

ground (PGND) and pre-driver supply voltage (BSTC).

They also share a common oscillator generating a

sawtooth waveform for channel 1 and an dephased

sawtooth for channel 2. Channel 2 has both inputs of

the error amplifier uncommitted and available externally.

a) Two independent channels with either common

or different input voltages and different output volt-

ages. The two channels each have their own volt-

age feedback path from their own output. In this

mode, positive inputs of error amplifier 2 are con-

nected externally to Vref. If the application uses a

common input voltage, the sawtooth phase shift

between the channels provides some measure of

input ripple current cancellation.

Applications Information - Theory of Operation

Power Good

The controller provides a power good signal. This is an

open collector output, which is pulled low if the output

voltage is outside of the power good window.

Soft Start/Enable

The Soft Start/Enable (SS/ENA) pin serves several func-

tions. If held below the Soft Start Enable threshold, both

channels are inhibited. DH1 and DH2 will be low, turning

off the top FETs. Between the Soft Start Enable thresh-

old and the Soft Start End threshold, the duty cycle is

allowed to increase. At the Soft Start End threshold,

maximum duty cycle is reached. In practical applications

the error amplifier will be controlling the duty cycle be-

fore the Soft Start End threshold is reached. To avoid

boost problems during startup in current share mode,

both channels start up in asynchronous mode, and the

bottom FET body diode is used for recirculating current

during the FET off time. When the SS/ENA pin reaches

the Soft Start Transition threshold, the channels begin

operating in synchronous mode for improved efficiency.

The soft start pin sources approximately 25uA and soft

start timing can be set by selection of an appropriate

soft start capacitor value.