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SC2677B

Dual Synchronous Voltage Mode Controller with Current Sharing Circuitry

Semtech Corporation 

SC2677B

POWER MANAGEMENT

Application Information

Main Loop(s)

The SC2677B 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 ampli-

fier uncommitted and available externally. This allows the

SC2677B to operate in two distinct modes.

a) Two independent channels with either common

or different input voltages and different output

voltages. The two channels each have their own volt-

age feedback path from their own output. In this

mode, positive input of the error amplifier 2 is con-

nected externally to Vref. If the application uses a

common input voltage, the sawtooth phase shift be-

tween the channels provides some measure of input

ripple current cancellation.

b) Two channels operating in current sharing mode

with common output voltage and either common in-

put voltage or different input voltages. In this mode,

channel 1 operates as a voltage mode Buck controller,

as before, but error amplifier 2 monitors and ampli-

fies the difference in voltage across the output cur-

rent sense resistors of channel 1 and channel 2 (Mas-

ter and Slave) and adjusts the Slave duty cycle to

match output currents. To controller also works well

for using the output choke winding resistance as cur-

rent sensing element (please refer the application

schematic for details). The amount of the current of

the slave channel vs the master channel can be pro-

grammed according to the application. This feature

is especially useful when two input sources are used

and each source has its power budget.

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

functions. If held below the Enable threshold, both chan-

nels are inhibited. DH1 and DH2 will be low, turning off

the top FETs. Between the Soft Start Enable threshold

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 before the Soft

Start End threshold is reached. To avoid boost problems

during start-up in current share mode, both channels start

up in asynchronous mode, and the bottom FET body diode

is used for circulating current during the top 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 50uA and soft start timing can be set by

selection of an appropriate soft start capacitor value.

Frequency Set and Phasing

The switching frequency can be programmed by connect-

ing a resistor from the FREQ pin to AGND. The PHASING

pin controls the phase shift between the master sawtooth

and slave sawtooth which allows the adjustment of the

phase shift for maximum noise immunity by controlling

the timing between master and slave transition. A resis-

tive divider is used from the FREQ pin to AGND and the

divided voltage is fed to the PHASING pin as depicted.

The offset of the current sharing error amplifier is

R13

trimmed whthin the range of -2mV to 0mV. The po-

larity being such that the slave is OFF if the master

has no current.

R19

SC2677B

Power Good

© 2009 Semtech Corp.

7

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