SC2677B(2004) 데이터 시트보기 (PDF) - Semtech Corporation

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SC2677B
(Rev.:2004)

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. Both Error Amplifier inputs on Channel 2 in-

puts are uncommitted and available externally. This al-

lows 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 voltage feed-

back path from their own output. In this mode, posi-

tive input of the Error Amplifier 2 is connected exter-

nally to Vref. If the application uses a common input

voltage, the sawtooth phase shift between the chan-

nels 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 own power budget.

Soft Start/Enable

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

tions. If held below the Enable threshold, both channels

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 Am-

plifier will control the duty cycle before the Soft Start

End threshold is reached. To avoid boost problems dur-

ing startup in current share mode, both channels start

up in asynchronous mode, and the bottom FET body di-

ode is circulates current during the top FET off time. When

the SS/ENA pin reaches the Soft Start Transition thresh-

old, the channels begin operating in synchronous mode

for improved efficiency. The Soft Start pin sources ap-

proximately 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.

R13

R19

SC2677B

The offset of the current sharing Error Amplifier is

trimmed within the range of -2mV to 0mV. The Slave

is OFF if the Master has no current.

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 the Power Good window.

© 2004 Semtech Corp.

7

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