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 2004 Semtech Corp.

www.semtech.com

SC1175

POWER MANAGEMENT

NOTES

(1) Block 1 (top) is the Master and Block 2 (bottom) is the Slave in current sharing configuration.

(2) For independant operation there is no Master or Slave.

Main Loop(s)

The SC1175 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 inverted sawtooth for

channel 2. Each channel has its own current limit com-

parator. Channel 1 has the positive input of the error

amplifier internally connected to Vref. Channel 2 has

both inputs of the error amplifier uncommitted and avail-

able externally. This allows the SC1175 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-

Block Diagram

Applications Information - Theory of Operation

back path from their own output. In this mode, the

positive input of 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 control-

ler, as before, but error amp 2 monitors and amplifies

the difference in voltage across the output current

sense resistors of channel 1 and channel 2 (Master

and Slave) and adjusts the Slave duty cycle to match

output currents. Because of finite gain and offsets in

the loop, the resistor ratio for perfect current match-

ing is not 1:1. The Master and Slave channels still have