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ã 2000 Semtech Corp.

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SC1175

POWER MANAGEMENT

Applications Information

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.

Theory of Operation

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 chan-

nel 1 and an inverted sawtooth for channel 2. Each

channel has its own current limit comparator. Chan-

nel 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 com-

mon or different input voltages and different

output voltages. The two channels each have

their own voltage feedback path from their own

output. In this mode, the positive input of error

amplifier 2 is connected 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.

b) Two channels operating in current sharing

mode with common output voltage and either

common input voltage or different input voltages.

In this mode, channel 1 operates as a voltage

mode Buck controller, 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-

Block Diagram