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 2006 Semtech Corp.

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POWER MANAGEMENT

SC1176/SC1176A

NOTES

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

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

Main Loop(s)

The SC1176 is a dual, voltage mode synchronous

Buck controller, the two separate channels are identi-

cal 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 comparator. Channel 1 has the

positive input of the error amplifier internally connected

to Vref. Channel 2 has both inputs of the error ampli-

fier uncommitted and available externally. This allows

the SC1176 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

Block Diagram

Applications Information - Theory of Operation

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-