6 2004 Semtech Corp.
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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
