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POWER MANAGEMENT

SC2677

Main Loop(s)

Main Loop(s)

Main Loop(s)

Main Loop(s)

Main Loop(s)

The SC2677 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. Channel 2 has both inputs of the

error amplifier uncommitted and available externally. This

allows the SC2677 to operate in two distinct modes.

a) Two independent channels with either common

or different input voltages and different output volt-

ages. The two channels each have their own volt-

age feedback path from their own output. In this

mode, positive input of the error amplifier 2 is con-

nected 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.

It is possible to sequence the start up of the chan-

nel with an RC delay between the reference and

+IN2. The capacitor will be internally reset during

UVLO and soft start.

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 out-

put current sense resistors of channel 1 and chan-

nel 2 (Master and Slave) and adjusts the Slave duty

cycle to match output currents. To controller also

works well for using the output choke winding resis-

tance as current sensing element (please refer the

application schematic for details). The amount of

the current of the slave channel vs.. the master

channel can be programmed according to the

application. This feature is especially useful when

two input sources are used and each source has its

power budget.

The offset of the current sharing error amplifier is

trimmed whthin the range of -2mV to 0. The polar-

ity being such that the slave is OFF if the master

has no current.

Power Good

Power Good

Power Good

Power Good

Power Good

Applications Information - Theory of Operation

The controller provides a power good signal. This is an

open collector output, which is pulled low if the output

voltage is outside of the power good window.

Soft Start/Enable

Soft Start/Enable

Soft Start/Enable

Soft Start/Enable

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 be controlling the duty cycle before the Soft

Start End threshold is reached. To avoid boost problems

during start-up in current share mode, both channels start

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

ode is used for circulating current during the top FET off

time. When the SS/ENA pin reaches the Soft Start Tran-

sition threshold, the channels begin operating in synchro-

nous mode for improved efficiency. The soft start pin

sources approximately 50uA and soft start timing can be

set by selection of an appropriate soft start capacitor

value.

Frequency Set and Phasing

Frequency Set and Phasing

Frequency Set and Phasing

Frequency Set and Phasing

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.

U1

SC2677

R19

3.92K

R13

3.57K