SC2672STRT Datasheet SC2672STRT | P7-P9

 2004 Semtech Corp.

www.semtech.com

SC2672

POWER MANAGEMENT

Main Loop(s)

The SC2672 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.

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 inputs of error amplifier 2 are 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.

Applications Information - Theory of Operation

Power Good

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

The Soft Start/Enable (SS/ENA) pin serves several func-

tions. If held below the Soft Start Enable threshold, both

channels are inhibited. DH1 and DH2 will be low, turning

off the top FETs. Between the Soft Start Enable thresh-

old 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 amplifier will be controlling the duty cycle be-

fore the Soft Start End threshold is reached. To avoid

boost problems during startup in current share mode,

both channels start up in asynchronous mode, and the

bottom FET body diode is used for recirculating current

during the FET off time. When the SS/ENA pin reaches

the Soft Start Transition threshold, the channels begin

operating in synchronous mode for improved efficiency.

The soft start pin sources approximately 25uA and soft

start timing can be set by selection of an appropriate

soft start capacitor value.

8 2004 Semtech Corp.

www.semtech.com

SC2672

POWER MANAGEMENT

Power and signal traces must be kept separated for noise

considerations. Feedback, current sense traces and ana-

log ground should not cross any traces or planes carrying

high switching currents, such as the input loop or the

phase node.

The input loop, consisting of the input capacitors and

both MOSFETs must be kept as small as possible. All of

the high switching currents occur in this loop. The en-

closed loop area must be kept small to minimize induc-

tance and radiated and conducted emissions. Designing

for minimum trace length is not always the best approach,

often a more optimum layout can be achieved by keep-

ing loop area constraints in mind.

It is important to keep gate lengths short, the IC must be

close to the power switches. This is more difficult in a

dual channel device than a single and requires that the

two power paths run on either side of a centrally located

controller.

Grounding requirements are always conflicting in a buck

converter, especially at high power, and the trick is to

achieve the best compromise. Power ground (PGND)

should be returned to the bottom MOSFET source to pro-

vide the best gate current return path. Analog ground

(GND) should be returned to the ground side of the out-

put capacitors so that the analog circuitry in the control-

ler has an electrically quiet reference and to provide the

greatest feedback accuracy. The problem is that the dif-

ferential voltage capability of the two IC grounds is lim-

ited to about 1V for proper operation and so the physical

separation between the two grounds must also be mini-

mized. If the grounds are too far apart, fast current tran-

sitions in the connection can generate voltage spikes

exceeding the 1V capability, resulting in unstable and er-

ratic behavior.

The feedback divider must be close to the IC and be

returned to analog ground. Current sense traces must

be run parallel and close to each other and to analog

ground.

The IC must have a ceramic decoupling capacitor across

its supply pins, mounted as close to the device as pos-

sible. The small ceramic, noise-filtering capacitors on the

current sense lines should also be placed as close to the

IC as possible.

Layout Guidelines

 2004 Semtech Corp.

www.semtech.com

SC2672

POWER MANAGEMENT

Semtech Corporation

Power Management Products Division

200 Flynn Road, Camarillo, CA 93012

Phone: (805)498-2111 FAX (805)498-3804

Contact Information

Outline Drawing - SO-16

Land Pattern - SO-16

P1-P3 P4-P6 P7-P9

SC2672STRT

Mfr. #:

Buy SC2672STRT

Manufacturer:

Semtech

Description:

Switching Controllers DUAL SYNCH D//DC CNTRL SHR CIR

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New from this manufacturer.

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SC2672STRT