SC1175CSWTRT datasheet(21/23 Pages) SEMTECH

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 2004 Semtech Corp.

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SC1175

POWER MANAGEMENT

Power and signal traces must be kept separated for

noise considerations. Feedback, current sense traces

and analog 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

enclosed loop area must be kept small to minimize

inductance 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 keeping 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 provide the best gate current return path.

Analog ground (GND) should be returned to the ground

side of the output capacitors so that the analog

circuitry in the controller has an electrically quiet

reference and to provide the greatest feedback

accuracy. The problem is that the differential voltage

capability of the two IC grounds is limited to about 1V

for proper operation and so the physical separation

between the two grounds must also be minimized. If

the grounds are too far apart, fast current transitions

in the connection can generate voltage spikes exceed-

ing the 1V capability, resulting in unstable and erratic

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 possible. The small ceramic, noise-filtering capaci-

tors on the current sense lines should also be placed

as close to the IC as possible.

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