SC908 datasheet(28/30 Pages) SEMTECH

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SC908

Applications Information (continued)

a few volts of bias. This is especially true of inexpensive

dielectrics. Insufficient SVOUT capacitance can cause

rapid decay of output voltage between PSAVE bursts,

resulting in poor low-load efficiency, PSAVE/PWM mode

cycling, and other erratic behaviors.

Switcher Grounding and PCB Layout Consider-

ations

Poor layout can degrade the performance of the DC-DC

converter and can contribute to EMI problems, ground

bounce and resistive voltage losses. Poor regulation and

instability can result.

A few simple design rules can be implemented to ensure

good layout:

Place the inductor and filter capacitors as close

to the device as possible and use short wide

traces between the power components.

Route the output voltage feedback path away

from the inductor and LX node to minimize

noise and magnetic interference.

Maximize ground metal on the component side

to improve the return connection and thermal

dissipation. Separation between the SLX node

and GND should be maintained to avoid cou-

pling of switching noise to the ground plane.

Use a ground plane with several vias connecting

to the component side ground to further reduce

noise interference on sensitive circuit nodes.

•

Charger Grounding and PCB Layout Consider-

ations

While layout for linear devices is generally not as critical as

for a switching application, careful attention to detail will

ensure reliable operation.

Attaching the part to a larger copper footprint

will enable better heat transfer from the device,

especially on PCBs with internal ground and

power planes.

Place the input, output and bypass capacitors

close to the device for optimal transient response

and device behavior.

Connect all ground connections directly to the

ground plane. If there is no ground plane,

connect to a common local ground point before

connecting to board ground.

The DGND pin and PGND pin should be con-

nected directly to the PCB ground plane as close

to the part as possible. The thermal pad should

be connected to the ground plane with thermal

vias under the SC908.

The nodes indicated as AGND in the Block

Diagram should be connected together and to

the AGND pin. The AGND pin should be tied to

the DGND pin at a single point close to the

SC908.

Route the BSEN trace directly to the battery posi-

tive terminal connection on the PCB.

•

SC908 Datasheet(PDF) 28 Page - Semtech Corporation

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