SC418 datasheet(26/30 Pages) SEMTECH

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SC418

Applications Information (continued)

tor. For a conventional PWM constant-frequency con-

verter, as load increases the duty cycle also increases

slightly to compensate for IR and switching losses in the

MOSFETs and inductor. A adaptive on-time converter

must also compensate for the same losses by increasing

the effective duty cycle (more time is spent drawing

energy from V

IN as losses increase). Because the on-time is

essentially constant for a given V

OUT/VIN combination, to

offset the losses the off-time will reduce slightly as load

increases. The net effect is that switching frequency

increases slightly with increasing load.

PCB Layout Guidelines

As with any switch-mode converter, good PCB layout is

essential to achieving high performance. The following

guidelines will provide an optimum PCB layout.

The device layout can be considered in four parts.

Grounding for PGND and AGND

Power components

Low-noise analog circuits

Bypass capacitors

Grounding for PGND and AGND

A ground plane layer for PGND is recommended

to minimize the effects of switching noise, resis-

tive losses, and to maximize heat removal from

the power components.

A separate ground plane or island should be used

for AGND and all associated components. The

AGND island should avoid overlapping switching

signals on other layers (DH/DL/BST/LX).

Connect PGND and AGND together with a zero

ohm resistor or copper trace. Make the connec-

tion near the AGND and PGND pins of the IC.

Power Components

Use short, wide connections between the power

components.

Input capacitors and high-side MOSFETs

High-side and Low-side MOSFETs and inductor

(LX connection). Use wide copper traces to

provide high current carrying capacity and for

heat dissipation.

Inductor and output capacitors.

•

All PGND connections — the input capacitors,

low-side MOSFETs, output capacitors, and the

PGND pin of the SC418.

An inner layer ground plane is recom-

mended.

Each power component requires a short,

low impedance connection to the PGND

plane.

Place vias to the PGND plane directly near

the component pins.

Use short wide traces for the pin connections

from the SC418 (LX, DH, DL and BST). Do not

route these traces near the sensitive low-noise

analog signals (FB, FBL, TON, V

OUT).

Low-noise Analog Circuits

Low-noise analog circuits are sensitive circuits that are

referenced to AGND. Due to their high impedance and

sensitivity to noise, it is important that these circuits be

kept away from the switching signals.

Use a plane or solid area for AGND. Place all

components connected to AGND above this

area.

Use short direct traces for the AGND con-

nections to all components.

Place vias to the AGND plane directly near

the component pins.

Proper routing of the V

OUT sense trace is essential

since it feeds into the FB resistor divider. Noise

on the FB waveform will cause instability and

multiple pulsing.

Connect the V

OUT sense trace directly to

the output capacitor or a ceramic bypass

capacitor.

Route this trace over to the VOUT pin,

carefully avoiding all switching signals

and power components.

Route this trace in a quiet layer if possible.

Route this trace away from the switch-

ing traces and components, even if the

trace is longer. Avoid shorter trace routing

through the power switching area.

If a bypass capacitor is used at the IC side

of the V

OUT sense trace, it should be placed

near the FB resistor divider.

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