FP6329ASOGTR datasheet(14/16 Pages) FITIPOWER

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FP6329/A/B-1.0-APR-2010

FP6329/A

85T

fitipower integrated technology lnc.

Application Information (Continued)

Layout Notice

When

designing

high

frequency

switching

regulated power supply, layout is very important.

Using a good layout can solve many problems

associated with these types of supplies.

The

problems due to a bad layout are often seen at high

current levels and are usually more obvious at large

input to output voltage differentials.

Some of the

main problems are loss of regulation at high output

current

and/or

large

input

output

voltage

differentials, excessive noise on the output and

switch waveforms, and instability.

Using the simple

guidelines that follow will help minimize these

problems.

(1) Inductor

Always try to use a low EMI inductor with a ferrite

type closed core.

Open core can be used if they

have low EMI characteristics and are located a bit

away

from

the

low

power

traces

and

components.

(2) Feedback

Try to put the feedback trace as far from the inductor

and noisy power traces as possible.

You would also

like the feedback trace to be as direct as possible

and somewhat thick.

These two sometimes involve

a trade-off, but keeping it away from inductor EMI

and other noise sources is the more critical of the

two.

It is often a good idea to run the feedback trace

on the side of the PCB opposite of the inductor with a

ground plane separating the two.

(3) Filter Capacitors

When using a low value ceramic input filter capacitor,

it should be located as close to the VIN pin of the IC

as possible.

This will eliminate as much trace

inductance effects as possible and give the internal

IC rail a cleaner voltage supply.

Sometimes using a

small resistor between VCC and IC VCC pin will more

useful because the RC will be a low-pass filter.

Some designs require the use of a feed-forward

capacitor connected from the output to the feedback

pin as well, usually for stability reasons.

(4) Compensation

If external compensation components are needed

for stability, they should also be placed closed to

the

IC.

Surface

mount

components

are

recommended here as well for the same reasons

discussed for the filter capacitors.

(5) Traces and Ground Plane

Make all of the power (high current) traces as short,

direct, and thick as possible.

It is a good practice

on a standard PCB board to make the traces an

absolute

minimum

15mils

(0.381mm)

per

Ampere.

The inductor, output capacitors, and low

side switch should be as close to each other

possible.

This will reduce lead inductance and

resistance as well which in turn reduces noise

spikes, ringing, and resistive losses which produce

voltage errors.

The grounds of the IC, input

capacitors, output capacitors, and low side switch

should be connected close together directly to a

ground plane.

It would also be a good idea to

have a ground plane on both sides of the PCB.

For multi-layer boards with more than two layers, a

ground plane can be used to separate the power

plane

and

the

signal

plane

for

improved

performance.

It is good practice to use one

standard via per 200mA of current if the trace will

need to conduct a significant amount of current

from one plane to the other.

Due to the way

switching regulators operate, there are power on

and power off states.

During each state there will

be a current loop made by the power components

that are currently conducting.

Place the power

components so that during each of the two states

the current loop is conducting in the same direction.

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