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FP6326

Synchronous Buck PWM DC-DC Controller

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fitipower integrated technology lnc.

FP6326/A

Application Information (Continued)

Calculating Formula

VOUT=VFB x ((R3/R2) + 1)

[ ] L ≥ VIN(min) - VDS(SAT) - VOUT × TON(max)

ΔIL

I = I - I RIPPPLE LOAD(max) LOAD(min)

COUT

ESR = [ VRIPPLE ]

ΔI L

VDC -Rating ≥1.5 × VOUT

CIN

IIN(rms) = IOUT × D(1 D)

VDC-rating ≥1.5 × VIN(max)

TON /(TON +TOFF )

D

VOUT / VIN

ΔIL

2 x 10% x IO

IIN(rms)

IOUT × D (1 D )

Layout Notice

When designing a 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 to 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

more 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 of 15mils (0.381mm) per

Ampere. The inductor, output capacitors, and

output diode (In synchronous case, means the 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 output diode (or

switch, if applicable) 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 (where the power traces and

components are) and the signal plane (where the

FP6326/A-1.1-JUL-2009

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