FP6329ASOGTR datasheet(11/16 Pages) FITIPOWER

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FP6329/A/B-1.0-APR-2010

FP6329/A

85T

fitipower integrated technology lnc.

Application Information

Introduction

The FP6329/A integrated circuit is a synchronous

PWM controller; it operates over a wide input voltage

range.

Being low cost, it is a very popular choice of

PWM controller.

This section will describe the

FP6329/A application suggestion.

The operation

and the design of this application will also be

discussed in detail.

Design Procedures

This section will describe the steps to design

synchronous buck system, and explains how to

construct basic power conversion circuits including

the design of the control chip functions and the basic

loop.

(1) Synchronous Buck Converter

Since this is a buck output system, the first quantity

to be determined is the duty cycle value.

The

formula calculated the PWM duty ratio; apply to the

system which we propose to design:

Duty ratio D =

VO +VDS(sat), Lowside N

VIN - VDS(sat), Highside N

, 0= D= 1

+ VDS(sat),Lowside N

TON

(2) Inductor Selection

To find the inductor value it is necessary to consider

the inductor ripple current.

Choose an inductor

which operated in continuous mode down to 10

percent of the rated output load:

ΔI

L = 2 x 10% x IO

The inductor “L” value for this system is connected to

be:

L ≧

(VIN - VDS(sat) – VO) x DMIN

ΔI

L x fS

If the core loss is a problem, increasing the

inductance of L will be helpful.

But large inductor

values reduce the converter’s response time to a

load transient.

(3) Output Capacitor Selection

The output capacitor is required to filter the output

noise and provide regulator loop stability.

When

selecting an output capacitor, the important capacitor

parameters are Equivalent Series Resistance (ESR),

the RMS ripples current rating, the voltage rating,

and capacitance value.

For the output capacitor,

the ESR value is the most important parameter.

The ESR can be calculated from the following

formula.















RIPPLE

ΔI

ESR

An aluminum electrolytic capacitor's ESR value is

related to the capacitance and its voltage rating.

most case, higher voltage electrolytic capacitors

have lower ESR values.

Most of the time,

capacitors with much higher voltage ratings may be

needed to provide the low ESR values required for

low output ripple voltage.

The capacitor voltage rating should be at least 1.5

times greater than the output voltage, and often

much higher voltage ratings are needed to satisfy

the low ESR requirements needed for low output

ripple voltage.

(4) Input Capacitor Selection

The RMS current rating of the input capacitor can

be calculated as below:

)

(

OUT

IN((rms)





This capacitor should be located close to the IC

using short leads and the volt age rating should be

approximately 1.5 times the maximum input voltage.

(5) Output N-channel MOSFET Selection

The

current

ability

the

output

N-channel

MOSFETs must be at least more than the peak

switching current IPK.

The voltage rating VDS of the

N-channel MOSFETs should be at least 1.25 times

the maximum input voltage.

Choose the low

RDS-ON MOSFETs for reducing the conduction power

loss.

Choose the low CISS MOSFETs for reducing

the switching loss.

But most of time, the two

factors

are

trade-off.

Consider

the

system

requirement and define the MOSFETs rating.

The

MOSFETs must be fast (switch time) and must be

located close to the FP6329/A using short leads and

short printed circuit traces.

In case of a large

output current, we must layout a copper to reduce

the temperature of these two MOSFETs.

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