FP6787 datasheet(17/20 Pages) FITIPOWER

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FP6787/A-1.3-JAN-2012

FP6787/A

85T

fitipower integrated technology lnc.

Application Information

Inductor Selection

Although small physical size and high efficiency are

major concerns, the inductor should have low core

losses at 640 kHz and series resistance (DCR,

copper wire resistance).

The minimum inductor

value, peak current rating and series resistance will

affect the converter efficiency, maximum output load

capability, transient response time and output voltage

ripple.

The inductor selection depends on input

voltage, output voltage and maximum output current,

Very high inductor value minimize the current ripple

and therefore reduce the peak

current,

which

decreases core losses in the inductor and conduct

losses in the entire power path.

However, large

inductor values also require more energy storage and

more turns of wire.

The size of inductor will become

bigger and increase conduct losses.

Low inductor

values decrease the size but increase the current

ripple and the peak current.

Choosing the inductor

values is based on the application.

The inductor selection depends on the switching

frequency and current ripple by the following formula:

VIN

fOSC

VIN

VOUT

where fOSC is the 640kHz switching frequency of the

FP6787/A. IL is the inductor ripple current.

In addition, it is important to ensure the inductor

saturation current exceeds the peak value of inductor

current in application to prevent core saturation.

Calculate the ripple current at the operating point and

the peak current required for the inductor:

IL VO 1

/ L fOSC

IL MA

IO max

Rectifier Diode Selection

A high-speed diode is necessary due to the high

switching

frequency.

The

Schottky

diode

recommended because of their fast recovery time and

low forward drop voltage for better efficiency.

The

forward drop voltage of Schottky diode will result in

the conduction losses in the diode, and the diode

capacitance (CT or CD) will cause the switching

losses.

Therefore, it is necessary to consider both

forward voltage drop and diode capacitance for diode

selection.

In addition, the reverse voltage rating of

this diode should 1.3 times of the maximum output

voltage.

The rectifier diode must meet the peak

inductor current requirement.

Flying Capacitor Selection

Increase of flying capacitor value results in a rise of

output

capability

with

smaller

ripple

voltage,

therefore flying capacitor is an important component

in charge pump system.

The voltage rating for

flying capacitor value is given by:

VCFL

VIN N

Where VCFLY is the voltage rating of charge pump

flying capacitor, N is number of charge pump

stages.

Output Capacitor Selection

The FP6787/A is permissible in using ceramic

capacitor for TFT LCD panel application.

The

value of capacitor depends on acceptable voltage

ripple.

Selecting

output

capacitor

must

consider the output ripple voltage and the ripple

current.

The ESR of capacitor is a major factor to

the output ripple.

For lower output voltage ripple,

the low ESR ceramic capacitor is recommended.

The ripple voltage is given by:

VO IL ESR

fOSC CO

The common aluminum-electrolytic capacitors have

high ESR and should be avoided.

Ceramic

capacitors have the lowest ESR in general.

Input Capacitor Selection

The input capacitor can reduced peak current and

noise at power source.

It should have 10

μF at

least and can be increased for better input voltage

filtering.

For better input bypassing, low ESR

ceramic capacitor is recommended for better

performance.

Output Capacitor of Charge Pump Selection

The capacitor which tenfold flying capacitor is

suitable for the output capacitor of charge pump.

FP6787 Datasheet(PDF) 17 Page - Fitipower Integrated Technology Inc.

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