FP6791C datasheet(11/13 Pages) FITIPOWER

11 / 13 page

FP6791C-1.1-FEB-2012

FP6791C

85T

fitipower integrated technology lnc.

Applications Information (Continued)

Where

expected efficiency at that operating point.

The value can be taken form an appropriate curve in

the typical operating characteristics.

∆I

L=inductor

ripple current, IL(MAX) =inductor peak current.

addition, the following equation used here assumes

a constant K, which is the ratio of the inductor

peak-to-peak AC current to average DV inductor

current.

A good compromise between the size of

the inductor versus loss and output ripple is to

choose a K 0.3 to 0.5.

The peak inductor current is

given by：

∆

UT(MAX) V UT

V (M )

Where K=ratio of the inductor peak-to-peak AC

current to average DC inductor current,

∆I

L=inductor

ripple current.

The inductor value is then given by:

)

MAX

(

OUT

OSC

)

MIN

(









Where

uty cycle

V (M )

VF V UT

(MAX)

ds(on)

VF V UT

VF = Catch diode forward drop

f = Switching frequency

The inductor

’s saturation current rating should

exceed IL(MAX) and the inductor DC current rating

should exceed IIN(DC,MAX).

Rectifier diode selection

The diode is the largest source of loss in DC-DC

converters.

A high speed diode is necessary due to

the high switching frequency.

The Schottky diodes

are recommended because of their fast recovery

time and low forward drop voltage for better

efficiency.

The forward drop voltage of the Schottky

diode will result in the conduction losses in the diode,

and the diode capacitance 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

output and peak inductor current requirement.

Output Capacitor Selection

The capacitor on the output side (COUT) is used for

sustaining the output voltage when the external

MOSFET or diode is switched on and smoothing the

ripple voltage.

Select an appropriate capacitance

value based on the load condition.

For lower

output

voltage

ripple,

the

low

ESR

ceramic

capacitor is recommended.

The output voltage

ripple consists of two components.

One is the

pulsating output ripple current through ESR, and the

other is the capacitive ripple caused by charging

and discharging.

RIPPLE

ESR

RIPPLE

































OSC

OUT

ESR

Where

∆V

O=output

voltage ripple,

∆I

L=inductor

ripple current, IL(MAX) = inductor peak current.

The optimal capacitor differs depending on the

inductor value, wiring, and application (output load),

so select the capacitor after performing sufficient

evaluation under the actual usage condition.

Input Capacitor Selection

The capacitor on input side (CIN) can stabilize the

input voltage and minimize peak current ripple form

the power source for better efficiency.

The value

of the capacitor depends on the impedance of the

input source used.

For better input bypassing, low

ESR ceramic capacitor is recommended for better

performance.

External Switch Transistor

An enhancement N-channel MOSFET or a bipolar

NPN transistor can be used as the external switch

transistor.

For high efficiency, it

’s ideal to use a

MOSFET with a low RDS-ON and small input

capacitance.

It is a more efficient switch than a

bipolar NPN transistor.

The RDS-ON and input

capacitance generally share a trade-off relationship.

The RDS-ON is efficient in a range which the output

current is relatively great during low frequency

switching, and the input capacitance is efficient in a

rang which the output current is middling during

high frequency switching.

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