FR9761 datasheet(10/12 Pages) FITIPOWER

10 / 12 page

FR9761-1.1-MAR-2012

FR9761

85T

fitipower integrated technology lnc.

Application Information

Output Voltage Setting

The output voltage VOUT is set by using a resistive

divider from the output to FB.

The FB pin regulated

voltage is 0.925V.

Thus the output voltage is:

VO T=0. 25

Table 1

Recommended Resistance Values

VOUT

12V

121

KΩ

44.2

KΩ

3.3V

26.1

KΩ

2.5

16.9

KΩ

1.8V

9.53

KΩ

Place resistors R1 and R2 close to FB pin to prevent

stray pickup.

Input Capacitor Selection

The use of the input capacitor is controlling the input

voltage ripple and the MOSFETS switching spike

voltage.

Because the input current to the step-down

converter is discontinuous, the input capacitor is

required to supply the current to the converter to keep

the DC input voltage.

The capacitor voltage rating

should be 1.25 to 1.5 times greater than the

maximum input voltage.

The input capacitor ripple

current RMS value is calculated as:

II (RMS =IO T

Where D is the duty cycle of the power MOSFET.

A low ESR capacitor is required to keep the noise

minimum.

Ceramic capacitors are better, but

tantalum or low ESR electrolytic capacitors may also

suffice.

When

using

tantalum

electrolytic

capacitors, a 0.1

μF ceramic capacitor should be

placed as close to the IC as possible.

Output Capacitor Selection

The output capacitor is used to keep the DC output

voltage and supply the load transient current.

Low

ESR capacitors are preferred.

Ceramic, tantalum

or low ESR electrolytic capacitors can be used

depending on the output ripple requirements.

Add

a 100

μF or 470μF Low ESR electrolytic capacitor

when operating in high input voltage range (VIN >

20V).

It can improve the device’s stability. The

output ripple voltage

ΔV

OUT is described as:

VO T

FOSC

VO T

VO T= I

R SR

FOSC CO T

Where ΔI is the peak-to-peak inductor ripple current,

FOSC is the switching frequency, L is the inductance

value, VIN is the input voltage, VOUT is the output

voltage, RESR is the equivalent series resistance

value of the output capacitor, and the COUT is the

output

capacitor.

When

using

the

ceramic

capacitors, the RESR can be ignored and the output

ripple voltage ΔV

OUT is shown as:

VO T=

FOSC CO T

When using tantalum or electrolytic capacitors,

typically 90% of the output voltage ripple is

contributed by the ESR of output capacitors.

The

output ripple voltage ΔVO T can be estimated as:

VO T= I R SR

Output Inductor Selection

The output inductor is used for storing energy and

filtering output ripple current.

But the trade-off

condition often happens between maximum energy

storage and the physical size of the inductor.

The

first consideration for selecting the output inductor is

to make sure that the inductance is large enough to

keep the converter in the continuous current mode.

That will lower ripple current and result in lower

output ripple voltage.

A good rule for determining

the inductance is setting the peak-to-peak inductor

ripple current ΔI almost equal to 30% of the

maximum

load

current.

Then

the

minimum

inductance can be calculated with the following

equation:

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