FR9710 datasheet(10/12 Pages) FITIPOWER

10 / 12 page

FR9710-1.1-MAR-2012

FR9710

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 1.23V.

Thus the output voltage is:

VOUT=1.2

R2 recommended value is 10kΩ, so R1 is

determined by:

R1= .1

VOUT 1.2 kΩ

Table 1 lists recommended values of R1 and R2 for

most used output voltage.

Table 1

Recommended Resistance Values

VOUT

68 k

Ω

22 k

Ω

3.3V

16.8 k

Ω

10 k

Ω

1.8V

4.7 k

Ω

10 k

Ω

1.2 V

4.7 k

Ω

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

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:

IIN(RMS =IOUT

D 1 D

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:

VOUT

FOSC L

VOUT

VIN

VOUT= I

R SR

FOSC COUT

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:

VOUT=

FOSC COUT

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 ΔV

OUT can be estimated as:

VOUT= I R SR

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