FR9808 datasheet(10/13 Pages) FITIPOWER

10 / 13 page

FR9808-1.3-OCT-2012

FR9808

85T

fitipower integrated technology lnc.

Application Information

Output Voltage Setting

The output voltage VOUT is set using a resistive

divider from the output to FB.

The FB pin regulated

voltage is 0.805V.

Thus the output voltage is:

O T=0.805

Table 1 lists recommended values of R1 and R2 for

most used output voltage.

Table 1

Recommended Resistance Values

VOUT

R1 (1%)

R2 (1%)

30.9

kΩ

5.76

kΩ

3.3V

30.9

kΩ

9.76

kΩ

2.5V

4.99

kΩ

2.32

kΩ

1.8 V

4.99 kΩ

3.92 kΩ

1.2 V

4.99 kΩ

10 kΩ

Resistors R1 and R2 should be placed close to the

FB pin to prevent stray pickup.

Input Capacitor Selection

The use of the input capacitor is filtering 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:

(RMS = O T

O T

Where D is the duty cycle of the power MOSFET.

This function reaches the maximum value at D=0.5

and the equivalent RMS current is equal to IOUT/2.

The following diagram is the graphical representation

of above equation.

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.

It is recommended that the input EC capacitor should

be added for applications if the FR9808 will suffer

high spike input voltage (ex. hot plug test).

It can

eliminate the spike voltage and induced the IC

damage from high input voltage stress (see Note1).

FR9808

330

μF/25V

EC x1

VIN

18V to 21V

μF/25V

MLCC x1

Output Capacitor Selection

The output capacitor is used to keep the DC output

voltage and supplies the load transient current.

When operating in constant current mode, the output

ripple is determined by four components:

R PPL

t = R PPL C t + R PPL SR t

+ R PPL ( SL t + O S t

The following figures show the form of the ripple

contributions.

VRIPPLE(ESR)(t)

VRIPPLE(ESL) (t)

VRIPPLE(C) (t)

(t)

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