FR9801 datasheet(11/14 Pages) FITIPOWER

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FR9801-0.5-JUL-2013

FR9801

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 0.8V.

Thus the output voltage is:

T=0.

Table 2 lists recommended values of R1 and R2 for

most used output voltage.

Table 2

Recommended Resistance Values

VOUT

30.9k

Ω

5.76k

Ω

3.3V

30.9k

Ω

9.76k

Ω

2.5V

4.99k

Ω

2.32k

Ω

1.8V

4.99k

Ω

3.92k

Ω

1.2V

4.99k

Ω

10k

Ω

Place resistors R1 and R2 close to 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

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 =

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 or 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.

When operating in constant current mode, the

output ripple is determined by four components:

R PPLE t = R PPLE C t + R PPLE ESR t

+ R PPLE(ESL t +

SE t

The following figures show the form of the ripple

contributions.

VRIPPLE(ESR)(t)

VRIPPLE(ESL) (t)

VRIPPLE(C) (t)

VNOISE (t)

VRIPPLE(t)

(t)

1.5A

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