FR9855 datasheet(12/16 Pages) FITIPOWER

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FR9855-Preliminary 0.3-JAN-2016

FR9855

85T

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 0.765V. Thus the output voltage equation

is:

O T 0. 6

Table 3 lists recommended values of R1 and R2 for

most used output voltage.

Table 3

Recommended Resistance Values

VOUT

118k

Ω

21k

Ω

3.3V

69.8k

Ω

21k

Ω

1.8V

28k

Ω

21k

Ω

1.2V

12k

Ω

21k

Ω

1.05V

7.87

kΩ

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 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:

C N(RM

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

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.

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 PPL

R PPL C t

R PPL

R t

R PPL (

L t

The following figures show the form of the ripple

contributions.

VRIPPLE(ESR)(t)

VRIPPLE(ESL) (t)

VRIPPLE(C) (t)

VNOISE (t)

VRIPPLE(t)

0.5

1.5

2.5

10 20 30 40 50 60 70 80 90

D (%)

(t)

5.5A

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