FR9886 データシートの表示（PDF） - Unspecified

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FR9886

23V, 2.5A, 340Khz Synchronous Step-Down DC/DC Converter

Unspecified 

fitipower integrated technology lnc.

Application Information

FR9886

85T

VRIPPLE(ESR)=IL  R(ESR)

  VRIPPLE(ESL)=IL  L(ESL)  FOSC 

1

D 1 D

VRIPPLE(C)=IL 

1

8  COUT  FOSC

IL



VOUT

FOSC  L



1

VOUT

VIN





Where Δ IL 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, R(ESR) is the equivalent series

resistance value of the output capacitor, L(ESL) is the

equivalent series inductance value of the output

capacitor and the COUT is the output capacitor. The

following diagram is an example to graphical

represent Δ IL equation.

L=4.7uH

L=6.8uH

L=10uH

VOUT=3.3V, FOSC=340KHz

Low ESR capacitors are preferred. Ceramic,

tantalum or low ESR electrolytic capacitors can be

used depending on the output ripple requirement.

When using the ceramic capacitors, the ESL

component is usually negligible.

It is important to use the proper method to eliminate

high frequency noise when measuring the output

ripple. The figure shows how to locate the probe

across the capacitor when measuring output ripple.

Removing the scope probe plastic jacket in order to

expose the ground at the tip of the probe. It gives a

very short connection from the probe ground to the

capacitor and eliminating noise.

Probe Ground

VOUT

GND

Ceramic Capacitor

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. The inductance value

should be determined to set the peak-to-peak

inductor ripple current Δ IL around 20% to 50%

of the maximum load current. Then the

inductance can be calculated with the following

equation:

  IL= 0.2~0.5  IOUT(MAX)

  L 

VIN-VOUT







FOSC

VOUT

 ΔIL



VIN





To guarantee sufficient output current, peak

inductor current must be lower than the FR9886

high-side MOSFET current limit. The peak

inductor current is as below:

IPEAK=IOUT(MAX)+ ΔIL

2

FR9886- 1.0-APR-2011

11

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