FR9855 datasheet(13/16 Pages) FITIPOWER

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FR9855-Preliminary 0.3-JAN-2016

FR9855

85T

Application Information (Continued)

R PPL (

O T

FO C L

O T

R PPL (

R PPL (C

O T

O C2

L CO T

O T

Where FOSC is the switching frequency, L is the

inductance value, VIN is the input voltage, ESR is the

equivalent series resistance value of the output

capacitor, ESL is the equivalent series inductance

value of the output capacitor and the COUT is the

output capacitor.

Low

ESR

capacitors

are

preferred

use.

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.

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

inductor

peak-to-peak ripple current:

O T

FO C L

O T

A good compromise value between size and

efficiency is to set the peak-to-peak inductor ripple

current Δ

equal to 30% of the maximum load

current. But setting the peak-to-peak inductor ripple

current

L between 20%~50% of the maximum load

current is also acceptable. Then the inductance can

be calculated with the following equation:

L 0.3

O T(MA

O T

N FO C

External Diode Selection

For 5V input applications, it is recommended to add

an external boost diode. This helps improving the

efficiency. The boost diode can be a low cost one

such as 1N4148.

FR9855

VIN

BST

VIN

1N4148

REG Capacitor Selection

Connect a 1uF ceramic capacitor between the REG

and GND, This helps stabilize the internal regulator

voltage.

Ceramic Capacitor

VOUT

GND

Probe Ground

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