FP6375 datasheet(11/13 Pages) FITIPOWER

11 / 13 page

FP6375-1.0-JUL-2013

FP6375

85T

fitipower integrated technology lnc.

Application Information (Continued)

PPL ( S )

F SC L

PPL ( SL)

PPL (C)

SC2

L C

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 to use.

Ceramic,

tantalum or low ESR electrolytic capacitors can be

used depending on the output ripple requirements.

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.

Remove 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 eliminates 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:

F SC L

The following diagram is an example to graphically

represent

L equation.

VOUT=1.8V, FOSC=1MHz

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 Δ

between 20%~50% of

the

maximum load current is also acceptable.

Then

the inductance can be calculated with the following

equation:

L 0.

(MA )

F SC

To guarantee sufficient output current, peak inductor

current must be lower than the FP6375 high-side

MOSFET current limit.

The peak inductor current

is shown as below:

P A

(MA )

0.2

0.4

0.6

0.8

1.2

2.5

3.5

4.5

5.5

VIN (V)

Ceramic Capacitor

VOUT

GND

Probe Ground

L=2.2

μH

L=1.2

μH

L=1.8

μH

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