FP6791C datasheet(12/13 Pages) FITIPOWER

12 / 13 page

FP6791C-1.1-FEB-2012

FP6791C

85T

fitipower integrated technology lnc.

Applications Information (Continued)

Select

MOSFET

whose

RDS-ON

and

input

capacitance are optimal depending on the usage

conditions.

The input voltage is supplied for the

gate voltage of the MOSFET, so select a MOSFET

with a gate withstanding voltage that is equal to

maximum usage value of the input voltage or higher

and drain withstanding voltage that is equal to the

output voltage and diode voltage or higher.

enhancement N-channel MOSFET can be selected

by the following guidelines:

1. Low RDS-ON.

2. Low gate threshold voltage.

3. Rated continuous drain current should be larger

than the peak inductor current.

4. Low gate capacitance.

If a MOSFET with a threshold near the UVLO

detection voltage is used, a large current may flow

and stop the output voltage from rising and possibly

generating heat in the worst case.

Select a

MOSFET with a threshold that is sufficiently lower

than the UVLO detection voltage value.

Feed-forward Capacitor Selection

The feed-forward capacitor (CF) is used to improve

the performance of internally compensated DC-DC

converter.

To optimize transient response, a

feed-forward capacitor value is chosen such that the

gain and phase of the feedback increases the

bandwidth of the converter, while still maintaining an

acceptable phase margin.

In general, capacitor

causes the loop gain to crossover too high in

frequency and the feed-forward capacitor phase

contribution is insufficient, resulting in unacceptable

phase margin or instability.

The following process

outlines a step by step procedure for optimizing the

feed-forward capacitor.

Determine the crossover frequency of converter.

2. Once

the

crossover

frequency is

known,

the

equation allows calculation of feed-forward capacitor

value which prompts a good compromise between

bandwidth

improvement

and

acceptable

phase

margin.

The feed-forward capacitor is selected by the

following formula:





















crossover

Layout Recommendation

For high frequency switching power supplies, the

device

’s performance including efficiency, output

noise, transient response and control loop stability

are dramatically affected by PCB layout.

There are some general guidelines for layout:

1. Place the external power components (the input

capacitors,

output

capacitors,

inductor

and

diode, etc.) in close proximity to the device.

The traces which connect to these components

should be as short and wide as possible to

minimize parasitic inductance and resistance.

2. Place output capacitor next to Schottky diode as

possible.

3. Place input capacitor close to the VIN pin.

4. The input and output capacitors

’ ground should

be wide and short enough to connect to a ground

plane.

5. The feedback network should sense the output

voltage directly form the point of load, and be as

far away from noisy loop as possible.

6. The compensation circuit should be away from

the power loops and should be shielded with a

ground trace to prevent noise coupling.

7. Place the resistor close to RDUTY and ROSC

pin.

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