FR9801 datasheet(13/14 Pages) FITIPOWER

13 / 14 page

FR9801-0.5-JUL-2013

FR9801

85T

fitipower integrated technology lnc.

Application Information (Continued)

Feedforward Capacitor Selection

Internal compensation function allows users saving

time in design and saving cost by reducing the

number of external components.

The use of a

feedforward capacitor C4 in the feedback network is

recommended to improve the transient response or

higher phase margin.

FR9801

VOUT

For optimizing the feedforward capacitor, knowing

the cross frequency is the first thing.

The cross

frequency (or the converter bandwidth) can be

determined by using a network analyzer.

When

getting the cross frequency with no feedforward

capacitor

identified,

the

value

feedforward

capacitor C4 can be calculated with the following

equation:

C4=

FCR SS

Where FCROSS is the cross frequency.

To reduce transient ripple, the feedforward capacitor

value can be increased to push the cross frequency

to higher region.

Although this can improve

transient response, it also decreases phase margin

and causes more ringing.

In the other hand, if more

phase margin is desired, the feedforward capacitor

value can be decreased to push the cross frequency

to lower region.

In general, the feedforward

capacitor range is between 10pF to 1nF.

External Boost 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.

FR9801

VIN

BST

VIN

1N 4148

PCB Layout Recommendation

The device’s performance and stability are

dramatically

affected

PCB

layout.

recommended to follow these general guidelines

shown as below:

Place

the

input

capacitors

and

output

capacitors as close to the device as possible.

The traces which connect to these capacitors

should be as short and wide as possible to

minimize parasitic inductance and resistance.

Place feedback resistors close to the FB pin.

Keep the sensitive signal (FB) away from the

switching signal (LX).

Multi-layer PCB design is recommended.

VIN

VOUT

GND

Figure 25. FR9801 Recommended Layout Diagram

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