FR9701 datasheet(10/12 Pages) FITIPOWER

10 / 12 page

FR9701-1.1-FEB-2012

FR9701

85T

fitipower integrated technology lnc.

Application Information (Continued)

Selecting the Input Capacitor

The input capacitor reduces the surge current drawn

from the input supply and the switching noise from

the device.

The input capacitor impedance at the

switching frequency should be less than the input

source

impedance

prevent

high

frequency

switching current from passing through the input.

Ceramic capacitors with X5R or X7R dielectrics are

highly recommended because of their low ESR and

small

temperature

coefficients.

For

most

applications, a 10μF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps the output voltage ripple

small, and a 47

μF ceramic capacitor with X5R or

X7R dielectrics is recommended for its low ESR

characteristics.

External Bootstrap Diode

An external bootstrap diode is recommended if the

input voltage is less than 5V or if there is a 5V system

rail available.

This diode helps improving the

efficiency.

Low cost diodes, such as 1N4148, are

suitable for this application.

FR9701

VIN

1N4148

Rectifier Diode

Use a Schottky diode as the rectifier to conduct current

when the high-side power MOSFET is off. The

Schottky diode must have current rating higher than

the maximum output current and the reverse voltage

rating higher than the maximum input voltage.

PCB Layout Recommendation

The

device’s

performance

and

stability

are

dramatically

affected

PCB

layout.

recommended to follow these general guidelines

shown as below:

1. 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.

2. Place VIN capacitors close to the VIN pin.

3. Place feedback resistors close to the FB pin.

4. Keep the sensitive signal (FB) away from the

switching signal (SW).

VIN

VOUT

GND

Figure 17. FR9701 Recommended Layout Diagram

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