FAC1483FE datasheet(10/13 Pages) FS

10 / 13 page

FAC1483FE

2012.03. 26

10/13

Revision No : 0

Function Description

Component Selection

Setting the Output Voltage

The output voltage is set using a resistive voltage

divider from the output voltage to FB pin.The voltage

divider divides the output voltage down to the

feedback voltage by the ratio:

Where VFB is the feedback voltage and VOUT is the

output voltage.Thus the output voltage is:

R2 can be as high as 100kΩ, but a typical value is

10kΩ. Using the typical value for R2, R1 is determined

by:

For example, for a 3.3V output voltage, R2 is 10kΩ,

and R1 is 26.1kΩ.

Inductor

The inductor is required to supply constant current to

the output load while being driven by the switched

input voltage. A larger value inductor will result in less

ripple current that will result in lower output ripple

voltage. However,the larger value inductor will have a

larger physical size, higher series resistance, and/or

lower saturation current. A good rule for determining

the inductance to use is to allow the peak-to-peak

ripple current in the inductor to be approximately 30%

of the maximum switch current limit. Also, make sure

that the peak inductor current is below the maximum

switch current limit. The inductance value can be

calculated by:

Where VOUT is the output voltage, VIN is the input

voltage, fS is the switching frequency, and ∆IL is the

peak-to-peak inductor ripple current.

Choose an inductor that will not saturate under the

maximum inductor peak current. The peak inductor

current can be calculated by:

Where ILOAD is the load current.

The choice of which style inductor to use mainly

depends on the price vs. size requirements and any

EMI requirements.

Optional Schottky Diode

During the transition between high-side switch and

low-side switch, the body diode of the lowside power

MOSFET conducts the inductor current. The forward

voltage of this body diode is high. An optional Schottky

diode may be paralleled between the SW pin and

GND pin to improve overall efficiency. Table 1 lists

example Schottky diodes and their Manufacturers.

Part Number Voltage/Current Vendor

B130

30V, 1A

SK13/SS13

Firstsilicon

MBRS130

30V, 1A

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low ESR

capacitors for the best performance. Ceramic

capacitors are preferred, but tantalum or low-ESR

electrolytic capacitors may also suffice. Choose X5R

or X7R dielectrics when using ceramic capacitors.

VFB = Vout (R2/R1+R2)

VOUT = 0.923 (R1+R2/R2)

R1 = 10.83 * (Vout-0.923) (㏀)

30V, 1A

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