FAC2786FE

2015.01. 26

11/13

Revision No : 0

Application Information

Setting Output Voltage

The regulated output voltage is determined by:

RR

To prevent stray pickup, please locate resistors R1 and R2

close to TD2786.

Inductor Capacitor Selection

Use small ceramic capacitors for high frequency decoupling

and bulk capacitors to supply the surge current needed each

time the N-channel power MOSFET (Q1) turns on. Place the

small ceramic capacitors physically close to the VIN and

between the VIN and GND.

The important parameters for the bulk input capacitor are

the voltage rating and the RMS current rating. For reliable

operation, select the bulk capacitor with voltage and current

ratings above the maximum input voltage and largest RMS

current required by the circuit. The capacitor voltage rating

should be at least 1.25 times greater than the maximum

input voltage and a voltage rating of 1.5 times is a

conservative guideline. The RMS current (IRMS) of the bulk

input capacitor is calculated as the following equation:

where D is the duty cycle of the power MOSFET.

For a through hole design, several electrolytic capacitors

maybe needed. For surface mount designs, solid tantalum

capacitors can be used, but caution must be exercised with

regard to the capacitor surge current rating.

Output Capacitor Selection

An output capacitor is required to filter the output and

supply the load transient current. The filtering requirements

are the function of the switching frequency and the ripple

current (DI). The output ripple is the sum of the voltages,

having phase shift, across the ESR and the ideal output

capacitor. The peak-to-peak voltage of the ESR is calculated

as the following equations:

The peak- to-peak voltage of the ideal output capacitor is

calculated as the following equations:

peak-to-peak output voltage (