FAC1483FE

2012.03. 26

11/13

Revision No : 0

Since the input capacitor (C1) absorbs the input

switching current it requires an adequate ripple current

rating. The RMS current in the input capacitor can be

estimated by:

capacitor whose RMS current rating greater than half

of the maximum load current.

The input capacitor can be electrolytic, tantalum or

ceramic. When using electrolytic or tantalum

capacitors, a small, high quality ceramic capacitor, i.e.

0.1μF, should be placed as close to the IC as possible.

When using ceramic capacitors, make sure that they

have enough capacitance to provide sufficient charge

to prevent excessive voltage ripple at input. The input

voltage ripple for low ESR capacitors can be

estimated by:

Where C1 is the input capacitance value.

Output Capacitor

The output capacitor is required to maintain the DC

output voltage. Ceramic, tantalum, or low ESR

electrolytic capacitors are recommended. Low ESR

capacitors are preferred to keep the output voltage

ripple low. The output voltage ripple can be estimated

by:

the equivalent series resistance (ESR) value of the

output capacitor.

In the case of ceramic capacitors, the impedance at

the switching frequency is dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated by:

In the case of tantalum or electrolytic capacitors,the

ESR dominates the impedance at the switching

frequency. For simplification, the output ripple can be

approximated to:

The characteristics of the output capacitor also affect

the stability of the regulation system. The FAC1483 can

be optimized for a wide range of capacitance and ESR

values.

Compensation Components

FAC1483F employs current mode control for easy

compensation and fast transient response. The

system stability and transient response are controlled

through the COMP pin. COMP pin is the output of the

internal transconductance error amplifier. A series

capacitor-resistor combination sets a pole-zero

combination to control the characteristics of the control

system.

The DC gain of the voltage feedback loop is given by:

load resistor value.

The system has two poles of importance. One is due

to the compensation capacitor (C3) and the output

resistor of the error amplifier, and the other is due to

the output capacitor and the load resistor. These poles

are located at:

1

1