Function Description(Cont.)

the inductor ripple current, and RESR is the equivalent

series resistance of the output capacitors.

Output Recti�ier Diode

The output rectifier diode supplies the current to the

inductor when the high-side switch is off. To reduce

losses due to the diode forward voltage and recovery

times, use a Schottky rectifier.

Table 1 provides the Schottky rectifier part numbers

based on the maximum input voltage and current rating.

Choose a rectifier who’s maximum reverse voltage rating

is greater than the maximum input voltage, and who’s

current rating is greater than the maximum load current.

Feedforward Capacitor (CFF)

For output voltages greater than approximately 8V, an

additional capacitor is required. The compensation

capacitor is typically between 100 pF and 33 nF, and is

wired in parallel with the output voltage setting resistor,

R1. It provides additional stability for high output

voltages, low input-output voltages, and/or very low ESR

output capacitors, such as solid tantalum capacitors.

This capacitor type can be ceramic, plastic, silver mica,

etc.(Because of the unstable characteristics of ceramic

capacitors made with Z5U material, they are not

recommended.)

Note:In PCB layout. Reserved an area for CFF.

Over Current Protection (OCP)

The cycle by cycle current limit threshold is set between

3A and 4A. When the load current reaches the current

limit threshold, the cycle by cycle current limit circuit

turns off the high side switch immediately to terminate

the current duty cycle. The inductor current stops rising.

The cycle by cycle current limit protection directly limits

inductor peak current. The average inductor current is

also limited due to the limitation on peak inductor current.

When the cycle by cycle current limit circuit is triggered,

the output voltage drops as the duty cycle is decreasing.

Thermal Management and Layout

Consideration

In the FAC1513 buck regulator circuit, high pulsing current

flows through two circuit loops. The first loop starts from

the input capacitors, to the VIN pin, to the VOUT pins, to

the filter inductor, to the output capacitor and load, and

then returns to the input capacitor through ground.

Current flows in the first loop when the high side switch is

on. The second loop starts from the inductor, to the

output capacitors and load, to the GND pin of the

FAC1513, and to the VOUT pins of the FAC1513. Current

flows in the second loop when the low side diode is on.

In PCB layout, minimizing the two loops area reduces the

noise of this circuit and improves efficiency. A ground

plane is recommended to connect input capacitor, output

capacitor, and GND pin of the FAC1513.

In the FAC1513 buck regulator circuit, the two major

power dissipating components are the FAC1513 and

output inductor. The total power dissipation of converter

circuit can be measured by input power minus output

power.

The power dissipation of inductor can be approximately

calculated by output current and DCR of inductor.

The junction to ambient temperature can be got from

power dissipation in the FAC1513 and thermal impedance

from junction to ambient.

FAC1513

2011.06. 17

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Revision No : 1