FAC1483FE

2012.03.26

12/13

Revision No : 0

The system has one zero of importance, due to the

compensation capacitor (C3) and the compensation

resistor (R3). This zero is located at:

The system may have another zero of importance, if

the output capacitor has a large capacitance and/or a

high ESR value. The zero,due to the ESR and

capacitance of the output capacitor, is located at:

In this case (as shown in Figure 14), a third pole set by

the compensation capacitor (C6) and the

compensation resistor (R3) is used to compensate the

effect of the ESR zero on the loop gain. This pole is

located at:

The goal of compensation design is to shape the

converter transfer function to get a desired loop gain.

The system crossover frequency where the feedback

loop has the unity gain is important. Lower crossover

frequencies result in slower line and load transient

responses,while higher crossover frequencies could

cause system instability. A good rule of thumb is to set

the crossover frequency below one-tenth of the

switching frequency.

To optimize the compensation components, the

following procedure can be used.

1. Choose the compensation resistor (R3) to set the

desired crossover frequency.

Determine the R3 value by the following equation:

typically below one tenth of the switching frequency.

2. Choose the compensation capacitor (C3) to achieve

the desired phase margin. For applications with typical

below one-forth of the crossover frequency provides

sufficient phase margin.

Determine the C3 value by the following equation:

Where R3 is the compensation resistor.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator, the applicable

conditions of external BST diode are:

Duty cycle is high:

In these cases, an external BST diode is

recommended from the output of the voltage regulator

to BST pin, as shown in Fig.14

Figure14.Add Optional External Bootstrap Diode to Enhance

Ef�iciency

The recommended external BST diode is IN4148, and

the BST cap is 0.1~1μF.

1

1

2∏*C6*R3

1

FAC1483

R3 =

2∏*C2*fc

ⅹ

Vout

ⅹ

2∏*C2*0.1*fs

Vout

<

3. Determine if the second compensation capacitor

(C6) is required. It is required if the ESR zero of the

output capacitor is located at less than half of the

switching frequency, or the following relationship is

valid:

If this is the case, then add the second compensation

ESR zero. Determine the C6 value by the equation: