3A 32V Synchronous Rectified Step-Down

Converter

EC3286

Page 12 of 14

4J23N-Rev.F001

The characteristics of the output capacitor also affect the stability

of

the

regulation

system. The EC3286 can be optimized for a wide range of capacitance and ESR values

Compensation Components

EC3286 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:

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:

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:

frequency.

2. Choose the compensation capacitor (C3) to achieve the desired phase margin. For

applications with typical inductor values, setting the compensation zero, fZ1, below one-forth of

the crossover frequency provides sufficient phase margin. Determine the C3 value by the

following equation: