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3I13N-Rev.P001

2.4A/32V Synchronous Rectified

Step-Down Converter

EC3278

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 10), 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 inductor

of the crossover frequency provides sufficient phase

margin.

Determine the C3 value by the following equation:

Where R3 is the compensation resistor.

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

zero. Determine the C6 value by the equation:

External Bootstrap Diode

An external bootstrap diode may enhance the efficiency

of the regulator, the applicable

conditions of external BST diode are:



VOUT=5V or 3.3V; and



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.10

Figure10.Add Optional External Bootstrap Diode to Enhance

Efficiency

The recommended external BST diode is IN4148, and the

BST cap is

0.1~1μF.