AMC3202DMF datasheet(9/11 Pages) ADDTEK

9 / 11 page

AMC3202

DD074_B -- AUGUST 2006

The VIN ripple is determined by the product of the inductor current ripple and the ESR of input capacitor, and the

VOUT ripple comes from two major sources, namely ESR of output capacitor and the charging/discharging of the output

capacitor. Ceramic capacitors have the lowest ESR, but too low ESR may cause loop stability problems. Aluminum

Electrolytic capacitors exhibit the highest ESR, resulting in the poorest AC response. One option is to parallel a ceramic

capacitor with an Aluminum Electrolytic capacitor.

Frequency Compensation

The goal of frequency compensation is to achieve desirable transient response and DC regulation while ensuring

the stability of the system. A typical compensation network, as shown in the typical application circuit, provides a

frequency response of two poles and one zero. The loop frequency compensation is performed on the output of the error

amplifier (COMP pin) with a series RC network. The main pole is formed by the series capacitor and the output

impedance of the error amplifier. The series resistor creates a zero, which improves loop stability and transient response.

A second capacitor is sometimes used to reduce the switching frequency ripple on the COMP pin.

where, RO= error amplifier output resistance;

Soft Start

Through the addition of an external circuit, a soft-start function can be added to the AMC3202. Soft-start circuitry

prevents the COMP pin from slamming high during startup, thereby inhibiting the inductor current from rising at a high

slope. Referring to the figure shown in the following, the soft-start circuitry requires a minimum number of components

and allows the soft-start circuitry to be activated any time when the EN pin is used to restart the converter.

V CC

CO M P

4uA

R P

C P2

C P1

V IN

V EN

C SS

D SS

Resistor RP and capacitors CP1 and CP2 form the compensation network. At turn on, the voltage at the COMP pin

starts to come up, charging capacitor CSS through internal transistor Q, clamping the voltage at the COMP pin such that

switching begins when COMP reaches the COMP threshold, typically 1.05V. Therefore, CSS slows down the startup of

the circuit by limiting the voltage on the COMP pin. The soft-start time increases with the size of CSS.

Diode DSS discharges CSS when the voltage on the EN pin is low. If the shutdown function is not used with this

part, the cathode of DSS should be connected to VIN.

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