AMC3202 Datasheet, PDF (9/11 Pages) ADDtek Corp – 1.5A 280kHz BOOST REGULATORS

AMC3202
The V_IN ripple is determined by the product of the inductor current ripple and the ESR of input capacitor, and the
V_OUT 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.
f _P1
=
1
2πC _P1 R_O
where, R_O= error amplifier output resistance;
f _Z1
=
1
2πC _P1 R_P
f_P2
=
1
2πC _{P 2} R_P
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 IN
V EN
D SS
EN
4
5
V CC
4uA
C SS
3
SS
Q
COMP
1
C P2
C P1
RP
Resistor R_P and capacitors C_P1 and C_P2 form the compensation network. At turn on, the voltage at the COMP pin
starts to come up, charging capacitor C_SS 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, C_SS slows down the startup of
the circuit by limiting the voltage on the COMP pin. The soft-start time increases with the size of C_SS.
Diode D_SS discharges C_SS when the voltage on the EN pin is low. If the shutdown function is not used with this
part, the cathode of D_SS should be connected to V_IN.
Copyright © 2006 ADDtek Corp.
9
DD074_B -- AUGUST 2006