Feb21-07

SP7661 Evaluation Manual

©2007 Sipex Corporation

Page 5 of 11

Figure 8. Current Limit Re-start Rate

TYPE III LOOP COMPENSATION DESIGN

The open loop gain of the SP7661EB can be divided into the gain of the error amplifier

Gamp(s), PWM modulator Gpwm, buck converter output stage Gout(s), and feedback resistor

divider Gfbk. In order to cross over at the selecting frequency fco, the gain of the error amplifier

must compensate for the attenuation caused by the rest of the loop at this frequency. The goal

of loop compensation is to manipulate the open loop frequency response such that its gain

crosses over 0dB at a slope of –20dB/dec. The open loop crossover frequency should be higher

than the ESR zero of the output capacitors but less than 1/5 to 1/10 of the switching frequency

fs to insure proper operation. Since the SP7661EB is designed with Ceramic Type output

capacitors, a Type III compensation circuit is required to give a phase boost of 180

° in order to

counteract the effects of the output LC underdamped resonance double pole frequency.

Soft Start