Mar12-07 RevD

SP7663 Evaluation Board Manual

©2007 Sipex Corp.

Page 5 of 16

Figure 8. Startup into no load

TYPE III LOOP COMPENSATION DESIGN

The open loop gain of the SP7663EB 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 SP7663EB 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.

Iout (5A/div)