SP6136_06 Datasheet, PDF (5/9 Pages) Sipex Corporation – Evaluation Board Manual

LOOP COMPENSATION DESIGN
The open loop gain of the SP6136EB 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 crossover at the selected frequency fc, the gain of the error amplifier
has to 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 of the switching frequency
fs to insure proper operation. Since the SP6136EB 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 under damped resonance double pole frequency.
Figure 11. SP6136EB Voltage Mode Control Loop with Loop Dynamic
Rev 5/01/06
SP6136 Evaluation Manual
Page 5 of 9
Copyright © 2006 Sipex Corporation