SP6138_06 Datasheet, PDF (5/9 Pages) Sipex Corporation – Evaluation Board Manual

LOOP COMPENSATION DESIGN
The open loop gain of the SP6138 Evaluation Board 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 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 of the switching frequency fs to insure proper operation.
Since the SP6138EB 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.
Figure 11. SP6138EB Voltage Mode Control Loop with Loop Dynamic
Aug 24-06
SP6138 Evaluation Manual
Page 5 of 9
©2006 Sipex Corporation