Aug 24-06

SP6138 Evaluation Manual

©2006 Sipex Corporation

Page 5 of 9

LOOP COMPENSATION DESIGN

The open loop gain of the SP6138 Evaluation Board can be divided into the gain of the

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

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