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.
IOUT 2A/div
Soft Start
VOUT
