Use auto-zero comparator techniques to improve PWM performance

摘要

Comparators are subject to input offset errors that can be either systematic or layout-related. Two applications of comparators in a PWM controller are detecting when there is no current in the inductor and detecting when there is no current in the main control-loop comparator. During the discontinuous conduction mode of the converter, the comparator must detect when the inductor current reverses through the lower-power MOSFET. This is done by comparing the drain voltage of the MOSFET during its on time to the ground reference. The use of an auto-zeroing comparator will eliminate the offset variation and, therefore, improve the accuracy of the zero-current detection. For the main PWM comparator, the use of an auto-zeroing comparator eliminates the control-loop offset between the error amplifier and the ramp-crossing level, thereby improving the overall system performance, especially at duty cycles that are less than 5 percent.