Product Specification

PE99151

Page 13 of 15

Document No. DOC-29314-2 │ www.psemi.com

©2010-2013 Peregrine Semiconductor Corp. All rights reserved.

One requirement of the voltage control loop stability

analysis and design is to maintain significant attenuation

at the switching frequency of the converter. If this

requirement is not met, the switching noise can enter the

control loop and loop stability will be lost. To ensure

sufficient attenuation at the switching frequency, a unity

gain cross over frequency one decade below the

switching frequency is recommended. Additional margin

may be desired depending on the application

requirements.

The pole created by the load and the output capacitor is

dependent on load current. The load current can vary,

likely all the way down to zero load current. When this

happens the output pole frequency falls arbitrarily low. It

is clear that this highly variable pole will not be sufficient

to set a dominant pole in the loop to ensure stability.

The recommended compensation technique is a

combined pole-zero compensation network. The zero is

set to cancel the variable load pole at minimum load so

that the load pole does not affect phase margin of the

system when the pole location is at it's minimum value,

possibly even in the bandwidth of the control loop. With

the load pole canceled by the additional zero, the added

pole in the compensation network acts as a stable

dominant pole. This dominant pole location can be

and for the required phase margin and loop bandwidth.

With this compensation technique, the zero location is

to cancel the load pole at minimum load.

If the compensation network resistor is selected to be

operational transconductance amplifier (OTA), the

dominant pole location can then be approximated as 1/

dominant pole location dependent on the loosely

controlled error amp output impedance. However, as the

error amp output impedance drops, the DC gain of the

system and the dominant pole location reduce together.

The result is that the unity gain cross over frequency (set

by the dominant pole) is independent of OTA output

impedance variation over process and temperature.

A compensation network design spreadsheet is

available in the PE9915x Design Tool.

Figure 12. PE99151 Control Loops