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1 Introduction
Application Report
SLVA289 – January 2008
Optimizing Transient Response of Internally Compensated
dc-dc Converters With Feedforward Capacitor
Brian Butterfield.................................................................................................. PMP - Portable Power
ABSTRACT
This application report describes how to choose the feedforward capacitor value (Cff) of
internally compensated dc-dc power supplies to achieve optimum transient response.
The described procedure in this application report provides guidance in optimizing
transient response by increasing converter bandwidth while retaining acceptable phase
margin. This document is intended for all power supply designers who want to optimize
the transient response of a working, internally compensated dc-dc converter.
Contents
1 Introduction .......................................................................................... 1
2 Feedback Network With and Without the Feedforward Capacitor ........................... 2
3 Conclusion ......................................................................................... 11
4 References ......................................................................................... 11
List of Figures
1 Feedback Network Consisting of Two Bias Resistors Used to Set Output Voltage ....... 2
2 Standard Feedback Divider Transfer Function ................................................. 2
3 Feedback Network With Addition of Feedforward Capacitor ................................. 3
4 Standard Feedback Divider With Feedfoward Capacitor Transfer Function ................ 3
5 Internally Compensated Converter Without Feedforward Capacitor ......................... 4
6 Tip and Barrel Measurement Technique ........................................................ 5
7 Voltage Transient in Response to Load Transient Without Feedforward Capacitor ....... 5
8 Step Response vs Loop Phase Margin .......................................................... 6
9 Loop Gain and Phase Plot of TPS61081 Circuit Without Feedforward Capacitor ......... 6
10 Voltage Transient in Response to a Load Transient With 82-pF Feedforward Capacitor 8
11 Loop Gain and Phase Plot of TPS61081 Circuit With 82-pF Feedforward Capacitor ..... 8
12 Voltage Transient in Response to a 0 to 100% Load Transient With 1000-pF
Feedforward Capacitor ............................................................................. 9
13 Loop Gain and Phase Plot of TPS61081 Circuit With 1000-pF Feedforward Capacitor .. 9
14 Voltage Transient in Response to Load Transient With 33-pF Feedforward Capacitor . 10
15 Loop Gain and Phase Plot of TPS61081 Circuit With 33-pF Feedforward Capacitor ... 10
Internally compensated dc-dc converters allow designers to save time in the design and in debug
processes by minimizing the number of external components that they must select. This simplification
inherently narrows a designer's ability to optimize the transient response of the converter. However, with
some internally compensated converters, the use of a feedforward capacitor in the feedback network is
recommended, but only general guidance is provided for choosing this capacitor value to improve
SLVA289 – January 2008 Optimizing Transient Response of Internally Compensated dc-dc Converters With Feedforward Capacitor 1
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