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Methods for Accurate Droop Compensation During Load Transient Conditions

IP.com Disclosure Number: IPCOM000010153D
Publication Date: 2002-Oct-28
Document File: 8 page(s) / 221K

Publishing Venue

The IP.com Prior Art Database

Abstract

Key Words: Droop Compensation, Converter Response, Converter Transient Response, Converter Output Error Background A power converter's output needs to provide a nearly constant output voltage to a load. In some applications, the converter is required to maintain a nearly constant output voltage for a sudden step change in load current. For example, a computer has a "power-saving mode" in which a number of circuit sections are turned off to reduce current consumption to near zero. Where required, these sections are turned on, resulting in the load current increasing to a high value within a few hundred nanoseconds.

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Methods for Accurate Droop Compensation During Load Transient Conditions

Key Words: Droop Compensation, Converter Response, Converter Transient Response, Converter Output Error

Background

A power converter’s output needs to provide a nearly constant output voltage to a load.� In some applications, the converter is required to maintain a nearly constant output voltage for a sudden step change in load current. For example, a computer has a "power-saving mode" in which a number of circuit sections are turned off to reduce current consumption to near zero. Where required, these sections are turned on, resulting in the load current increasing to a high value within a few hundred nanoseconds.


Problem Solved by the Invention


� � � � When there is a change in load current, some deviation in the converter's output voltage is unavoidable. The capacitance and the equivalent series resistance (ESR) of the output capacitor affects the magnitude of the deviation.� With the quest for faster responding outputs, measurement using only inductor current results in substantial error of the output voltage.

Load Current = IC + IL� � �

Where: IC is current from capacitor and

        � � � � �         � � IL is current from Inductor

This error is due to the fact that during the transient state most of the load current contribution is from the buffer capacitor.� Only during steady state does the inductor contribute the total load current. Note: it is impossible to change the current across the output inductor instantaneously.� So in order to provide accurate compensation to accommodate faster devices, a means to account for the capacitor current contribution is needed.

        � � � � � Some popular methods of current measurement for a Buck converter include: inductor current sensing, Rdson current sensing, and the use of either high-side or low-side sense resistors. All these methods measure the current flowing through the output inductor (or combined current through output inductors for a multi-phase converter), so they cannot correctly represent load current during transients.

Detailed Description of the Method

Shown in figure 1 is a low cost and efficient method for measuring the total load current for buck/boost converters to achieve accurate droop compensation.� To determine the portion of load current contributed by the buffer capacitor it is necessary to measure the voltage drop across the internal series resistance (ESR) of the buffer capacitor.� Since direct measurement of the voltage across the buffer capacitor ESR is not possible, the method shown here adds a capacitor/resistor network in parallel to the buffer capacitor with its internal E...