Current-Mode Vs. Voltage-Mode Control in Synchronous Buck Converters

Focus:

Synchronous buck converters designers with current-mode control (CMC) are generally viewed as having better feedback loop response than those designed with voltage-mode control (VMC). This article compares the performance and design requirements of CMC and VMC, making the case that newer converters designed with high-frequency VMC can provide comparable feedback loop response to CMC. The feedback loop performance of two example converters (3.3-V input, 1.8-V output at 10 A, 600-kHz switching frequency) is compared. The use of Type 3 compensation in the VMC converter is shown to provide closed-loop performance comparable to that of the CMC converter. Topics briefly discussed include selection of compensation components for the VMC converter and its slower response to high-speed load transients. The article goes into detail about the differences in current-sensing requirements for VMC and CMC (VMC requirements are simpler, can save power); describes several different locations and methods for measuring current, explains how these do or don't work with each control method, and their advantages and disadvantages. Article concludes with a summary of the pros and cons of the two control methods.

What you’ll learn:

• How to understand the differences in operation and design requirements for synchronous buck converters built using voltage-mode control and current-mode control
• How to choose between voltage-mode control and current-mode control when designing synchronous buck converters
• How to understand different options for current-sensing in buck converters

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Author & Publication:

Brian Lynch, Texas Instruments Incorporated, Analog Zone, Apr 01 2007

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