AN3939| Application Note
Maxim/Dallas > App Notes > POWER-SUPPLY CIRCUITS Keywords: Average Current-Mode, ACMC, Multimedia, Telematics
Oct 18, 2006
APPLICATION NOTE 3939
DC-DC Controllers Use Average-Current-Mode Control for Infotainment Applications
Auto infotainment products (multimedia and telematics) are demanding more power and challenging a vehicle's power-management systems. This application note explains average-current-mode control (ACMC) technology for power management. The article demonstrates that this technology minimizes several power-management problems, including efficiency, size, EMI, transient response, design complexity, and overall cost. The MAX5060/ MAX5061 will serve as examples of the technique.
Introduction
As high-performance microprocessors demand more power in auto multimedia and telematics, (e.g., infotainment products), so do some of the well-known design problems like noise susceptibility, EMI, and loop compensation. Average-current-mode control (ACMC) helps to relieve these problems, especially in auto infotainment applications. This application note describes ACMC and explains its benefits over a current-mode control design for infotainment applications. The MAX5060/MAX5061 illustrate ACMC, and the information presented here supplements the basic narrative in the product data sheet.
Define the Design Goals
Each auto infotainment application presents a unique set of technical and commercial requirements for power management. The most important design considerations are efficiency, size, EMI, transient response, design complexity, and cost. All of these parameters are indirectly related to the power-supply switching frequency, an important parameter which is chosen to balance all these requirements.
Advantage of ACMC
For converters with relatively high output currents (5A to 25A), lowering the value of the current-sense resistor in current-mode control (CMC) techniques helps boost efficiency. In this article, CMC implies constant-frequency with peak current-sensing. There is, however, a drawback to this approach: CMC makes the converter increasingly susceptible to noise. In extreme high-current cases, even the best PCB layout cannot adequately suppress noise superimposed on the current-sense signal. One way to circumvent this problem by using voltagemode control, VMC, an old but proven technique. VMC improves noise immunity and efficiency, but requires a certain amout of loop compensation design to achieve acceptable performance.
ACMC Design Basics
The ACMC technique combines the noise immunity and efficiency of VMC with the stability and performance characteristics of CMC. Figure 1 illustrates a functional block diagram of a buck converter operated in ACMC.
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