Project Analog - April 2008 - (Page 11)

requirements including cost, power output, power supply design and signal characteristics. A half-bridge output stage requires positive and negative power supply rails, whereas a full-bridge will operate from a single power supply, produces a higher output for a given switch rating and allows the circuit to be referenced to ground. Power design For audio engineers The design of the MOSFET gatedriver circuitry for Class D amplifiers involves power design disciplines that are unfamiliar to many audio engineers. Because a Class D amplifier cannot be turned on gradually, it is difficult to build a finely tuned and well-protected power switching stage. Design flaws can quickly result in catastrophic failure of the circuit that make fault finding at best time-consuming and, at worst, impossible. On the whole, the power switching stage is difficult to perfect. A plug-in solution may be desirable, to which the designer can add a half-bridge or full-bridge output stage, using MOSFETs optimized for Class D. This approach allows audio design engineers to focus on other aspects of the amplifier design to differentiate its performance in the marketplace. These aspects include the optimization of the feedback loop to reduce output distortion and the damping factor to achieve the desired acoustic qualities. Other important features and amplifier properties are closely linked with the power switching stage design, and designers need to have sufficient control over these aspects if they are to deliver outstanding features and maximize value for end users. These include techniques to eliminate EMI caused by the switching stage, the use of a selfoscillating PWM to prevent switching noise from contaminating the audio output, the provision of overcurrent protection, and the implementation of click/pop noise reduction. integrated design New integrated gate driver ICs address these needs and provide Contents Viewpoint Calculating Power Dissipation Accelerating Amplifier Design Driving Power MOSFETs Analog news fig 1 Board sizE of 120 W class a/B aMPlifiEr coMParEd to 120 W class d dEsign. Figure 2 shows the basic topology of an open-loop, Class D audio amplifier. The input audio signal is compared with a high-frequency sawtooth wave, producing a square wave that is pulse-width-modulated according to the audio signal. Setting the sawtooth frequency much higher than the maximum audio signal 11 · prOjECT ANAlOg · Apr 08 frequency, in the region of 400 kHz, helps to reduce distortion and simplify output filter design. The PWM-equivalent of the audio signal is then used to drive the amplifier output stage. This may be a full-bridge or half-bridge design, depending on other system Microchip analog page MOSFET driver overview Sample center microchipDIRECT Reference designs/ app notes Technical training http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/mosfetdriver http://www.microchip.com/mosfetdriver http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423

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Project Analog - April 2008 Contents Viewpoint About Project Analog Sponsor Calculating Power Dissipation in a MOSFET Driver Accelerating Design of >50-W Class D Amplifiers Driving Power MOSFETs in Switch Mode Power Supplies Synergistic MOSFET Solutions Trends in MOSFET Gate Drivers Analog News—Analog news from multiple sources Enter to win an iPhone Contact Project Analog Sponsor Treelink Microchip Advanced Parts Selector (MAPS)

Project Analog - April 2008

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