Project Analog - April 2008 - (Page 14)

fig 1 (figurE 3 in an786) Protection against shoot-through current is still another consideration, especially in higher speed SMPS designs. Shoot through currents are usually caused by excessively long driver rise, fall or propagation delay times, causing both the high side and low side MOSFETs to be on for a brief instant. Current “shoots through” from the supply input to ground, significantly degrading the overall supply efficiency. The use of dedicated MOSFET drivers minimizes this problem in two ways. First of all, MOSFET gate drive rise and fall times must be symmetrical, and as short as possible. A driver like the TC4427A has a specified tR and tF of approximately 25 ns into a 1000 pF load. A higher peak output current driver may be selected to achieve more aggressive rise and fall times if so desired. Second, the propagation delay times through the driver must be short (and matched for higher speed designs) to ensure symmetrical turn-on and turn-off delays of both the high side and low side MOSFET. Microchip Technology’s TC4427A for example, has rising and falling edge propagation delay times matched to within 2 ns. These delays track each other with both voltage and temperature, and is among the best available. These concerns (and related cost and reliability concerns) usually point in the direction of an external, dedicated driver, as opposed to an integrated or external discrete component driver solution. Portable coMPuter suPPly One common application that exploits the design benefits of dedicated MOSFET drivers is a switching power supply for portable systems, such as those found in notebook computer applications. The circuit topology of a high efficiency, synchronous buck converter is shown in Figure 1. It accepts an input voltage range of 5V to 30V to accommodate AC/DC adapters (14V to 30V) or a battery supply (7.2V to 10.8V). The TC1411N acts as a low side driver, and is powered from a +5V supply to minimize turn-off delay due to gate “overdrive charge”. The high side driver in Figure 1 is a Contents Viewpoint Calculating Power Dissipation Accelerating Amplifier Design Driving Power MOSFETs Analog news Microchip analog page MOSFET driver overview Sample center microchipDIRECT Reference designs/ app notes Technical training fig 2 (figurE 4 in an786) 14 · prOjECT ANAlOg · Apr 08 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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