Project Analog - February 2009 - (Page 20) 16-v, 250 ma lDo comEs iN 3-PiN soT-223 power converter running at less than full load. The derating diagrams in the datasheets define the maximum load at any given temperature within the operating temperature range. The derating curves are in reality not so linear as they are declared in most datasheets. However, reliable manufacturers will always err on the safe side so that the values given can be safely relied on in practice. RTHcase-ambient = RTHcase-HS + RTHHS-ambient where: RTHcase-ambient Thermal impedance from the case to the ambient surroundings RTHcase-HS Thermal impedance from the case to the heat sink RTHHS-ambient Thermal impedance from the heat sink to ambient The value of RTHHS-ambient incorporates the thermal resistance of the heat sink as well as the thermal resistance of any thermally conductive paste or silicon pads used for a better thermal contact to the case. If these aids are not applied, then a value of about 0.2 K/W must be added to the thermal resistance of the heat sink alone. When establishing the value of RTHHS-ambient it is also necessary to know how much air is being blown across the heat-sink fins. These values are most often given in linear feet per minute (lfm) and declared by the fan manufacturer. The conversion to m/s is 100 lfm = 0.5 m/s. This has been a basic overview for all users confronted with a thermal management issue, but it has covered the main points that need to be considered. If however, the results of your calculations or measurements are borderline, then the issue must be examined in more depth. So, for example, there is a difference in thermal performance between vertically and horizontally mounted modules, between static air and freely convecting air and with air at low atmospheric pressures. However, investigating all of these factors would take up too much space in this article. Contents Viewpoint Selecting the Right dc/dc Converter Selecting the Right Battery System Understanding the Buck Switchmode Power Converter Analog news Housed in a 3-pin SOT-223 package, the 250-mA MCP1703 LDO features a quiescent current of 2 µA, an input voltage up to 16 V, an over-temperature accuracy of ± 2%, overcurrent protection, and thermal shutdown. The part is also available in 3-pin SOT-89 and SOT-23A packages. The part is also available in 3-pin SOT-89 and SOT-23A packages. microchip Technology, chandler, az reader/literature inquiries 888-mcu-mchP www.microchip.com/analog If the converter has a plastic case, then the next largest case size with the same power rating could be chosen to increase the available surface area. However, care must be taken not to compromise on efficiency otherwise no net gain will be made. If the converter has a metal case, then adding a heat sink is can be very effective, particularly in conjunction with a forced-air cooling system. If a heat sink is used with fan cooling, then the thermal resistance equation becomes: Microchip analog page Sample center microchipDIRECT Reference designs/ app notes Technical training 20 · PRojeCt ANAlog · VolUme 3 / NUmBeR 1 http://www.microchip.com http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://www.microchip.com/analog 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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