Project Analog - December 2007 - (Page 6)

10% Capacity Loss (%) 9% 8% 7% 6% 5% 4% 3% 2% 1% 0% 0.0% 0.2% 0.4% 0.6% fig. 1 caPacity loss vs. undErchargE voltagE adapter will have a typical range of 9V to 18V. conStant current charge rate and accuracy The choice of topology for a given application may be determined by the desired constant current. Many high constant current, or multiple cell applications rely on a switchmode charging solution for improved efficiency and less heat generation. Linear solutions are desirable in low to moderate fast charge current applications for their superior size and cost considerations. However, a linear solution purposely dissipates excess power in the form of heat. The tolerance on the constant current charge becomes extremely important to a linear system. If the regulation tolerance is loose, pass transistors and other components will need to be oversized adding size and cost. In addition, if the constant current charge is low, the complete charge cycle will be extended. Contents Viewpoint Charging lithium-ion batteries New trends in battery-powered portable devices Low power design: LDO thermal considerations 0.8% Percent Undercharge(%) load current being drawn from the wall cube. In the US, the AC mains input voltage can vary from 90 VRMS to 132 VRMS for a standard wall outlet. Assuming a nominal input voltage of 120 VRMS, the tolerance is +10%, –25%. The charger must provide proper regulation to the battery independent of its input voltage. The input voltage to the charger will scale in accordance to the AC mains voltage and the charge current: VO = √ 2 x VIN x a – 1o (REQ + RPTC) – 2 x VFD REQ is the resistance of the secondary winding plus the reflected resistance of the primary winding (RP/ a2). RPTC is the resistance of the PTC, and VFD is the forward drop of the bridge rectifiers. In addition, transformer core loss will slightly reduce the output voltage. Applications that charge from a car adapter can experience a similar problem. The output voltage of a car An understanding of the charging characteristics of the battery and the application’s requirements is essential in order to design an appropriate and reliable battery charging system. Analog news Microchip analog page Battery charger overview Sample center microchipDIRECT Reference designs/ app notes Technical training 6 · prOject AnALOg · Dec 07 http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/batterychargers http://www.microchip.com/batterychargers 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

Table of Contents Feed for the Digital Edition of Project Analog - December 2007

Project Analog - December 2007 Contents Viewpoint About Project Analog Sponsor4 Charging Lithium-Ion Batteries: System Considerations New Trends In Battery-Powered Portable Devices Low Power Design: LDO Thermal Considerations Runtime Measurements for a Hybrid Power Source Battery-Powered System Design Considerations Analog News—Analog News from Multiple Sources Enter to Win an iPhone Contact Project Analog Sponsor Mindi™ Battery & Power Circuit Simulator Microchip Advanced Parts Selector (MAPS)

Project Analog - December 2007

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