Project Analog - December 2007 - (Page 12)

Equation 6: T1 – T2 = PD x Θ12 Equation 7: Θ12 = (T1 – T2)/PD Where: T1 = Temperature of Point 1 T2 = Temperature of Point 2 PD = Power dissipated in the device Relating this model to an IC, we can say that the device’s thermal resistance from junction to ambient fig. 2 thermal resistance Contents Viewpoint Charging lithium-ion batteries New trends in battery-powered portable devices Low power design: LDO thermal considerations Analog news fig. 1 ldo power dissipation fig. 3 heat transfer—thE thErmal rEsistancE can noW bE WrittEn as shoWn in Equation 10. (ΘJA) is equal to the junction temperature minus ambient, divided by power dissipation, or as expressed in Equation 8. When calculating power dissipation, it is critical that worst case conditions be used. This means maximum VIN, ILOAD, and IGND, and minimum VOUT values. Equation 2 is more accurately written as Equation 3: Equation 3: PDMAX = (VINMAX – VOUTMIN) x ILOADMAX + VINMAX x IGNDMAX thermaL reSIStance Heat flows from a high temperature (T1) to a relatively lower one (T2) at a rate determined by the thermal resistance (Θ12) between the two points (see Figure 2). The thermal resistance is the temperature rise (in °C) for every watt dissipated for the system in question. This results in the expressions in Equation 6 and Equation 7. Equation 8: ΘJA = (TJ – TA)/PD Equation 9: TJ = (ΘJA x PD) + TA Heat is transferred from the die (heat source) to the air, through several material interfaces. The thermal resistance between these interfaces comprises the ΘJA of the system. These interfaces are typically the dieto-package (ΘJC), package-to-heat sink (ΘCS), and heat sink-to-air (ΘSA) (see Figure 3). The thermal resistance can now be written as shown in Equation 10. Equation 10: ΘJA = ΘJC + ΘCS + ΘSA Microchip analog page Battery charger overview Sample center microchipDIRECT Reference designs/ app notes Technical training 12 · 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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