Project Analog - December 2007 - (Page 11)

Low power Design: LDO thermal considerations By Paul Paglia Microchip Technology / Chandler, AZ / www.microchip.com IntroductIon Battery-operated equipment (most notably cell phones and notebook computers) have created a strong demand for linear regulators in small packages. While such packages save space, they also have poor heat transfer characteristics. To minimize power dissipation, these regulators are designed to work with very low input/output voltage differentials, hence the name “low dropout regulators” or LDOs. dISSIpatIng heat Like other power devices, LDOs dissipate heat generated in the die by convection at rates determined by the thermal resistances in the system. Heat dissipation by convection is determined by the thermal resistance from the junction to ambient (ΘJA). Typically, heat sinks and/or forced air techniques may be used to decrease ΘJA, but not without impacting system size and cost. In addition to convection, heat is also removed from the LDO by conduction (i.e., through any portion of the package that is in contact with the circuit board). In this 11 · prOject AnALOg · Dec 07 Contents Viewpoint Charging lithium-ion batteries New trends in battery-powered portable devices Low power design: LDO thermal considerations Analog news case, increasing copper trace size and improving thermal interface (using thermal grease or films) significantly improves conduction cooling efficiency. Ldo poWer dISSIpatIon Determining the power dissipated by an LDO involves a straight forward calculation. The current entering the LDO can only go two places: through the pass device to the output (IOUT); or through the internal bias circuitry to ground (IGND). See Figure 1. The conservation of power states that power in must equal power out. Consequently, input power is equal to the power delivered to the load plus the power dissipated in the LDO, (Equation 1): Equation 1: PIN = POUT + PLDO The power dissipation of the LDO is expressed in Equation 2: Equation 2: PD = (VIN – VOUT) x ILOAD + VIN x IGND Microchip analog page Battery charger overview Sample center microchipDIRECT Reference designs/ app notes Technical training http://www.microchip.com 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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