AN817

AN817 Vishay Siliconix A Discrete Approach to Battery Charging for Cellular Phones Guy Moxey and Michael Speed INTRODUCTION All portable cordless appliances must receive power from an external source, whether it’s a wall cube adapter, car charger, or docking station. This external source will then charge, in a predetermined fashion, the equipment’s internal battery. In the case of a portable phone, the power management system will incorporate charging control circuitry to regulate the voltage supplied to the battery from the external charger. External charging equipment—whether wall cubes or chargers that utilize car cigarette lighters—will supply a continuous but unregulated voltage to the phone, typically 4.2 V for a single Lithium-ion (Li+) cell. A typical charging design is explored in Appendix A. Charge control for a Li+ cell is most commonly implemented by a discrete MOSFET in series with a Schottky diode, controlled via the onboard power management ASIC or system microprocessor. Integration of these two discrete power components into a single power package, such as the ChipFETt, reduces size and simplifies the assembly. Charger To this end, moving away from a separate Schottky diode and MOSFET to the single package integration of both devices, as in the Vishay Siliconix LITTLE FOOT PlusTM, may have significant advantages. However, in an integrated package both components operate in a highly dissipative manner, making the choice of package a critical decision. The LI...