AN3623| Application Note
Battery-Charger PWM Input Has Watchdog Protection
BATTERY MANAGEMENT MICROPROCESSOR SUPERVISOR CIRCUITS
Sep 01, 2005
APPLICATION NOTE 3623
Battery-Charger PWM Input Has Watchdog Protection
Adding watchdog protection to the charger-input circuitry of a mobile phone guards against damage when the baseband processor stalls or ceases software execution. Mobile-phone batteries are usually charged through a proprietary charging algorithm in the baseband controller. The phone's charger input is connected to the internal battery through a p-channel switch of low on-resistance, controlled by a pulse-width modulation (PWM) signal from the baseband controller (Figure 1). To minimize power dissipation and consequent thermal problems in the phone, the charging supply (usually a wall cube) is current limited and specified according to the battery's chemistry and charge-recovery requirements.
Figure 1. Typical charger-input circuitry for a mobile phone. Note that the (almost) direct connection between charger and battery could cause damage if the baseband processor should stall for any reason. What is needed is a circuit that monitors the PWM input and disables the series power switch after a predetermined delay interval. The circuit should be independent of the processor, and should allow charging to restart when the PWM signal returns. To provide this protection, add a microprocessor supervisor (one that includes a watchdog circuit for software execution) and a usually open, SPST analog switch (Figure 2). The two ICs shown are available in 5-lead SOT23 packages to minimize space. R4, D2, and C1 protect the ICs by limiting VCC to a maximum of 5.1V. The R4 value isn't critical, because quiescent current for the added circuit is very low (30uA). Choose the R4 value to provide just enough current (0.5mA) to activate the Zener diode's "knee" characteristic.
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