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Maxim > Design Support > Technical Documents > Application Notes > Analog Switches and Multiplexers > APP 3191
Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 3191
Keywords: battery charging, watchdog timer, protection, switch, reset, supervisor, PWM circuit
APPLICATION NOTE 3191
Protecting Against Shorts Between a Battery and
the PWM Wall Cube Charging Supply
Jun 25, 2004
Abstract: This appnote provides a simple solution to prevent damage in a cell-phone's battery-charging
system should the baseband processor lock up. The circuit monitors the processor and intervenes to
prevent a short between the battery and the PWM charger supply.
A mobile phone's battery charging chores are handled in part by its baseband controller, which includes
a proprietary charging algorithm. This algorithm generates a PWM signal that controls Q2, the low-on-
resistance p-channel MOSFET shown in Figure 1. This transistor connects a wall cube present at the
Charger Input terminal to the battery. In order to reduce power dissipation and thermal problems within
the phone, the wall cube includes a current limit specified to meet the requirements of the battery's
specific chemistry and the time period in which the battery must be fully recharged.
Figure 1. Battery charging is controlled by the baseband µP. If the µP stalls, the chances of damaging
the battery by overcharging are high.
This charging scheme can present a problem if the baseband processor locks up. Damage can occur if
the charger is connected directly to the battery for any length of time. What is needed is a simple add-on
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