June Birth Magoncia

user Electronics Engineer

city Innovuze Solutions, Inc.

  • Single 3.7V/4.2V Li-Ion/Li-Polymer Battery Charger with Thermal Cutoff

  • Created: Jan 23, 2017

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This reference design features a circuit that can be used to charge a single 3.7V or 4.2V lithium-ion or lithium-polymer battery. Lithium-ion and lithium-polymer batteries are now widely used in different electronic devices such as mobile phones, notebook computers, battery-powered electric vehicles, etc. Since they are very light and have four times energy density, they are more preferred to be used than nickel cadmium or nickel metal hydride batteries. However, lithium-ion or lithium-polymer batteries must be charged properly in order to avoid issues like overcharging and overheating that lead into fire accident.

The circuit in this design is just simple and only uses few components. The charger is based on the MCP73833 device that can charge up to 1A. The MCP73833 can accept 3.75V as minimum supply voltage and 6V as maximum. In this circuit, the MCP73833 device was designed to accept 5V supply through the USB connector or the DC barrel jack connector. The MCP73833 charging current is determined by the value of the resistor connected between its PROG pin and the ground. The charging current in this circuit is set to 500mA by placing a 2kΩ resistor. The VBAT pins serve as the battery charge control outputs of the MCP73833. The battery terminals can be connected to the JST connector or to the screw terminal. The MCP73833 also includes status indication and power good pins that can be used to illuminate LEDs to indicate the charge cycle states of the device like shutdown, standby, charging, charge complete, temperature fault, timer fault, and system test mode. An optional NTC thermistor can be connected to the MCP73833 THERM pin to monitor temperature. But if not used, a plain resistor can be used instead of a thermistor.

In this design, an AA85A10 thermal cutoff (TCO) device was used to protect the circuit and the battery from overtemperature conditions. The AA85A10 acts as a closed switch under normal operation but opens when temperature hits 85°C. It must be mounted near to the heat source in order for it operates effectively. The circuit in this reference design can be used to charge single lithium-ion or lithium-polymer cell with rating up to 6600mAh.