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Li-Ion Battery Charger and Load Sharing System with MCP73837

10/04/2012 Category: Analog, Tutorial

Batteries often serve as the main energy source for portable electronic devices. Although they depend on batteries, portable consumer electronic products, such as GPS devices and multi-media players, often consume energy directly from an ac-dc wall adapter or accessory power adapter (or “Auto Adapter”) when the battery is low or the device is in a stationary mode.

Due to their cost effectiveness over their useful life, rechargeable batteries are often used for the power source of the portable electronic device.

However, most of the time, batteries are designed to be recharged while the devices are still in the operational mode. An end user can extend the run time while refilling the energy back to the battery for the next mobile action. The power source now has to supply the device while charging the battery. The battery can deliver energy to the system load when the power source is absent as depicted in Figure below.

Load Sharing System
click for large image

This circuit shows how to design a simple load sharing system using Microchip’s MCP73837 device for cost-sensitive applications. The system load consumes a maximum current of 500 mA when all applications are running at the same time. A 950 mAh rated Li-Ion battery is available to operate the example portable system for nearly two hours during intensive load operations. The actual battery run time may vary based on the system load, battery age, and environmental conditions.

Li-Ion Battery Charger
click for large image

The input power should supply the system load and charge the battery when a battery is present in the system. When the input power source is removed, the system is supported by the battery. When the system load and the battery draw more energy than the supply can offer, the system load takes priority over the battery charger.

Design Specifications
System Load Input Voltage Range:
– 4.5V – 6.5V from ac-dc adapter (1A)
– 5V from USB port (100 mA/500 mA)
– 3V – 4.2V from 1-cell Li-Ion battery (950 mAh)
Constant Charge Current:
– 0.5C (The battery manufacturer recommended value)
– 100 mA / 500 mA (Charge from USB port)
Precondition Current:
– 0.1C or recommended value
Termination Current:
– 0.07C
Charge Status and Power Good Indicators
Safety Timer: Turn charger off after 6 hours of continuous charging