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How to manage power in single-cell battery system

Posted: 22 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:lithium-ion? lithium-polymer? single-cell battery? linear regulator? ESD?

Most mobile and wearable electronic devices, such as smartphones, fitness trackers and headsets, employ a rechargeable single cell battery based on the widely used and well-understood lithium-ion or lithium-polymer technology as energy source. They offer great power density in small sizes but need versatile protection against high charges and discharge currents, over- and under-voltage protection as well as short circuit protection. Otherwise the battery can become defective and cause serious damage. This article provides a solution for devices powered by a single-cell battery including battery monitoring.

Figure 1 shows the block diagram of a fully featured battery power solution. The red lines mark the power path, so basically everything in context with charging and discharging the battery. All blue lines indicate signals, which are processed by the microcontroller, which can either be inputs, outputs or bidirectional.

Figure 1: Block diagram.

The circuit can be supplied either by a USB port or another external power source with +5.0V. The power multiplexer TPS2113A is configured so that it automatically selects the adapter input as the power source if available. Otherwise the USB source is used to charge the battery. The battery charger IC, the bq24050, is a linear regulator based device dedicated for single cell charging, especially charging from a USB power source. It charges a Varta CoinPower CP1654 coin-cell style battery with a nominal voltage of 3.7V and a capacitance of 100mAh. To prevent any damage to the battery, the battery protection device bq29707 monitors the voltage and current. It disconnects the charger or load, if the safe operation area is exceeded. The bq27410 fuel gauge implements measurement of the batteries charge status. The system is controlled and monitored by the MSP430F5510 microcontroller. It offers an integrated USB interface, which is used as serial-to-USB converter for communication with a computer.

A small GUI shows the voltage and average current parameters and helps the designer during evaluation of the system. To complete the system, the TLV70033 linear regulator powers the microcontroller and the TPD4S012 ESD solution protects the USB lines on the charger side.

The battery charger bq24050 uses the data lines (D+, D-) of the USB bus to automatically detect if it is connected to a USB port of a computer or a dedicated USB charger. If pull-down resistors on both lines are present, it indicates a USB port and sets the maximum charge current to 100mA to make sure not to overload a low power USB port. If the D+ and D- lines are shorted, a dedicated USB charger adapter is attached and the maximum charge current is set to a level programmed by a resistor on pin ISET of the IC. If the required charge current is higher than 100mA, while it is supplied by a USB port, the state of pin ISET2 needs to be changed to give the current control back to ISET. The microcontroller gets the information if the circuit is powered by a USB port or an external power source from the status pin of the power multiplexer TPS2113A. If it is powered from a USB port, the microcontroller toggles pin ISET2 of the charger IC, to give back the control of the maximum charge current to pin ISET, where the current is adjustable by jumpers between 15mA and 200mA.

Of course, the USB port has to support those higher currents, otherwise it is limited to 100mA internally. As the charger is based on a linear regulator, the maximum charging current is the same as the maximum input current. Two LEDs indicate if a battery is charged or if charging is finished.

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