Power management design for single-cell battery system
Introduction
The energy source for mobile and wearable electronic devices, such as smartphones, fitness trackers and headsets, is typically a rechargeable single cell battery based on the widely used and well-understood lithium-ion or lithium-polymer technology. They offer great power density in small sizes but need versatile protection against high charges and discharge currents, over- and undervoltage 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.
Overview
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.
Next: Functionality
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.
Functionality
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.
Lithium-ion batteries are very sensitive regarding the minimum and maximum voltages as well as the charge and discharge currents. It has to be assured that the voltage during charging is not too high and that the battery is disconnected from the load if a certain lower threshold is hit. Very high currents or even a short of the battery can cause serious problems resulting in burning. Thus, the dedicated battery protection bq29707 monitors all the critical parameters during operation and disconnects the battery from the charger or load if the safe operation area is left. The over- and undervoltage thresholds are fixed and depend on the version of the IC, whereas the maximum current is set by a shunt.
Figure 2 – Discharging with 200mA
Figure 2 shows the discharge cycle of a fully charged battery with a constant current of 200mA. As soon as the load is attached, the battery voltage drops from 4.1V to about 4.0V. Then, the voltage decreases slowly to 2.80V within 25 minutes. To prevent an over-discharge of the battery, the battery protection disconnects the battery from the load once the lower threshold of 2.80V is hit.
Next: Charging
Figure 3 – Charging with 200mA
Figure 3 shows the charging cycle of a fully discharged battery with the maximum charging current set to 200mA. The current falls almost linearly during the charging time, because the voltage of the battery increases, therefore, the voltage difference between the charger (4.20V) and the battery becomes smaller. After charging is finished, the charger releases the battery and the voltage settles at about 4.1V.
The user of a mobile device usually wants to have an overview of the battery’s state-of-charge to estimate the remaining runtime. The bq27410 fuel gauge is utilized to gather all the information about the battery’s health like average current (charge – discharge), battery voltage and remaining capacitance. Before its first use, it needs to be calibrated with Texas Instruments “bq Evaluation Software” to give precise measurement results. During the calibration process the output voltage and load current have to be measured precisely. These parameters are then stored in the calibration memory of the device to eliminate any tolerances and the influence of the copper resistance of the printed circuit board.
Next: Board and links
The MSP430F5510 microcontroller, with an integrated USB interface, shows up as a virtual COM port on a computer. It reads the data from the fuel gauge via I2C and sends it to a GUI, which shows all the relevant parameters of the battery for easy evaluation of the battery power system.
Figure 4 – Board Photo
Conclusion
For a safe and reliable operation of a battery-powered device, some precautions for the battery itself have to be taken. The charging algorithm needs to match the battery’s chemistry and protection regarding overcharge, over-discharge and too high currents is essential. The reference design presented provides charging, protection and monitoring for a single coin-cell battery to supply mobile devices. For monitoring and development purposes, the board can be attached to a computer via USB to show the relevant parameters of the battery on a GUI. Further information including schematic, layout, bill of material and software is available on TI.com with PMP5722 as reference.
About the author
Matthias Ulmann was born in Ulm, Germany, in 1980. He was awarded a degree in electrical engineering from the University of Ulm in 2006. After working for several years in the field of motor control and solar inverters specialized in IGBT-drivers, he joined TI’s Analog Academy for a one-year trainee program. Since 2010 he has worked in the European design services group as a reference design engineer in Freising, Germany. His design activity includes isolated and non-isolated DC/DC converters for all application segments.
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