13 May 12

The new STC3105 battery monitor from STMicroelectronics provides the precision voltage and current measurements needed to implement a low-cost ‘gas gauge’ for portable electronic devices, such as in smart phones, tablets, and digital cameras.

Accurate predictions of the battery state-of-charge and time-to-empty can be made while monitoring the charge/discharge status of the battery. These features can extend battery life, extend operating time between charges, and help avoid unnecessary recharging. All these features are contained in a 2 x 3 x 0.8 mm package while consuming only a few micro-amps of current.

The STC3105 uses precise voltage and current measurements of the battery to estimate the battery’s state-of-charge (SOC).

A Coulomb counter is used to keep track of the state-of-charge and to track when the battery is charging or discharging.  This Coulomb counter method is combined with the current and open-circuit voltage measurements to estimate the battery capacity and to supervise and track the charge/discharge status. The microcontroller can program the Coulomb counter, set the SOC base register value and read the charge register through the I2C interface. The voltage monitoring accuracy is about 0.5%, and by using an external sensing resistor, Coulomb counter accuracy is 1%.

A battery relaxation timer is used to measure the time interval that initiates when the battery charger is removed or when the current falls below a programmable load current threshold, which means the battery is in a light-load condition. The relaxation counter register is read from the I2C interface and can be checked to make sure that the battery voltage had enough time to relax from the last time the current was high or charging. The timer also indicates when an OCV (Open Circuit Voltage) measurement should be made for SOC correction.

A programmable alarm output provides a warning signal of a low State-Of-Charge (SOC) condition and/or a low battery voltage. The SOC and voltage thresholds are both programmable through the I2C interface.

The STC3105 also helps to extend the battery runtime and lifespan with several power-saving features that not only reduce its own power consumption, but the total power consumed by the application. There are two operating modes, the active mode, which draws 100 μA, and the power-saving mode, which draws 50 μA. There are also two sleep modes, the standby mode, which draws 2 μA, and the power-down mode, which draws 1 μA.

This device can initiate the necessary power-management functions when the battery is nearly empty without involving the system microcontroller, and free the microcontroller for other tasks or enable it to spend a longer time in a sleep mode. This can further increase battery life by decreasing overall power consumption, which will extend the time between charges.