Chip scale Li-Ion protection for wearables
Wearable devices incorporate a small rechargeable Li-Ion battery with a capacity in the range from 50 to 100 mAh and have new challenging requirements for its internal components, like an ultra-small package size, low current consumption in On and Standby mode but also sophisticated safety circuits. The R5441 and R5443 fully comply with these requirements. The R5443 is the smallest version and has a board area of 1.05 mm2 and a thickness of 0.36 mm, around half the size of protection devices in a 1.4 x 1.4 mm DFN package.
The two devices have a current consumption of 3.5 µA (R5441) and 2.5 µA (R5443). As soon the battery voltage decreases below the over-discharge voltage threshold, the product enters a standby mode and disables several internal circuits to lower the current consumption to 0.04 µA, ensuring further discharge of the battery is kept to a minimum.
Li-Ion batteries are safe in general when used according to the specifications but can become a potential hazard in case of an internal or mechanical malfunction. Overheating, emitting fumes or even worse can be the result; therefore, the protection IC monitors the charge and discharge process and interrupts as soon the process is at risk of going beyond the safe operation range of the battery cell.
Both the R5441 and R5443 have an improved over-charge voltage threshold accuracy of ±10 mV versus ±30 mV for regular products. The idea behind this feature is to increase the charge voltage whilst maintaining the same safety level. As a result, the battery voltage will increase with 40 mV and extends the battery lifetime, which is important for applications with a small battery capacity.
Traditionally one makes use of the MOSFET on-resistance in order to measure the current flow of the battery pack. However, the on-resistance level is most inaccurate as it depends on various criteria such as ambient temperature, gate voltage and MOSFET type. In order to achieve a higher standard of current sensing, the R5441 and R5443 make use of an external current sense resistor. Such resistor can be ordered with your preferred resistance value and accuracy level. The great advantage of using an external resistor is that the over-current threshold margin improves significantly in comparison to the MOSFET current sense method.
The R5441 has an additional circuit to monitor the temperature of the battery pack by connecting an external NTC resistor to the Tin pin. In case of an over-temperature event, the charging or discharging process is interrupted. It is even possible to set different temperature threshold settings for the over-charge and over-discharge threshold.
Optional latch and auto-release functions for the over-charge and over-discharge monitoring circuits are available. A latch function keeps the charge or discharge process interrupted until a load or charger is connected. The auto-release function releases the interruption at the same condition and when the release threshold is exceeded.
Circuit delay times are fixed internally and do not need any external components. With 0V battery charge function, it is possible to prevent recharging when the battery voltage decreases below the deep discharge threshold. In such case, it would take a long time to recharge the battery and/or it may cause excessive heat dissipation.