COVER STORY New ideas and
advanced solutions for Li-Ion protection ICs
Hans Adams, manager Semiconductor Support Centre, Ricoh Europe (Netherlands) B.V. talks about Li-Ion battery protection
I
n recent years, the number of applications using Li-Ion batteries has increased significantly. Initially, single cell batteries were used in mobile phone applications but nowadays they are used in almost any kind of portable device. Ricoh has always been a pioneer in developing Li-Ion protection ICs and production started around 1995. So far, smartphones were considered as one of the smallest or thinnest devices using Li- Ion batteries but this situation has changed in the last few years with the introduction of rechargeable smart watches, hearing aid instruments and fitness trackers. As a result, the required battery capacity shrinked from a 3000mAh for smart phones to around 50mAh for these so-called wearable
devices. Such applications have new challenging requirements for its internal components, like ultra-small package size, low current consumption in on and standby mode but also sophisticated safety circuits. The new R5441 and R5443 were especially designed for this purpose and both products are embedded in a wafer level chip scaled package (WLCSP) to minimise board space. The R5443 is the smallest version and requires a board space area of only 1.05mm2 and has a thickness of 0.36mm, it is considered as one of the smallest single cell Li-Ion protection ICs available today. Around 53 per cent smaller compared to the popular protection ICs in a 1.4 x 1.4mm DFN package.
Li-Ion battery protection Li-Ion batteries are safe in general when used according to the specifications but can become a potential hazard in the case of an internal or mechanical failure. Overheating, emitting fumes or even worse can be the result and also the manufacturer’s reputation can become seriously harmed when a disaster strikes. The Li-Ion battery protection IC monitors the charge and discharge process and intervenes as soon as the process is at risk of going beyond the safe operation range of the battery cell. The protection ICs can be used on the application’s circuit board when used with a bare battery cell but can also be integrated in a laminated battery pack. In particular the rechargeable Li-Ion batteries have strict operation limits to keep the battery cell in optimal condition and to prevent a critical situation when overloaded. The R5441 and R5443 include several detection circuits and their outputs control two external MOSFETs that are connected in series with the battery wiring.
Protection circuits • Over-voltage: When the battery voltage exceeds the over-charge voltage threshold for a period of time, the chip will interrupt the charge process preventing an overload condition. • Over-discharge voltage: In the event that the over-discharge voltage threshold is exceeded for a period of time, the chip will interrupt the discharge process and enters a standby mode. It disables some internal circuits to lower the current consumption and ensuring further discharge of the battery is kept to a minimum. • Discharge over-current: When the discharge current exceeds the maximum threshold for a period of time, the protection circuit becomes active. • Charge over-current: The protection
circuit is triggered as soon the charge current exceeds the threshold for a period of time. • Short current: The short protection
circuit is triggered at a higher discharge current level compared to the discharge over-current level and has a shorter delay time.
High accuracy over-charge voltage threshold
Both the R5441 and R5443 have an improved over-charge voltage threshold accuracy of ±10mV versus ±30mV 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 operation lifetime will increase with 40mV that is important for applications with a small battery capacity. The image shows more details and compares a standard versus an improved accuracy level.
Resistor current sense versus MOSFET current sense 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. Design
8 September 2018 Components in Electronics
www.cieonline.co.uk
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