An Ultra-Compact and Efficient Li-ion Battery Charger

Circuit for Biomedical Applications

TOPIC:

INTRODUCTION

Analog Li-ion battery charging circuit intended for operation in a wirelessly rechargeable medical implant.

Why Li-ion batteries are preferred in medical implants?

High performance in both energy and power densities.

Wide variety of shapes and sizes efficiently fitting the devices they power.

Much lighter than other energy-equivalent secondary batteries.

No memory effect.

Components are environmentally safe as there is no free lithium metal.

Battery longevity is a primary concern in implanted medical devices.

Battery longevity, in turn, is highly sensitive to the accuracy of the final charging voltage on the battery.

If the Li-ion battery is overcharged, dangerous thermal runaway can occur.

Deeply discharging the Li-ion battery below 3 V can permanently reduce the cell’s capacity.

BACKGROUND

Problems encountered in previous charger designs

Unnecessarily complex control circuitry needed.

This requires more circuit area and power consumption.

Requires a sense resistor in order to detect end-of-charge.

Needs precision on-chip resistor fabrication.

Theoretical Li-ion charging profile

• trickle-charging

• constant current charging

• constant voltage charging

• end-of-charge

Theoretical Li-ion charging profile

Trickle-charge region:- During trickle-charge, the battery is charged with a small amount of current.

Constant current region:- Above 3.0 V, the battery may be charged at higher currents.

Constant voltage region:- As the battery voltage approaches 4.2 V, the charging profile enters the constant voltage region.

End-of-charge region:- Charging current should be decreased until a certain threshold is met.

Simplified battery charger block diagram

The OTA compares the battery voltage to the 4.2 V band gap reference.

As the battery voltage reaches 4.1 V the OTA enters the linear region.

The current gain stage is simply composed of current mirrors.

All current mirrors in this design including those in the OTA are of the Wilson Current Mirror type in order to reduce channel length modulation error.

BLOCK DIAGRAM

Wilson Current Mirror

A current mirror is a circuit designed to copy a current through one active device.

The wilson current mirror circuit eliminates the base current mis-match of the conventional current mirror.

Wilson Fudh current source

simple current mirror Wilson current mirror

The simple current mirror has two main imperfections:

1.The output current differs from the input one because of the two base currents that the transistors Q1 and Q2 "suck" from the input current.

2.The output current varies when the output (load) voltage changes because of the Early effect in BJT or channel length modulation error in MOSFETS.

Disadvantages of simple current mirror

The operational transconductance amplifier (OTA) is an amplifier whose differential input voltage produces an output current.

It uses the differential input voltage to produce a gain in current as the output signal.

The operational transconductance amplifier produce current, while the standard op amp would produce voltage.

Operational Transconductance Amplifier

Schematic symbol for the OTA.

Different types of OTA.

OTA and trickle-charge circuit schematic

If the battery voltage is less than 3 V, the Trickle Charge Flag is low enabling M1, thus transistor M2 conducts some current.

The reduction in charging current during trickle-charge is proportional to the ratio of W/L of M2 to the W/L of M6.

Once the battery voltage crosses the 3 V threshold, the Trickle Charge Flag goes high disabling the current path through M1 and M2.

End-of-charge current comparator

The end-of-charge is detected by comparing the output of the OTA to a reference current;

The End-of-charge Output signal goes low when the OTA output is higher than IREF.

When the End-of-charge Output signal is high, the last stage of current mirrors in the current gain block is disabled.

Trickle charge threshold detector

This low-power detector circuit is to determine when the battery reaches the 3 V threshold.

This circuit is used to detect critically low battery voltage.

when critical threshold is reached, the detector circuit cuts off power to the load.

The designed threshold detector consumes only 3 µW.

Advantages over other charger designs

The circuit naturally transitions between constant current (CC) and constant voltage (CV) charging regions.

Achieving an efficiency of greater than 75%.

This design does not require sense resistors to determine end-of-charge.

This design represents a simple, analog, power- and area-efficient version of previous, more complicated and power-hungry designs.

System Performance during trickle-charge

The battery was charged with 1.5 mA and 2.2 mA during trickle-charge and constant current, respectively.

the design can easily be modified if a higher charging current is required.

System Performance during constant current, constant voltage, and end-of-charge.

The transition between constantcurrent and constant voltage is Continuous.

The end of charge is reached when the current is approximately 0.26 mA.

Has an accuracy of 99.8%.

Comparison of this design with previous Li-ion charger

circuits in the literature.

Most of the literature uses the maximum power efficiency during charging as a figure of merit for battery chargers.

Design Power Efficiency Layout Area

Charge-Pump Technique

67.9% 1.96 mm2

Hysteresis-current-controlled buck converter

82% 2.6 mm2

A Multi-Mode LDO-Based Li-ion Battery Charger

72.3% Not Specified

This Work 75% 0.15 mm2

CONCLUSION

A novel design for a Li-ion battery charger that simplifies the control circuit by using the tanh output current profile of an OTA has been presented and experimentally verified.

This design does not require the use of sense resistors to determine the end-of-charge point, reducing layout area and charging errors due to resistor variability.

efficiency can be further improved if one designs the circuit to operate with a lower supply voltage or with an adaptive supply that varies with battery voltage.

REFERENCES D. Linden and T. B. Reddy, Handbook of Batteries. New York: Mc-Graw Hill, 2002, ch. 35.

Y. S. Hwang, S. C. Wang, F. C. Yang and J. J. Chen, “New Compact CMOS Li-ion Battery Charger Using Charge-Pump Technique for Portable Applications,” IEEE Trans. on Circuits and Systems - part I: Regular papers, Vol. 54, No. 4, pp. 705-712, Apr. 2007.

F. C. Yang, C. C. Chen, J. J. Chen, Y. S. Hwang and W. T. Lee,” Hysteresis-current-controlled buck converter suitable for Li-ion battery charger,” Proc. of IEEE International Conference on Communications, Circuits and Systems (ICCCAS), pp. 2723-2726, Guilin, China, June 2006

P. Li and R. Bashirullah, “A Wireless Power Interface for Rechargeable Battery Operated Medical Implants,” IEEE Trans. Circuits Syst. II, Exp. Briefs, vol. 54, no. 10, pp. 912–916, Oct. 2007.

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