Li-ion battery charger circuit for biomedical devices - IEEE Xplore

2015 International Conference on Advanced Technologies for Communications (ATC)

Li-ion battery charger circuit for biomedical devices Dat Nguyen(1), Cuong Dao(1), Hao Nguyen-Van(1), Long Nguyen(1), Sang-Gug Lee(2), Loan Pham-Nguyen(1) (1)

IC Design Lab, SET, Hanoi University of Science and Technology

(2)

Nice Lab, Korea Advanced Institute of Science and Technology Email: [email protected]

Abstract— In advance of biomedical devices, the demands of integrated power harvesting system for rechargeable battery applications are underlying issues. This paper presents a wireless power system for Li-Ion battery charger including voltage doubler (AC-DC), voltage linear regulator (LDO) and charging control loop applying multi charging phases method to minimize battery effect. Proposed Li-ion battery charger with input voltage is 6Vp-p at frequency 4MHz, DC output voltage is 4.1V and maximum DC output current is 1.5mA is designed using TSMC 130nm technology for implanted biomedical devices.

Constant End of voltage charge

Constant current

Vhigh Imax

Current

Voltage

Keywords— rectifier, regulator, LDO, control loop, battery model, Li-ion battery.

Vlow

I. INTRODUCTION In recent years, Li-ion battery has been widely used in consumer electronics, electric vehicles, military electronics, as well as biomedical devices. Li-ion battery offers several advantages such as: rechargeable many times, high specific energy, high energy density, no memory effect [1]. In implanted biomedical devices, Li-ion cells are taken into account of biocompatibility, longer life, ultra-high safety, and unbeatable reliability in the challenging operating conditions within the human body [2]. In this paper, QL0003I rechargeable Li-ion cell specifications are utilized to establish battery model in simulation environment and other parameters in charging control loop circuit. From Li-ion battery specifications [3], Li-ion battery for biomedical has a charging profile with three stages: constant current, constant voltage and end of charge (Fig. 1). Constant current stage: In standard Li-ion cells, when battery voltage is in between voltage level VL and VH, a large constant current is applied to charge the battery (0.2 – 1C) [4]. The battery capacity is expressed by C and measured as Ampere-hours (Ah). In this mode, charging time can be reduced by maximizing charging current value of Li-ion battery’s capacity. However, in order to retain initial battery capacity, charging current is set at 0.5C (1.5mA) [3]. Constant voltage stage: If battery voltage reaches battery’s specification value (4.0V), charging current will be reduced to a predicted cut-off current. Consequently, when battery voltage is 4.1V, battery will reach its full capacity and charging process is completed.

0

Fig. 1. Li-ion battery charging profile

In this paper, we study a wirelessly powered battery charging circuit using TSMC 130nm CMOS process. This circuit is designed for implantable biomedical applications. In section II, system description with block diagrams structure and functions will be discussed. Post-layout simulation results and concluding remarks are provided in Sections III and IV, respectively. II. SYSTEM DESCRIPTIONS

Rectifier

Regulator

Control loop

Model Li-ion battery

Battery charger Implant into the human body

Fig. 2. Block diagram of battery charger system

Fig. 2 shows block diagram of studied rechargeable Li-ion battery charger system. Each of the sub-block in this system will be discussed further in the following sections. In charging process, an external 4MHz - power supply is rectified by a voltage doubler circuit and then regulated by linear regulator to supply a stable voltage. This supply voltage is used to power the battery charging control loop circuit which drives charging current following charging profiles, and providing battery-safety protection as well. The test bench for charger system in time domain is battery model in which battery output voltage follows charging current and time.

End-of-charge stage: Because Li-ion cells are degraded irreversibly by overcharge, and may vent if overcharge, charging current must be terminated once the voltage reaches the highest recommended battery voltage and fully charged. With 3mAh Li-ion battery, cut-off current is about 0.06mA [4].

978-1-4673-8374-5/15/$31.00 ©2015 IEEE

Charging time

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be turned off completely, the obtained power conversion efficiency is over 85%.

A. Rectifier VC

C1

Vin

M2

C21

M22

SP2

Vout

R1

SP1

B. Regulator Regulator block is used to provide a regulated voltage for control loop block. Based on the target, input voltage of regulator is from 4.6V to 5V while the output voltage of regulator is fixed at 4.5V. Low dropout voltage (LDO) topology is selected to meet the requirement for maximum output current of regulator (1.5mA). The proposed LDO shown in Fig. 4 has 4 main blocks: pre-regulator, bandgap voltage reference (BGR), error amplifier, pass transistor and feedback system.

C2

SN2 M1 R2

gnd

C11

M11

SN1

Fig. 3. AC/DC rectifier with switching IVC

The input voltage of charging block is of 3V and frequency of 4MHz. The output of charging block requires a DC voltage of 4.1V to charge battery so the voltage doubler topology is chosen for rectifier block. Fig. 3 shows a voltage doubler topology employing switch internal voltage cancellation (IVC) technique, which has been proposed in our previous work [5], to obtain low drop voltage over two active diodes M1, M2. Moreover, the switch IVC technique does not consume as much power as a conventional comparator does. Therefore, the power consumption of rectifier block is minimized allowing to increase power conversion efficiency PCE of rectifier block. M1, M2, C1, C2 are the main elements in voltage doubler structure. There are two IVC blocks, the first block for M1 including M11, SN1, SP1, R1, C11 and the second one for M2 grouping M22, SN2, SP2, R2, and C21. The operation of the first IVC block is as follow. During the ON state of M1, SN1 is OFF and SP1 is ON. M11 and R1 create a voltage divider structure which generates a voltage VG1§Vthn applied to the gate of M1. In contrast, SN1 is ON and SP1 is OFF during the OFF state of M1. So the gate of M1 is connected to ground then M1 is turned off completely. The operation of the second IVC block for M2 is similar to that of the first IVC block for M1. As M1, M2 can

First, pre-regulator has bootstrap and op-amp sub-blocks. Bootstrap sub-block adopt cascade topology to generate output voltage VA that is less dependent on Vin LDO. Because of the instabililty of bootstrap circuit when Vin LDO varies so a capacitor C1 is added to improve its transient behavior of bootstrap circuit. The op-amp sub-block is designed to keep VA=VB to generate an output voltage of pre-regulator or input voltage of BGR VC that is less dependent on Vin LDO. Preregulator is required to feed a well-regulated voltage into BGR and to generate bias voltages for following block. The first function allows gain of op-amp in BGR not need to be high, so the value of capacitor for compensation is small. Therefore, the implemented area is reduced. The second function of preregulator block enables more flexible choices of bias voltage levels for following blocks without any additional bias voltage blocks. Second, the BGR has two sub-blocks called start-up circuit and bandgap core. The start-up circuit helps bandgap core avoid start-up problem because two branches of bandgap core can carry zero currents. Moreover, start-up circuit can be turned on during starting time of LDO and turned off for the

Vin LDO M1

M3

Bias1

M2

M13

M14 M18

Bias2

M4 M9

M28

VC M19

M23

M11

VD M15

VB R2

M6 M17

M7

M8

M30 M25

Bias2

R7 C2 R8

M22

R3

R9

R5

M21 R4

Q1

Q2

C1

Op-amp

Pre-regulator

Vout LDO

M31

M26

M27

M32

M33

R1

Bootstrap

Pass transistor

C3 M20

M5

M34

R6

M16

M12

M29

M24

M10

VA

Bias1

Start-up

Bandgap core

BGR

Error amplifier

Fig. 4. Proposed Low dropout voltage circuit

363

M35

R10

Iload


remaining time. At initial period, output voltage of BGR VD is 0V. M19, M20 are on, while M21 are turning M22 on. That will turn M23, M24 on then two branches of bandgap core bring nonzero current, so VD increases. When VD reaches a specific voltage, M21 turns on and M19, M20 turn off then M22 turns off. At this time, startup circuit is disconnected from the bandgap core. In the bandgap core, Capacitor C2 is connected in parallel with R6 to improve the power supply rejection of bandgap at frequency greater than 1MHz. The bandgap core generates output voltage approximately 1.2V.

charging current to several micro Ampere. When current through M9 increases, the voltage of RB will be higher than RA. Thus the output voltage of EOC detector will be low that will turn off power MOSFET and terminates charging current.

+ C_CAP f(Icharge)

Third, error amplifier uses folded cascode op-amp as input stage and the output stage uses common source with diode connected load to reduce output resistance of error amplifier. This pushes the second pole of LDO at higher frequency. For frequency compensation purpose, capacitor C3 is connected between output of LDO and the gate of current mirror M32, M33 that generates a low impedance node to create a left half plane (LHP) zero.

M1

M5

M6

M7

M8

M12

M10

M11

M13

Power MOS

Bulk protection

A1

R2

M14

)

Post-layout simulations (using Cadence) are carried out to verify the operation of layout of battery charger circuit. In this simulation, the value of C_CAP in Li-ion battery model is set at 5μF to reduce the simulation time and initial voltage of battery is 2V. Fig. 7 shows the output voltage of three main blocks of charger circuit: rectifier, LDO, control loop and output current to charge battery in time domain. At the start up stage, the initial output voltage of rectifier, LDO, and control loop (VBAT) is 2V because of the initial voltage of battery. After that, the voltages start to ramp up: Vout Rectifier from 2V to 4.6V, Vout LDO from 2V to 4.5V, VBAT from 2V to 3.6V, and the IBAT increases from 0 to 1.5mA. Once IBAT reaches 1.5mA, charger circuit enters the constant current stage. In constant current stage, IBAT remains approximately at 1.5mA, VBAT increases from 3.6V to 4V. Thanks to the high stability of LDO, Vout LDO is kept at 4.5V. Vout Rectfier also maintains constant at 5V as the output current of rectifier block is almost unchanged during constant current stage. After VBAT reaches 4V, charger circuit goes into constant voltage stage. In this stage, IBAT start reducing significantly from 1.5mA to nearly 0. The significant decreasing of IBAT leads to the increasing output resistance of rectifier, so the output voltage of rectifier increases from 5V to 5.9V. Again, thanks to the high stability of LDO, although the input voltage of LDO increases and the output current of LDO decreases, Vout LDO is always at 4.5V. This allows control loop operate precisely. Once VBAT reaches 4.1V, constant voltage stage ends and charger circuit goes into the last stage, end of charge stage. At the end of charge stage, IBAT is 0A and VBAT is 4.1V, Vout LDO is 4.5V, Vout Rectfier is 5.9 and charger circuit is disconnected from Li-ion battery by turning off pass transistor of control loop circuit in Fig. 5.

IBAT

VBIAS

M9

VREF

R1

A2

SOC

III. POST LAYOUT SIMULATION RESULTS

EOC

M4

- f(V

As shown in Fig. 6, battery model includes 3 components: capacitor (C_CAP), current control current source (CCCS), voltage control voltage source (VCVS). Capacitor C_CAP represents the state of charge by its voltage (0 to 1). In order to simulate Li-ion battery charger system in time domain, battery model is utilized. In charging process, CCCS generates a current depend on charging current. Capacitor is charged by CCCS current making its voltage raise from 0 to 1V or stateof-charge from 0 to 100%. Meanwhile, capacity voltage controls VCVS to output battery open voltage. The value of battery voltage followed state-of-charge is extract from [6].

C. Charging control loop and Battery model

M3

-

+

Fig. 6. Li-ion battery model design

Finally, pass transistor acts as a variable resistor to control the current to generate a constant output voltage. This controlled current flows through chain of resistors R7, R8, R9, R10 then establishes constant voltages. These constant voltages are used as VBIAS, VREF for charging control loop as shown in Fig. 5.

M2

VSOC

Icharge

M15

Fig. 5. Charging control loop circuit

Fig. 5 shows the design of battery control loop circuit including four main blocks: unit current, constant currentconstant voltage detector (comparator A1), end-of-charge detector (comparator A2) and charging current generator. The circuit is powered (VREG) at 4.5V by a low dropout voltage. Unit current block generates reference current to bias for other branch in circuit. CC-CV detector (A1) compares battery voltage (VBAT) and reference voltage (VREF=4.1V) to reduce charging current in Constant Voltage stage. End-of-charge detector (A2) comparing voltage of RA and RB, outputs end-ofcharge signal to terminate charging current. Initially, during CC stage, VBAT is smaller than reference voltage VREF, so M9 is off and the current across M10 is large. This current is entirely mirrored via current mirror of charging current generator block to supply the battery with a current of about 1.5mA. Charging current of power MOSFET varies from 1.55mA to 1.4mA due to channel length modulation as cell voltage (VBAT) increase from 2.7V to 4.1V. When battery voltage is equal to VREF, CC-CV detector (A1) will turn M9 on and charging process to Constant Voltage stage and reduces

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IV. CONCLUSION A completed integrated charger circuit is designed in TSMC 130nm technology. Post layout simulation confirms that the proposed charger circuit operates with high stability and accuracy in following Li-ion battery charging profile. Layout of charger circuit occupies an area of 195 x 157 μm2 (Fig.8). Comparing with other published studies [7][8][9], the proposed circuit offers good performances regarding power efficiency (77%) and EOC current (16μA). V. ACKNOWLEDGMENT We would like to express our special thanks to NICE lab, Korea Advanced Institute of Science and Technology (KAIST) for their financial and technical support. REFERENCES Fig. 7. Post-layout simulation results of battery charger circuit with (a) output voltage of Rectifier, (b) output voltage of LDO, (c) Li-ion battery voltage, (d) charging current to Li-ion battery in time domain

[1] [2]

LDO

[3] CONTROL LOOP

[4] [5]

[6] RECTIFIER Fig. 8. Layout of charger circuit

[7]

TABLE I. Battery charger circuit benchmark

Technology Battery capacity Regulated voltage Charging current EOC current Battery voltage Efficiency

[7] 0.35μm 350mAh

[8] 0.6μm

[9] 0.5μm 25mAh

This work 0.13μm 3mAh

4.4V

4.1V

5V

4.5V

350mA 17.5mA 4.104V

1.5mA 4.1V

2.5mA 0.26mA 4.21V

1.5mA 16 μA 4.1V

93.1%

73%

75%

77%

[8]

[9]

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D. Linden and T. B. Reddy, Handbook of Batteries. New York: Mc-Graw-Hill, 2002, ch. 35, pp 35.2. Boone B. Owens & P. S. S. Prasad, University of Minnesota, “The use of Lithium batteries in biomedical devices”, 9th discussion meeting of new battery conceptive division, Kyoto University, 9/1989. Mikito Nagata, Ashok Saraswat, “Miniature pin-type lithium batteries for medical applications”, Journal of Power Source, pp.762-765, 2005. S. Dearborn, “Charging Li-ion batteries for maximum run times”, Power Electronics Technology, pp. 40-49, Apr. 2005. Dat Nguyen, Toan Nguyen, Long Nguyen, “A high power conversion efficiency rectifier with new internal VTh cancellation topology for RFID applications”, IEEE Fifth International Conference on Communications and Electronics (ICCE), pp. 313-316, August 2014. Min Chen, Gabriel A. Rincon-Mora, “Accurate Electrical Battery Model Capable of Predicting Runtime and I-V Performance”, IEEE Transactions on Energy Conversion, Vol 21, No 2; pp 504-511, June 2006. Jader A. De Lima, “A Compact and Power-Efficient CMOS Battery Charger for Implantable Devices”, Symposium on Integrated Circuits and Systems Design (SBCCI), pp. 1-6, Sept 2014. Pengfei Li, Rizwan Bashirullah, “Wireless Power Interface for Rechargeable Battery Operated Medical Implants”, IEEE Transactions on circuits and systems, pp. 912-916, October 2007. Bruno Do Valle, Christian T. Wentz, Rahul Sarpeshkar, “An Ultra-Compact and Efficient Li-ion Battery Charger Circuit for Biomedical Applications” Proceeding of 2010 IEEE International Symposium on Circuit & System (ISCAS), pp. 1224-1227, June 2012.