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Research ArticleA High Efficiency Li-Ion Battery LDO-BasedCharger for Portable Application
Youssef Ziadi and Hassan Qjidaa
CED-ST, LESSI, Faculty of Sciences Dhar el Mahraz, Sidi Mohamed Ben Abdellah University, BP 1796, 30003 Fez, Morocco
Correspondence should be addressed to Youssef Ziadi; youssef.ziadi@usmba.ac.ma
Received 2 June 2015; Revised 2 August 2015; Accepted 12 August 2015
Academic Editor: Yuh-Shyan Hwang
Copyright © 2015 Y. Ziadi and H. Qjidaa. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.
This paper presents a high efficiency Li-ion battery LDO-based charger IC which adopted a three-mode control: trickle constantcurrent, fast constant current, and constant voltage modes. The criteria of the proposed Li-ion battery charger, including highaccuracy, high efficiency, and low size area, are of high importance. The simulation results provide the trickle current of 116mA,maximum charging current of 448mA, and charging voltage of 4.21 V at the power supply of 4.8–5V, using 0.18 𝜇m CMOStechnology.
1. Introduction
As we know, portable devices have become the main appli-cations of advanced technical products. Due to their smallsize, light weight, and recharge ability, Li-ion batteries arewellsuited to portable electronicmanufactures such as cell phonesand PDAs [1] because of their high energy density, long cyclelife, high voltage, and absence of memory effects. But the lifeof a rechargeable battery depends not only on charging time,but also on overcharging control and charging strategies [2].To avoid the battery from overcharging, the charging processneeds a specificmethod supporting the constant current (CC)and the constant voltage (CV) modes in order to charge thebattery by a degrading current [3–5].
In literature, there are several architectures of batterychargers with different approaches of power control methodsin order to adapt the supply voltage. The same are char-acterized by high efficiency such as the DC/DC converter,switching-capacitor, or switching mode power supply [1, 6–8], but they are not suitable for single chip integration and,sometimes, they have low accuracy despite their efficiency.On the other hand, the same are using charge pump as anadaptive supply voltage [3]; however this one is distinguishedby its high current ripple and low efficiency. LDO-basedcharger is characterized by a low current ripple and it canbe integrated into the chip without descript components [9],
but itsmajor problem is the low efficiency. In thisworkwewillimprove the efficiency of an LDO-based charger, using thepower transistor as a variable current source and minimizingits dropout.
One of the important missed criteria in several works[1, 3, 6, 8–12] is the control of temperature during chargingand the lack of trickle current charge mode in [1, 13] whichis necessary for the battery protection when it is fullydischarged. That makes this work a complete battery chargerchip with respect to all charge procedures and the batteryprotection.
This paper describes the architecture and simulationresults of a three-mode control high efficiency battery chargerintegrated circuit for Li-ion batteries with charging currentsfrom 116mA up to 448mA. In Section 2 we describe thetypical charging method of Li-ion battery. The architectureand the functionality of major blocks of the proposed inte-grated circuit are illustrated in Section 3. Simulation resultsare shown in Section 4. Finally, the conclusion is remarked inSection 5.
2. Li-Ion Battery Charger Method
Amongst the important criteria of Li-ion battery charger issafety of charging. Yet there are some limitations for the
Hindawi Publishing CorporationActive and Passive Electronic ComponentsVolume 2015, Article ID 591986, 9 pageshttp://dx.doi.org/10.1155/2015/591986
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2 Active and Passive Electronic Components
End of chargeCV
VoltageCurrent
4.2
Fast CCTrickle CC
2.9
C/40
0
Char
ging
vol
tage
(V)
Char
ging
curr
ent (
A)
Charging time
Ifcc
Itcc
Figure 1: Typical charging process of a Li-ion battery.
Charging start
Detect battery
YesTrickle currentcharging
NoYesFast constant
current charging
NoConstant voltage
charging
No
Yes Yes
End of charge
Over temperature?
voltage (Vbat)
Vbat < 2.9V?
Vbat < 4.2V?
Ichg < C/40
Figure 2: Battery charging flowchart.
current and voltage of charging; thus, the battery temperaturewill augment severely causing fatal troubles due to some ofthese limitations.
Notably, the typical charging current is 1C, in which thecurrent can completely charge a battery in one hour [14].
The typical charging profile of Li-ion batteries shownin Figure 1 is needed to achieve three fundamental modes:trickle constant current, fast constant current, and constantvoltage modes.
So far as the first trickle constant current mode is con-cerned, when the battery voltage (𝑉bat) is less than 2.9V,the internal resistance of Li-ion battery is getting larger; theLi-ion battery, accordingly, has to be charged by a trickleconstant current phase; this strategy is called an “overnightcharger” [15]; as for the second phase, once the value of𝑉bat is
larger than 2.9V, the process switches from the trickle currentto the fast constant current phase.
Ultimately, we use constant voltage to charge the batterywhen the battery voltage is greater than 4.2V, whilst thecharger is operating within constant voltage mode.
There are twomethods to terminate the charging process,the first of which is monitoring the minimum chargingcurrent at the CV stage. The charger terminates the chargingprocess when the charging current shrinks to the specifiedrange. To finish the charging process, the other one is basedon the maximum charging time [16].
In the proposed design, we have adopted the first methodto terminate the charging process; the battery is charging untilthe charging current is less than 1C/40.
The whole charging flow of our Li-ion battery charger isdesigned as shown in Figure 2.
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Active and Passive Electronic Components 3
PTAT
CV
M1
M2
End of charge
Li-io
n ba
ttery
Level shifter
2.9V comparator
4.2V comparatorIref2Iref1
A2
A3
A4
A0
Vref2
Vref1
R1
R2
R3
Vfb1
Vfb2
A1
1 : N
Vbat
Ichg
Vadapt
−+
−+
−+
−+
−+
Rsens
M1
M2
MP ≡Mcs
Mm
Figure 3: Simplified diagram of the proposed Li-ion battery charger.
3. Circuit Descriptions
Contrary to the other architectures which use a microproces-sor to control the differentmodes of the charge, our proposedarchitecture is a purely analogic support of the threemodes ofLi-ion battery charging, trickle constant current, fast constantcurrant, and constant voltage, as shown in Figure 3. It ismade of many blocks: LDO, current generator, current sense,temperature sense (PTAT circuit), and bandgap multioutput.The supplying tension 4.8V–5V is adapted by LDO regulatorbefore generating 𝑉adapt. The power transistor MP is equiva-lent to a variable current source; this technique has been usedin [10] with flyback converter. The MP control made by twocurrent sources 𝐼ref1 and 𝐼ref2 passed through a shifter levelcomposed of𝑀1, 𝑀2, 𝑀1, and𝑀2, so that the transistorMP provides a constant current order of 116mA and 448mAto order the trickle constant current and the fast constantcurrent modes. We use the integrator to control the constantvoltage mode. These modes are switched by Op 𝐴
2and 𝐴
3
that compares tensions 𝑉fb1 and 𝑉fb2 with 𝑉ref1. We use theoutput of 𝐴
1and 𝐴
4to stop charging when the battery is
full or when the temperature detected by the PTAT circuitexceeds 115 degrees.
Figure 4 shows the complete architecture of the proposedLi-ion battery charger. What distinguishes our architectureis the use of current like a parameter of command toswitch the power transistor MP between different modesof charge, and it is utilized like a variable current source.The current generator furnishes two reference currents 𝐼ref1and 𝐼ref2 which are copied by current mirror (𝑀4 and 𝑀3)to command the MP gate with level shifter (this currentgenerator will be discussed in Section 3.1). Unlike trickle
constant current and fast constant current (CC mode) witchare under the control of 𝐼ref1 and 𝐼ref2, CVmode is controlledby an integrator, following its principles [10]; when 𝑉fb1 isgetting larger gradually, the integrator output is decreasing,so the MP furnishes a current decrease until the end ofcharge; we could make use of voltage-to-current conversionto generate CV current.
The end of charge occurs when the current sense detectsthat 𝐼chg (charge current) equals 20mA or when the dietemperature transpasses 115∘C switching off PMOS (𝑀eoc)integrated into the current generator.
3.1. Current Generator. The current generator furnishes thereference currents, 𝐼ref1 and 𝐼ref2, to control the trickle CC andfast CC modes. We have opted for utilizing this architecture,as shown in Figure 5, which is not influenced by the tempera-ture variation [17]. It is formed by a conventional architecture(𝑀6-𝑀6-𝑀9-𝑀10) where the passive resistor is changedwith a PMOS transistor𝑀18 and its gate bias generator.Thereare two diode-connected NMOS transistors and one PMOStransistor that make up the gate bias generator, for eachmode(𝑀11, 𝑀12, and 𝑀7 for 𝐼ref1 mode and 𝑀13, 𝑀14, and 𝑀8for 𝐼ref2 mode). Whereas𝑀7 copies the reference current 𝐼,the gate voltage of𝑀18 is generated by the diode-connectedtransistors 𝑀11 and 𝑀12 to generate 𝐼ref1. We can generatethe reference current 𝐼ref1 or 𝐼ref2 by the control switches𝑀15and 𝑀16. We integrate a transistor 𝑀eoc switcher to cancelthe generation of 𝐼ref1 and 𝐼ref2 to end the charging process.
3.2. Bandgap Reference Multioutput. In our proposed design,four reference tensions would be prerequisite (𝑉ref1, 𝑉ref2,
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4 Active and Passive Electronic Components
M1
M2
PTATcircuit
M3M4
M17
LDO
Current generator
M18
M6M5
M9 M10
M15
M7
M11
M12
M16
M8
M13
M14
End of charge
CV integrator
Li-io
n ba
ttery
2.9V comp
4.2V comp
A2
A3
A5
A6
A4
A0
Vref2
Vref1
Vref3
Vref4
R1
R2
R3
Vfb1
Vfb2
A1
1 : N
Vbat
Ichg
VsupVadapt
Vdd
−+
−+
−+−
+
−+
−+
−+
Rsens
M1
M2
MP ≡
C1
C2
Mcs
Mm
Meoc
Figure 4: Complete architecture of the proposed Li-ion battery charger.
End of charge
M18
M5 M6 M7 M8
M9 M10
M11
M14M12
M13M15 M16
I
Iref2 modeIref1 mode
Iref
Vdd
2.9V compMeoc
Figure 5: Current generator circuit.
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Active and Passive Electronic Components 5
A B
M1 M2 M3 M4 M5 M6M8M9M7
M10M11M12
M14M15M16
M13
Vdd
Vref2 Vref3Vref1 Vref4
C1R8
R1R2
R3
R4 R5 R6 R7
A0
Q1 Q2
I1
I11 I12 I21
I22
I2 I3 I4 I5 I6− +
Figure 6: Bandgap reference multioutput.
𝑉ref3, and 𝑉ref4), hence making use of multioutput bandgapreference tensions. Figure 6 shows a high precision tempera-ture compensates CMOS bandgap reference [18].This latter isimproved in order to generate𝑉ref1 = 1.3 V,𝑉ref2 = 1.4V,𝑉ref3 =0.53mV, and 𝑉ref4 = 0.99mV reference voltages by means ofadding four out-stages.
The equation of the two generated currents, which areproportional to𝑉EB andΔ𝑉EB, to bias these four added stages,is the following equation:
𝐼1= 𝐼2= 𝛼𝐼3= 𝛽𝐼4= 𝛾𝐼5= 𝛿𝐼6. (1)
As shown in Figure 6, each current is divided into twoother currents passing through two branches containingresistor and bipolar transistor in such a way that
𝐼11= 𝐼22;
𝐼12= 𝐼21.
(2)
We made 𝑅2= 𝑅3, to make the voltage of 𝐴 equal to 𝐵.
The inputs of the operational amplifier are equalized.From Figure 6, 𝑉
𝐴= 𝑉EB1. Consider
𝑅3𝐼11= 𝑅2𝐼22,
𝑉𝐴= 𝑉𝐵,
𝑅2𝐼22= 𝑉𝐴= 𝑉𝐵= 𝑉EB1,
𝐼22=𝑉EB1𝑅2
,
𝐼21𝑅1= 𝑉𝐵− 𝑉EB2
= 𝑉𝐴− 𝑉EB2
= 𝑉EB1 − 𝑉EB2 = Δ𝑉EB,
𝐼21=Δ𝑉EB𝑅1
.
(3)
So, the outputs reference voltages of the proposed multi-output BGR can be obtained as
𝑉ref1 = 𝑅4𝐼5 =𝑅4𝐼2
𝛼
=𝑅4(𝐼21+ 𝐼22)
𝛼
=𝑅4
𝛼(Δ𝑉EB𝑅1
+𝑉EB1𝑅2
) ,
𝑉ref2 =𝑅5
𝛽(Δ𝑉EB𝑅1
+𝑉EB1𝑅2
) ,
𝑉ref3 =𝑅6
𝛾(Δ𝑉EB𝑅1
+𝑉EB1𝑅2
) ,
𝑉ref4 =𝑅7
𝛿(Δ𝑉EB𝑅1
+𝑉EB1𝑅2
) .
(4)
Unlike 𝑉EB1, which has a negative TC, Δ𝑉EB has a positiveTC. So, all outputs (𝑉ref1, 𝑉ref2, 𝑉ref3, and 𝑉ref4) becomealmost autonomous from temperature. Figure 7 shows thesimulation of these aforementioned output reference voltagesas a function of temperature over the range −40∘C to 120∘C.
3.3. Current Sense. To sense the current on power transistorMP we have used the 𝑀cs transistor in the current sensecircuit shown in Figure 8 to make the voltage of 𝑉sd of 𝑀cs
-
6 Active and Passive Electronic Components
V(V
)V
(V)
V(V
)V
(mV
)
1.391.3851.38
1.371.375
1.365
545.0540.0
530.0535.0
1.4351.425
1.411.41.0
0.9980.9960.9940.992
−50.0 −25.0 0.0 50.025.0 75.0 100.0 125.0
Temp. (∘C)Vref1Vref3
Vref2Vref4
Figure 7: Reference voltages versus temperature.
MP
Vbat
Ichg
Isens
Vadapt
Rsens
A0−
+
Mcs
Mm
Figure 8: Current sense circuit.
and MP equal. We have used the OP. The current of𝑀cs MPcan be described as the following equation:
𝐼𝑀cs=1
2𝜇𝑛𝐶𝑜𝑥
𝑊cs𝐿cs[2 (𝑉SG −
𝑉thp) 𝑉SD − 𝑉SD
2]
= 𝐼sens,
𝐼MP =1
2𝜇𝑛𝐶𝑜𝑥
𝑊𝑝
𝐿𝑝
[2 (𝑉SG −𝑉thp) 𝑉SD − 𝑉SD
2]
= 𝐼chg.
(5)
𝐼chg and 𝐼sens are proportional to their aspect ratios.Their ratiocan be described as
𝐼sens𝐼chg≅𝑊cs/𝐿cs𝑊𝑝/𝐿𝑝
=1
𝑁. (6)
Figure 9 shows the simulation results of sensing currents.
4. Simulation Results
The results simulation waveforms of the proposed Li-ionbattery charger are presented in Figure 10. The reference
-
Active and Passive Electronic Components 7
A(m
A)
A(𝜇
A)
500.0
400.0
300.0
200.0
100.0
0.0
−100.0
250.0
200.0
150.0
100.0
50.0
0.0
−50.0
0.0 2.0 4.0 6.0 8.0
Time (ks)
IchgIsens
Figure 9: Simulation results of sensing currents; from top to bottom, the waveforms are the charging current 𝐼chg and the sensed current 𝐼sens,respectively.
Ichg
Vbat
0.0 2.0 4.0 6.0 8.0
Time (ks)
A(m
A)
500.0
400.0
300.0
200.0
100.0
0.0
−100.0
V(V
)
4.25
4.0
3.75
3.5
3.25
3.0
2.75
2.5
4.21V
2.89V
20mA
Figure 10: Simulation results of output voltage and current; from top to bottom, the waveforms are the battery voltage 𝑉batt and the chargingcurrent 𝐼chg, respectively.
currents are, respectively, 116mA and 448mA for workingat the trickle constant current and fast constant currentmodes. The range voltages are presented as 2.5 V and 4.2 V,correspondingly. The stop current is 20mA that is equal toaround 1C/40 that we have defined. The transition between
CC and CV mode occurs, as shown in Figure 11. And as canbe observed in Figure 12, the system is stable.
Figure 13 shows a layout of the battery charger integratedcircuit. As expected, the Li-ion battery charger was designedusing 0.18𝜇m technology; most of the area is occupied by
-
8 Active and Passive Electronic Components
4.199V
VbatIchg
2.5
2.75
3.0
3.5
3.75
4.0
4.25
3.25V(V
)
A(m
A)
−100.0
10.0
120.0
230.0
340.0
450.0
8.07.5 8.257.25 7.75
Time (ks)
Figure 11: Transition from CC to CV mode.
V(d
eg.)
10−3 10−2 10−1 100 101 102 103 104 105
25.0
25.0
−100.0
−75.0
−50.0
−25.0
0.0
−100.0
−75.0
−50.0
−25.0
0.0
V(d
B)
Freq. (Hz)
Figure 12: Frequency response of battery charger.
the power PMOS pass device, in which the effective die areais 1.172mm2.
Table 1 summarizes the performance characteristics of theproposed battery charger IC herein presented. Apparently,this work presents an average power efficiency up to 87% andbetter performance in terms of maximum current charge andvery small chip size.
5. Conclusion
The presented Li-ion battery charger has been designed with0.18 𝜇m CMOS processes. The proposed charger is operating
Table 1: Summary of simulation results.
Topology Adaptive LDOTechnology 0.18𝜇mSupply voltage (V) 4.8–5
Efficiency (%) 87 at 4.8 V84 at 5VOutput voltage (V) 2.5–4.2Maximum charging current 448mAChip area 1.172mm2
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Active and Passive Electronic Components 9
Power MOS
MP LDOControl
Figure 13: Layout of proposed charger.
within trickle constant current, fast constant current, andconstant voltage triple mode with high power efficiency of87%, small chip size, and low consumption and it is suitablefor portable system as battery charger.
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper.
References
[1] M. Chen and G. A. Rincón-Mora, “Accurate, compact, andpower-efficient li-ion battery charger circuit,” IEEETransactionson Circuits and Systems II: Express Briefs, vol. 53, no. 11, pp. 1180–1184, 2006.
[2] D. Linden and T. B. Reddy, Handbook of Batteries, chapter 35,McGraw-Hill, New York, NY, USA, 2002.
[3] Y.-S. Hwang, S.-C.Wang, F.-C. Yang, and J.-J. Chen, “New com-pact CMOS Li-ion battery charger using charge-pump tech-nique for portable applications,” IEEE Transactions on Circuitsand Systems I: Regular Papers, vol. 54, no. 4, pp. 705–712, 2007.
[4] J. Buxton, “Li-ion battery charging requires accurate voltagesensing,”AnalogDialogue: AnalogDevices, vol. 31, no. 2, pp. 3–4,1997.
[5] C.-H. Lin, C.-Y. Hsieh, and K.-H. Chen, “A Li-ion battery char-ger with smooth control circuit and built-in resistance compen-sator for achieving stable and fast charging,” IEEE Transactionson Circuits and Systems. I. Regular Papers, vol. 57, no. 2, pp. 506–517, 2010.
[6] H.-Y. Yang, T.-H.Wu, J.-J. Chen, Y.-S. Hwang, andC.-C. Yu, “Anomnipotent Li-Ion battery charger with multimode controlledtechniques,” in Proceedings of the IEEE 10th International Con-ference on Power Electronics and Drive Systems (PEDS ’13), pp.531–534, April 2013.
[7] R. Pagano, M. Baker, and R. E. Radke, “A 0.18-𝜇 monolithic li-ion battery charger for wireless devices based on partial currentsensing and adaptive reference voltage,” IEEE Journal of Solid-State Circuits, vol. 47, no. 6, pp. 1355–1368, 2012.
[8] F.-C. Yang, C.-C. Chen, J.-J. Chen, Y.-S. Hwang, and W.-T.Lee, “Hysteresis-current-controlled buck converter suitable forLi-ion battery charger,” in Proceedings of the InternationalConference on Communications, Circuits and Systems (ICCCAS’06), pp. 2723–2726, Guilin, China, June 2006.
[9] P. H. V. Quang, T. T. Ha, and J.-W. Lee, “A fully integrated mul-timode wireless power charger IC with adaptive supply controland built-in resistance compensation,” IEEE Transactions onIndustrial Electronics, vol. 62, no. 2, pp. 1251–1261, 2015.
[10] J.-J. Chen, F.-C. Yang, C.-C. Lai, Y.-S. Hwang, and R.-G. Lee, “Ahigh-efficiency multimode Li-Ion battery charger with variablecurrent source and controlling previous-stage supply voltage,”IEEE Transactions on Industrial Electronics, vol. 56, no. 7, pp.2469–2478, 2009.
[11] J. A. De Lima, “A compact and power-efficient CMOS batterycharger for implantable devices,” in Proceedings of the 27thSymposium on Integrated Circuits and Systems Design (SBCCI’14), September 2014.
[12] P. Li and R. Bashirullah, “A wireless power interface forrechargeable battery operated medical implants,” IEEE Trans-actions on Circuits and Systems II: Express Briefs, vol. 54, no. 10,pp. 912–916, 2007.
[13] S.-H. Yang, J.-W. Liu, and C.-C.Wang, “A single-chip 60-V bulkcharger for series Li-ion batteries with smooth charge-modetransition,” IEEETransactions onCircuits and Systems. I. RegularPapers, vol. 59, no. 7, pp. 1588–1597, 2012.
[14] C.-C. Tsai, C.-Y. Lin, Y.-S. Hwang, W.-T. Lee, and T.-Y. Lee, “Amulti-mode LDO-based Li-ion battery charger in 0.35𝜇MCMOS technology,” in Proceedings of the IEEE Asia-PacificConference on Circuits and Systems (APCCAS ’04), vol. 1, pp. 49–52, IEEE, December 2004.
[15] R. C. Cope and Y. Podrazhansky, “The art of battery charging,”in Proceedings of the 14th Annual Battery Conference on Applica-tions and Advances, pp. 233–235, IEEE, Long Beach, Calif, USA,January 1999.
[16] S. Dearborn, “Charging Li-ion batteries for maximum runtimes,” Power Electronics Technology, vol. 31, no. 4, pp. 40–49,2005.
[17] S. S. Bethi, K.-S. Lee, R. Veillette, J. Carletta, and M. Willett, “Atemperature and process insensitive CMOS reference currentgenerator,” inProceedings of the IEEE 56th InternationalMidwestSymposiumonCircuits and Systems (MWSCAS ’13), pp. 301–304,2013.
[18] A. Dey and T. K. Bhattacharyya, “Design of a CMOS bandgapreference with low temperature coefficient and high powersupply rejection performance,” International Journal of VLSIDesign & Communication Systems, vol. 2, no. 3, pp. 139–150,2011.
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