Multi-String LED Driver with Accurate CurrentMatching and Dynamic Cancellation of Forward

Voltage Mismatch

Punith R. Surkanti, Disha Mehrotra, Manaswini Gangineni and Paul M. FurthVLSI Laboratory, Klipsch School of Electrical and Computer Engineering,

New Mexico State University, Las Cruces, NM 88003, USAEmail: {punith, mehrotra, manaswi and pfurth}@nmsu.edu

variations in the LED fabrication process, operating tempera­ture, device aging, and usage.

Current Matching Circuit

Abstract-In this paper, we present a multi-string LED driverwhich is capable of driving five parallel LED strings with twoseries LEDs in each string. The current in the LED strings isregulated using a boost converter with a hysteretic control loop.Accurate current matching is achieved using a regulated cascodecurrent mirror architecture. The loser-take-all circuit enablesdynamic cancellation of LED forward voltage mismatch and alsooptimizes efficiency. A PFM dimming block is designed to operatewith a 200 kHz clock and has 11 programmable dimming ratiosranging from 2:1 to 2048:1. This LED driver is implementedin 0.5-{tm CMOS process and operates with a Li-ion batterywith a voltage range 3 - 4.2 V. Efficiency at steady-state withno dimming and at 4:1 and 8:1 dimming ratios are 89%, 88%and 85.4%, respectively. This LED driver is suitable for backlightLED displays in mobile devices.

Index Terms- LTA, DCM, current matching, hysteretic con­troller, dimming, forward voltage mismatch.

I. INTRODUCTION

Off Chipie,

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DIMDimming Block

Most modern wireless devices use color displays. Thecolor screen backlight is generated from light-emitting diodes(LEDs) because the electrical efficiency and color spectrum ofLEDs are much better than their fluorescent counterparts [1]-[3].

All of the LEDs should emit the same intensity of lightin order to preserve the perceived color of the image whichrequires the same amount of current going through all theLEDs [4]. One way of achieving it is by connecting all ofthe LEDs in series. The drawback of this structure is thatit requires a very high voltage to drive them. Generating avery high voltage from a Li-ion battery in portable devicesis complicated and needs high voltage process that increasescost and area. An alternative structure is to use parallel LEDstrings with a limited number of series LEDs in each stringin order to limit the required drive voltage. This structure canbe implemented using low voltage processes, decreasing costand area.

An LED driver converts DC voltage into a regulated DCcurrent, generating necessary voltage levels to drive the LEDmodule [5]. In a multi-string LED system, the number ofLEDsper string and the total number of strings are dependent uponthe desired level of illumination. LED lighting systems requiresophisticated LED driver circuitry in order to ensure that thecurrent delivered to an LED string is constant irrespective of

978-1-5090-6389-5/17/$31.00 ©20 17 IEEE 229

Fig. I. Block-level architecture of proposed multi-string LED driver with aDC-DC boost converter.

The multi-string LED driver proposed in [6] uses a pulse­width modulation (PWM) control loop to drive the LEDstrings. In their control loop, an error amplifier with integratingcapacitor is used to stabilize the LED driver, thereby limitingthe bandwidth of the loop. Illumination is changed throughaveraging the drive current. Averaging the current in LEDsresults in higher mismatch in individual LED currents. An­other multi-string LED driver is proposed in [7], in whichfrequency compensation limits the bandwidth of the loop.This implementation also employs bipolar junction transistors,which increases the fabrication cost.

II. PROPOSED MULTI-STRING LED DRIVER

The proposed multi-string LED driver uses a hystereticcontrol loop with no error amplifier. Fig. 1 shows the block­level architecture of the proposed LED driver with a boostconverter driving five parallel LED strings with two seriesLEDs in each string. The number of series LEDs in each stringis limited by the choice of a low-voltage device process. Ahigh-voltage process allow more number of LEDs in series.The proposed architecture contains a current matching circuit,loser-take-all circuit, control block, and a dimming block. TheLED driver is powered by Li-Ion battery voltage VBATT .

A. Current Matching Circuit

In LED backlight applications, the light emitted shouldbe the same from all LEDs to avoid a perceived shift incolor [4]. The brightness of LEDs is proportional to the currentthrough it. In a single string LED architecture, the LEDs,by design, will have same current in all of them. On theother hand, in multi-string LED structure, if the voltage acrossthe strings were held constant and there existed variationsin the LED forward voltages, the currents would not be thesame [6]. Hence the need for an accurate current matchingcircuit to ensure minimum current mismatch between all theLED strings.

Fig. 2 shows the current matching circuit (CMC) for theproposed 5-string LED driver. The CMC is placed in serieswith each LED string. The boost converter is required togenerate an output voltage that ensures that voltages VGl toVC5 are sufficiently high for proper operation of the currentmatching circuit.

A current mirror replicates the input current IREF to eachof five different output currents, hEDl to hED5. The outputcurrents are a scaled version of input current,!REF. Currentmatching could be accomplished using a simple current mirror.High efficiency would be obtained using a simple currentmirror because it requires a low headroom of one VDSsat. Themajor drawback is the current mismatch. The current gain fromthe input to a single output is given by Ai = Ai,lDEAL(l+E).The error is given by E = A(VD S 2 - VDS 1 ), where A is channel­length modulation co-efficient. Changes in VDS causes linearmismatch in output current. To overcome this error a low­voltage cascode current mirror can be used. In a low-voltagecascode current mirror, the output headroom requirement is2VDssat and the output impedance is gmr; where gm and roare the transconductance and output resistance of the transistor,respectively [8]. Implementing a low-voltage cascode currentmirror in LED driver results in a degradation of efficiency dueto the higher output headroom requirement of 2VDSsat.

Regulated cascode current mirror is one technique that doesthe precise current mirroring. A differential opamp is used tomaintain the drain voltage of M l to M 5 equal to VDSREF.This ensures that the VDS of input transistor M o and that ofoutput transistors M l to M 5 are equal. This will make E ideallyequal to O. Hence there is no linear mismatch in lout. Thedifferential amplifier boosts the output impedance by factorA which is the open-loop gain of the opamp. This causesROUT to increase to Agmlcrolcrol. By decreasing VDSREF toa value less than VDSsat the output impedance of the transistorM 1 , r 01 decreases. But the additional differential amplifierrestores the output impedance to a value that is comparable tothat of a cascodc current mirror. As such, the output headroomof regulated cascode current mirror is VDSsat + V x , where V xis somewhat less than VDSsat. Using the regulated cascodecurrent mirror, then, increases the efficiency of the LED drivercompared to a low-voltage cascode mirror.

B. Loser-Take-All Circuit

The primary constraint in any multi-string LED architec­ture is the LED forward voltage mismatch exhibited betweenstrings due to manufacturing tolerances and temperature dif­ferences. Referring to Fig. 1, mismatch in the forward voltages

230

Ms

Fig. 2. Simplified schematic of five string current matching circuit imple­mented in proposed LED driver.

of the different LED strings leads to different voltage valueson nodes VCl to VC5.

In order for the current matching circuit to operate withaccuracy, we need to ensure that all transistors operate insaturation by maintaining a required voltage. This can beaccomplished using a loser-take-all circuit (LTA). The LTA'sprimary function is to sense the minimum voltage among all ofthe LED strings and convey that voltage to the boost controlloop. With a reference voltage V REF and LTA output as afeedback voltage, the boost control loop regulates the outputvoltage continuously such that VLTA is equal to V REF . SinceV LTA is the minimum voltage among VGl to VC5, it is clearthat VGl to VC5 2: VREF. Setting V REF as a minimumrequired voltage ensures accurate functionality.

Different topologies for LTAs have been reported in [9]­[11]. The implemented 5-input LTA's high-level schematic isshown in Fig. 3, and is adapted from [11]. The LTA constitutesa 5-cel1 LTA core circuit, 5 common-source amplifiers, and a5Xl multiplexer. In this application the input voltages for theLTA are in the range of 300 - 400 mV, thereby requiring PMOSinput transistors M(O,i) where i =1 to 5.

Fig. 3. Five-input Loser-Take-AU (LTA) circuit from [11], implemented inproposed LED driver.

Each LTA core cell contains a PMOS input transistor(M(O,l) to M(O,5), an NMOS diode-connected transistor

(M(1,l) to M(5,5)) and four positive feedback transistors fromother cell s.

The operation of the LTA can be explained as follows:assuming input voltage VGl is lower than the other fourvoltages, transistor M(O,l) will experience a higher currentthan the other M(O,i) transistors. As transistor M(O,l) sourcesa higher current, diode-connected transistor M(l,l) holdingvoltage VI will also sink a higher current. The diode-connectedtransistor M(l,l) will mirror this current to nodes V2 to V5 ofall the other cells through positive feedback transistors M(1,2),M(1,3), M(1,4) and M(1,5) and since the sinking current fromV2 to V5 is higher than the sourcing current, the voltage atthe node pulls down close to Vss . This turns OFF transistorsM(2,1), M(3,1), M(4,1) and M(5,1) and will maintain node VIat one Vcs above Vss. This operation will be the same for allthe other cells. The cell with the lowest input voltage amongVCl to V C5 will have a detection voltage that is one Vcsabove Vss while the other 4 cells' detection voltages will bepulled down to Vss. The common-source amplifiers with biascurrents equal to I BIAS /5 are used to generate wide-swingselect lines 8 1 to 85 for the multiplexer, based on the valuesof VI to V5 from the core block. The lowest input among VGlto VC5 is passed to the output using a 5Xl multiplexer.

The combination of current matching circuit and the LTAwith a DCM control loop enables the output voltage to dynami­cally cancel mismatch in the LED forward voltages. Variationsin LED forward voltages cause variations in voltages VCl

to VC5, which in turn affects V LTA via the LTA. Via thecontrol loop, the difference between VREF and V LTA forcesthe control loop to modify VOUT such that VCl to V C5 settlesback and V LTA is equal to V REF .

C. Dimming Control

In order to improve light load efficiency, LED driverstypically include dimming circuitry. The digital dimming blockin the proposed LED driver has been designed for a maximumdimming ratio of 2048:1. It has been adopted from from [12].The dimming block consists of a counter, multiplexer and gatedrivers. The input clock is a 200 kHz clock signal eLKIN.The dimming block generates pulses at 10 different frequencieswith constant ON-time. The output of the dimming block,DIM is used as a sleep signal to all of the other blocks.

III. SIMULATION RESULTS

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Fig. 4. Steady-state simulation result of LED driver with 100 mV LEDforward voltage variation between the strings at VBATT = 3.6 V and !LEDof 20rnA in each string.

A. Steady-State Operation

The steady-state output of the proposed multi-string LEDdriver is shown in Fig. 4. We note that the LED driver alwaysoperates in discontinuous conduction mode (DCM), since theinductor current iL stays at zero for part of each cycle. A newcycle is initiated only when V LTA drops below VREF . Wealso see in Fig. 4 that the output of the LTA, V LTA , alwaystracks the lowest voltage among all the LED strings. Finally,we note that the average LED currents of all of the stringsarc very close to each other with a mismatch of ±0.5%. Thesimulated efficiency of the proposed LED driver at steady-stateis 89% operated at a battery voltage of 3.6 V.

Fig. 5. Simulated step response of the LED forward voltages of lOamY, onoutput voltage and LED current in each sting at VBATT = 3.6 V

where 2VLED ,max is the maximum of voltage drop across twoseries LEDs.

The proposed multi-string LED driver is implemented ina 5 V 0.5-1l,ffi CMOS process to drive five parallel stringswith two series LEDs in each string. The driver operates at aswitching frequency of 1.5 MHz while providing LED currentsof 20 rnA in each string. The LED driver designed to operatewith a nominal Li-ion battery voltage range of 2.8 - 4.2 V. Ituses a 1 JiH inductor and a 470 nF capacitor. For simulations,we intentionally modified the LED forward voltages for all ofthe LEDs strings to measure the effect on the LED currents.The output voltage of the proposed LED driver is given by

VOUT = 2VLED ,max + V REF (1)

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The LED driver optimizes the output voltage to a requiredlevel such that it provides the required 20 mA current to allfive strings and maintains a voltage of V REF or higher on allof the voltages across the current matching circuit. Referringto Fig. 5, a simulation is performed where the LED driverexperiences a sudden change in LED forward voltage. Theoutput voltage follows the change in LED forward voltagesquickly. Moroever, Fig. 5 shows that VLTA tracks the lowestvoltage as per the change in the LED forward voltages and theLED currents of all LED strings have a maximum mismatchof ±0.5%.

The boost converter used in the proposed LED Driver wasoptimized driving a resistive load at an efficiency of 94.9%.The reported efficiency of 89% of the multi-string LED driver

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IV. CONCLUSION

REFERENCES

This paper describes a proposed multi-string LED driverthat regulates the current in 5 strings of 2 series LEDs. Asimulated mismatch error of ±0.5% between the LED stringsin achieved using a regulated cascode CMC. This LED driveris capable of dynamically cancelling mismatch in LED forwardvoltages. The simulated efficiency of 89% is achieved and upto 2048: 1 of PFM dim ratio has been implemented.

[I] D. G. Lamar, 1. S. Ziga, A. R. Alonso, M. R. Gonzlez, and M. M. H.Ivarez, "A very simple control strategy for power factor correctorsdriving high-brightness leds," IEEE Transactions on Power Electronics,vol. 24, no. 8, pp. 2032-2042, Aug 2009.

[2] 1. Y. Tsao, "Solid-state lighting: lamps, chips and materials for to­morrow," in (CLEO). Conference on Lasers and Electro-Optics, 2005.,vol. I, May 2005, pp. 143 Vol. 1-.

[3] 1. Moon and K. Oh, "Effects of light-emitting diode (led) configurationon luminance and color of an edge-lit backljght unit," Journal ofDisplayTechnology, vol. 11, no. 9, pp. 768-775, Sept 2015.

[4] H. Kim, 1. Liu, H. S. Jin, and H. J. Kim, "An led color control systemwith independently changeable illuminance," in INTELEC 2009 - 31stInternational Telecommunications Energy Conference, Oct 2009, pp.1-5.

[5] B. M. Lim, Y. H. Ko, Y. S. Jang, O. H. Kwon, S. K. Han, and S. G.Lee, "A 200-v 98.16 x0025;efficiency buck led driver using integratedcurrent control to improve current accuracy for large-scale single-stringled backlighting applications," IEEE Transactions on Power Electronics,vol. 31, no. 9, pp. 6416--6427, Sept 2016.

[6] c.-L. Chiu and K.-H. Chen, "A high accuracy current-balanced controltechnique for led backlight," in 2008 IEEE Power Electronics SpecialistsConference, June 2008, pp. 4202--4206.

[7] S. i. Hong, J. W. Han, D. H. Kim, and O. K. Kwon, "A double-loopcontrol led backlight driver ic for medium-sized 1cds," in 2010 IEEEInternational Solid-State Circuits Conference - (ISSCC), Feb 2010, pp.116-117.

[8] R. 1. Baker, CMOS Circuit Design, Layout, and Simulation, 3rd ed.Wiley-IEEE Press, 2010.

[9] M. Soleimani and M. Nazaraliloo, "Voltage-mode loser/winner-take­all circuits," in 2011 IEEE 54th International Midwest Symposium onCircuits and Systems (MWSCAS), Aug 2011, pp. 1--4.

[10] J. Ramirez-Angulo, 1. E. Molinar-Solis, S. Gupta, R. G. Carvajal,and A. J. Lopez-Martin, "A high-swing, high-speed cmos wta usingdifferential flipped voltage followers," IEEE Transactions on Circuitsand Systems II: Express Briefs, vol. 54, no. 8, pp. 66&-672, Aug 2007.

[11] P. R. Surkanti, V. Siripurapu, and P. M. Furth, "A high precisionand high speed voltage-mode loser/winner-take-all circuit," in 2015IEEE 58th International Midwest Symposium on Circuits and Systems(MWSCAS), Aug 2015, pp. 1--4.

r12] P. R. Surkanti and P. M. Furth, "High-efficiency, high-dimming ratioled driver," in 2013 IEEE 56th International Midwest Symposium onCircuits and Systems (MWSCAS), Aug 2013, pp. 360-363.

Dim:\lodr

Parameter RangeVBATT 3.0 - 4.2 VVLRO 25 V ± 100 mV

hEO 20.4 rnA ± 0.5%louT 102 rnA

Boost Efficiency 94.9%LED Driver Efficiency 89%

Headroom of CMC 300 mV

LED DRIVER SPECIFICATIONS AND SIMULATION RESULTSUMMARY

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TABLE 1.

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B. Dimming-Mode

The proposed LED driver is simulated for 4: 1 and 8: 1dimming ratios with VBATT of 3.6 V. The simulated outputvoltage, inductor current and LED currents are shown in Fig. 6.As we can see, in dim mode, the LED currents are zero and thecomplete LED driver is turned OFF, such that the output heldhigh. In active mode, the LED driver operates for a short burstwith 20 rnA of LED current in each string. The efficiency ofthe LED driver with dimming control for 4: 1 and 8: 1 dimmingratios is 88% and 85.4%, respectively.

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11 .••••••••• i.. i<i.•• ·•··········••••••••••• bJ.····..~~..••.••••••••••••••~......... , " ',......... . , .

. ,

is due to 29.8 mW of additional power loss in current matchingcircuit. The minimum voltage across the current matchingcircuit, VREF, is set to 300 mV and a maximum variationof 100 mV is added to the 5 strings. Table I summarizesparameters and simulated results of the proposed LED driver.

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Fig. 6. Simulation result of multi-string LED driver in (a) 4:1 and (b) 8:1dimming ratio at VBATT = 3.6 V.

232