a built-in self-test high-current led driver

8/8/2019 A Built-In Self-Test High-Current LED Driver

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A Built-In Self-Test High-Current LED DriverDo Hung Nguyen, Jaber Hasan, and Simon S. Ang

Abstract - In this paper, a built-in self-test (B1ST) highcurrent light-emitting-diode (LED) driver circuit is proposed.The circuit not only maintains different constant currents formultiple LED strings, but it also minimizes the conductionpower dissipation by keeping the power MOSFETs in theconstant-current controllers operating in the linear region.The proposed driver first acquires the current-voltage (1-V)data of the constant-currentpower MOSFETs and stores theminto memories. These stored1-V data, along with a duty cyclecontrol of the switching converter, are used to ensure thatthese power MOSFETs are operating in their linear region tominimize conduction power dissipation. The proposed circuitwas verified using PSP1CE with two to five LED strings.Simulation results show a maximum efficiency of95.6%.

Index Terms - LED driver, Built-in Self-Test.I. INTRODUCTION

Backlights for liquid-crystal-display (LCD) panels using theconventional co ld cathode fluorescent lamps (CCFLs) aregradually being replaced by light-emitting-diodes (LEDs) dueto their longer life time, high efficiency luminance, andimproved chromaticity [1]. LEDs are being used not only inpor table e lec tronics with LCD displays such as laptopcomputers, cameras, cell phones, and automobiles, but theyare also being used in general-purpose household andcommercial lighting, street lighting, signage, and traffic signallights.The brightness i lluminated by a LED is a function of itsforward current. In order to achieve a desired luminance,

LEDs are usually connected in a series configurationoperat ing from a similar driving current. By using a constantcurrent source, the effect of forward voltage variations on theLEDs is eliminated. As such, brightness dimming of the LEDstring can be implemented by controlling its forward current.There are two approaches to maintaining a constant current

in a LED st ring. The first is to use a constant-current powersupply with a stable feedback loop. The constant-currentsupply can be either a linear or swi tch-mode power supply.The linear current regulator is cost effective but it suffers frompoor efficiency because of the vol tage drop across its l inearpass device [2]. The drawbacks of the linear regulator can beovercome by the use of a switch-mode power supply [2].The second approach is the constant-current sinking

configuration. There are two topologies. The firsttopologyDo Hung Nguyen, Jaber Hasan, and Simon S. Ang are with the Department ofElectrical Engineering, University of Arkansas, Fayetteville, Arkansas, 72701,USA.978-1-4244-3870-9/09/$25.00 2009 IEEE

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employs a power MOSFET acting as a part of a currentmirror as shown in Fig. 1. However, the on-chip currentregulator is costly due to its large size, especially when manyLED strings are needed [3]. In addition, high currentconsumption in the LED string could damage the chip. Otherissues such as channel length modulation and "fal ling intotriode region" can also cause mismatch in the current mirror[3].

Fig. 1. LED driver using on-chip current mirror.The second topology of the constant-current s inking

conf igurat ion is shown in Fig. 2. In this topology, a referencevoltage is used to maintain a constant voltage drop across thesensing resistor, and this regulates the current flowing throughthe LED string. The main disadvantage of this configuration isthat the MOSFET may drift into saturation causing asignificant amount of conduction power loss.Oscillation may occur in the above two topologies [3]. This

happens when the drain-to-source voltage of the MOSFET issufficiently high during the saturation mode. The largeparasi tic inductances associa ted with the connect ion wirestogether with high saturated drain-to-source voltage of thepower MOSFET can drive the power MOSFET intooscillation during an increasing drain-to-source voltage [3].

LED, \J\LED, \J\

Fig. 2. LED driver using sense resistanceII . CIRCUIT DESCRIPTION

In this paper, a built-in self-test (BIST) LED driver circuitis proposed. The current-voltage (I-V) characteristics of thepower MOSFETs are first extracted using a built-in self-test

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. ~ _ . . . . o . . - ---L_--l._--'II '511S taR t5e\; WiU"Ulu:r_) ' 1 _ 1 : ' ) " ' I ' . ' : ' ) ,..... :

Fig. 9. Minimum analog voltage selector operation. V-Selector is theoutput.

3. 'n l1 l ~ " -" ....._... '" _ . _ _ .~ . I I K S .l In ~ . ' f t ....

= ~ f t 1 " , , 1 : ' ) "(",,r .tel)

Fig. 8. Input and output signals with the ADC feeds into a DAC.

Fig. 10 shows the SPICE simulation to illustrate theoperation in the testing mode. The ramp up phase as indicatedin the plot is to determine the saturation current and itscorresponding saturated drain-to-source voltage. After this,the testing mode goes into the ramp down phase to determinethe saturation edge voltage VDO by reducing to 95% of itssaturation current. As shown, the saturated drain-to-sourcevoltage drops to its saturation edge voltage VDO during thistransition. In these simulations, the sign of the currentflowing through the sense resistor is defined as negative as thedrain current flows into drain of the power MOSFETs. Theentire testing mode takes only 10 us,Fig. 11 shows the operation mode of the BIST LED driver.

A disturbance pulse of 2 V is added to one of LED strings at20 us, As shown, a positive disturbance causes the drainvoltage of the power MOSFET to reduce as shown in themiddle plot. As such, the decision circuit selects this voltageto control the DC/DC converter output. The output of theDC/DC converter decreases to ensure that the powerMOSFET is operating in its linear region to minimizeconduction power loss. At the same time, currents flowingthrough the LED strings are maintained at about 94.8% of thespecified 350 rnA. As shown in Fig. 11, for a 2 V disturbancestep with a rise time of 2JlS, the transient responses of LEDcurrent and DC/DC converter output did not reveal any signof oscillation.ED t

SwitchModeDC/DC LED tSelector Converter

To control dutyVDO cycle of DC/DCConverter

Amplification an d LogicComparationVoo =D- SWot>-Ol SW IV0 2 SW2

VM1N

Fig. 7. Operation mode circuit

Fig. 7 shows the operation mode BIST LED driver circuitwith two LED strings. As shown, the minimum analogvoltage selector feeds the information to change the duty cycleof the DC/DC converter according to a control law. Unlike[1], the duty cycle of the DC/DC converter is directlycontrolled by sensing the drain terminal at each of the powerMOSFET in the constant-current controller. As such, a moreprecise control of the drain-to-source voltage of the powerMOSFET is achieved. Since the duty cycle of the pulse-widthmodulator (PWM) is controlled using two separate controlloops, the stability of the system is improved.

VDI - - -

Fig. 6. Block diagram ofthe minimum voltage selector.

III. SIMULATION RESULTSThe power MOSFET SPICE model used in this simulation

is a 2N7002 n-channel MOSFET [6]. These power MOSFETsare being chosen since they are being used in a practicalML4876 LCD Backlight Controller ICs. The SPICE model forthe LED is modified from [7] for a high current operation at350 rnA. The 8 bits ADC and DAC used in the simulation areadapted from [8] for a 5 V supply system. Fig. 8 shows theinput and output signals with the output of the ADC fed intothe input of the DAC. The ADC is an 8-stage pipelinearchitecture using a sample and hold circuit. A 5 V referencevoltage is used which corresponds to a 19.53 mV resolution.The simulation result shows an acceptable integralnonlinearity (INL) for this application even though the twoend points have better resolutions than the middle input range.Fig. 9 illustrates the functionality of the simulated analog

minimum voltage selector. As can be seen, with the desiredsaturation edge voltage VDO, and the two drain-to-sourcevoltages of VD1 and VD2, the output V_Selector (bolded plot)is the minimum of these voltages.

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Fig. 10. Simulated plots: (top) current flowing through the powerMOSFET, (bottom) drain-to-source voltage across the power MOSFETduring the testing mode.

Fig. 11. (Top) Disturbance voltage of +2V and -2V added to the LEDstrings, (middle) Drain voltage at the powerMOSFET, (bottom) Forwardcurrent flowing through one of the LED strings.Table 1 summarizes the performance parameters of theproposed BIST LED driver with five LED strings compared tothe reported work of [1] and [9]. As can be seen, the designedBIST LED driver has a maximum efficiency of 95.6% at thelowest drain voltage of the power MOSFET and yet thecontroller is able to deliver the desired forward current of 350mA to all the LED strings. The minimum operation efficiencycorresponds to when the power MOSFETs are operating attheir saturation edge voltage, similar to those reported in [1]and [9]. The DC input range for this particular five LEDstrings BIST driver is from 11.5 V to 13.9 V. It should benoted that the DC input range will increase as the number ofLED strings increases. Obviously, the lower limit of theDC/DC converter output voltage has to be large enough tosupply the desired 350mA forward current to all the LEDstrings.

TABLE 1. COMPARISONWITH PREVIOUSWORKPerformance

This Work [1] [9]ParametersMaximum 95.6% 88% 92%EfficiencyMinimum 86% 85% 87%EfficiencyLED Current 350 rnA 350mA 350mA

DC/DC Voltage 11.5 V to 13.9 V 20 V to 30 V 2.7 V to 3.7 VNumber ofLEDs 5 10

per string

IV. CONCLUSIONThe concept of employing built-in self-test to determine the

current-voltage characteristics of power MOSFETs in theconstant-current controllers of a LED driver is demonstrated.The acquired current-voltage characteristics are used to ensurethat the power MOSFETs in the constant-current regulatorsoperate in the linear region of operation to minimizeconduction power dissipation. Stability of the BIST LEDdriver is enhanced with a separate control of the constantcurrent controller and the output voltage of the DC/DCconverter. Simulations verified that the proposed BIST LEDdriver was able to select the minimum drain voltage to controlthe duty cycle, and hence, to achieve a minimized conductionpower loss. At a 350 rnA forward current, a maximum powerefficiency of 95.6% can be attained for a 5-LED string BISTdriver. This BIST LED driver offers a much more flexible andpractical implementation due to the fact that there is no needto match the current-voltage characteristics for each of thepower MOSFET in the constant-current controller.

REFERENCES[1] Yuequan Hu, Jovanovic, and M. Milan, "A Novel LED Driver with

Adaptive Drive Voltage," Applied Power Electronics Conference andExposition, APEC. pp. 565-571. Feb 2008.

[2] S. S. Ang and Alejandro Oliva, Power-Switching Converters, SecondEdition CRC Press, 2005.

[3] Tse-Ju Liao, and Chern-Lin Chen, "Robust LED Backlight Driver withLow Output Voltage Drop and High Output Current Accuracy,"International Conference on Software Engineering and Technology,ICSET 2008.

[4] C. Stroud, P. Karunara tna, and E. Bradley, "Digi ta l Components forBuilt-in-Self-Test of Analog Circuit," ASIC Conference and Exhibit,1997. Proceedings, Tenth Annual IEEE International, 7-10 Sept. 1997Page(s):47 - 51.

[5] T. Lopez, and R. Elferich, "Measur ement Technique for the StaticOutput Characterization of High-Current Power MOSFETs," IEEETransactions on Instrumentation and Measurement, August 2007.

[6] 2N7002 N-Channel enhancement mode field effect transistor spicemodel: http://www.fairchildsemi.com/pf/2N/2N7002.html

[7] S. Mollet, "Analyze LED characteristics with SPICE", EDNwww.ednmag.com. Jan 18 2001.

[8] R. Jacob Baker, CMOS: Mixed Signal Circuit Design, IEEE PressSeries on Microelectronics Systems, John Wiley & SONS 2002.

[9] W. Y. Leung, T.Y. Man, and M. Chan, "A High-Power-LED Driverwith Power-Efficient LED-Current Sensing Circuit," 34th EuropeanSolid-State Circuits Conference, 2008 ..

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a built-in self-test high-current led driver

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