A RGB-Driver for LED Display Panels Jaber Hasan, Do Hung Nguyen, and Simon S. Ang

Department of Electrical Engineering University of Arkansas Fayetteville, AR 72701

Abstract—Red-Green-Blue (RGB) light-emitting diodes display panels are finding widespread use due to recent advances in the light-emitting diodes (LEDs) and their driver technology. This paper investigates a digital microcontroller based RGB-driver for application in display panels. The RGB-driver uses three different voltage sources from switch-mode power converters (SMPCs) as each RGB color requires different drive voltages. The proposed driver selects the minimum drain voltage of the MOSFETs of the current controllers for each color and uses this voltage to control the duty cycle of the SMPCs, and thus, maintains the minimum output voltages required to keep the MOSFETs in the current controllers in regulation. With a 5-V supply voltage, the efficiencies of the RGB LEDs are 91.5%, 95.7%, and 95.7%, respectively. Even though the driver was experimentally verified in a 3 x 3 RGB pixels, the concept can be extended to a larger display matrix.

I. INTRODUCTION

Due to the recent advancement in light-emitting diode (LED) technologies, LEDs are increasingly being used in LCD backlighting, automobiles, traffic lights, and general-purpose lighting [1] [2]. As the cost of LEDs is decreasing, LEDs are finding new applications such as in display panels and signage.

The complexity of driving RGB based LEDs in display panel is due to the large number of control nodes and RGB LEDs needed. For each RGB pixel, there are three constant current controls for dimming and three different supply voltages. Since each color of the RGB LEDs requires different drive voltages, each requires its independent driver. The most common and straightforward method is to employ independent current controllers for each RGB color [1] [2] as shown in Fig.1. The constant-current controllers can be either a linear or a switch-mode type. Linear current controller suffers from excessive power dissipation in its series-pass devices [1]. However, switch mode current controller requires a number of storage devices such as capacitors and inductors which lead to higher parts count and adding to the cost of the driver.

The LED-drivers implemented in TVs are either used for backlight or displays panels. For backlight, the LED driver operates at frequency lower than the

frequency that is required for display panels since the refresh rate in a LED display panels is about 240Hz, whereas in a backlight it is about 120Hz.

Figure 1. Independent current controlled LED-driver.

As shown in Fig. 2, linear current controllers are being used to control the current in the LED together with a SMPC supplying the required operating voltage [1] [2]. In this implementation, the output voltage of the SMPC, which is set by the feedback voltage, ensures that the linear current controllers for each RGB color LED are operating at the desired currents [1] [2]. However, this often yields the worst-case operating voltage, leading to a lower efficiency. Also, three different SMPCs are required due to different drive voltage of each color of RGB LEDs. In order to improve the efficiency of the RGB-driver using a SMPC, the lowest drain voltage of the MOSFET in the linear current controllers was detected using diodes for LED backlight applications [3]. The efficiency of this driver depends on the selection of the reference voltage and the

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temperature dependence of the sensing diodes [1] [2]. In order to maintain the linear current controllers in regulation, the reference voltage has to be selected for the worst-case condition, and hence leading to reduced efficiency. Additionally, the sensed voltage of the diode changes with operating temperature causing detrimental effects [1] [2].

Figure 2. LED-driver implemented in [3].

In this paper, a different approach to detect the

minimum drain voltage using a microcontroller with a multiplexer circuit for a RGB driver in display applications is presented. In this implementation, the efficiency of the driver is increased by eliminating the need for sensing diodes to detect the minimum drain voltage of the current controller. Additionally, this proposed circuit is implemented with dimming capabilities thus enabling each color pixel to be individually dimmed from 1% to 100%, thus enabling high contrast colors which are required for state-of-the-art display panels.

II. Proposed Driver

The proposed LED-driver is shown in Fig. 3 with N (N=1, 2, 3…) RGB LED strings. In this implementation, the proposed driver has two modes of operation – startup and operation modes. A PIC18F4431 microcontroller is used to control the duty cycles of the three switching converters using the minimum drain voltages of the MOSFET in the current controllers in each RGB color LEDs.

A digital microcontroller is being used to control the SMPC rather than an analog controller as implemented in [1] [2]. The primary reasons for implementing the driver digitally are its capability of adding extra features without additional circuitry, easy reconfiguration of control schemes by software, and accurate calculations [4]. However, digital control suffers from limitation due to analog-to-digital (ADC) as well as digital-to-analog (DAC) conversions.

During startup mode, in order to maintain the MOSFETs of the current controller in saturation – each

switching converter first outputs a high voltage. The gate-to-source (Vgs) voltages of the MOSFETs of all the current controllers are sensed and then the maximum Vgs of each color of RGB is selected by the microcontroller together with a multiplexer circuit and compare against reference voltage. If the value of the maximum Vgs of the MOSFETs of each color of the RGB is greater than the reference voltage, the output voltage of the SMPC is reduced until the maximum Vgs of the MOSFETs in the current controller of each color of the RGB is equal to the reference voltage. When the maximum Vgs of the MOSFET of each color of RGBs is equal to the reference voltage, the driver leaves the startup mode and enters the operation mode.

In the operation mode, the drain voltages of the MOSFETs in the linear-regulators of each color of RGBs are sensed directly using the multiplexer. The minimum drain voltage of each LED color is determined from the sensed voltages and set as the reference voltage for the SMPC. A proportional-integral-derivative (PID) control is implemented to maintain this reference voltage thus forcing the MOSFETs to operate at the boundary of the saturation region. Since the controller uses this voltage to control the duty cycle of the switching converters to output a minimum voltage necessary to maintain the MOSFETs in the current controller in regulation, an improved efficiency is thus achieved for each RGB color LED independently. Analog dimming is implemented to control the brightness of each individual RGB color by adjusting the reference voltage of the error amplifiers in the current controllers.

Figure 3. RGB LED-driver proposed in this paper. (Note – Similar circuits for green, and blue LED-driver.

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III. Dimming Control

Dimming is being implemented to provide brightness and contrast adjustments which are required in a display panel. There are two types of dimming - analog and pulse-width-modulation (PWM). In analog dimming, LED brightness of brightness of 50% is achieved by maintaining 50% of the maximum LED current in the string. For PWM dimming on the other hand, LED brightness of 50% is achieved by providing the maximum LED current at 50% duty cycle.

The main advantage of analog dimming is that it can be easily implemented by changing the reference voltage of the error amplifiers in the current controller. However, this method suffers from shifting of LED colors as the current across the LED are changing in this method [5]. Whereas, in PWM dimming, this problem does not exist as the current across the LED are not changing since the maximum LED current is provided at different duty cycles in this method to achieve dimming. However, this method suffers from noise and EMI interferences [6].

In this implementation of a 3x3 RGB pixel for display, it requires individual dimming control of each color of each pixel to create multiple spectrums of colors. A total of 27 dimming control signals are required for a 3x3 RGB pixel and for ease of implementation, an analog dimming is implemented in this LED-driver system.

The dimming control signal is generated using one 3-wire data bus (active-low Chip Select, Clock, and DATA) by connecting the serial-data-in (SI) of one digital-potentiometer (DP) to the serial-data-out (SO) of the other DP in order to communicate with multiple DPs as shown in Fig. 4. This form of connecting multiple devices in a same data bus in series is called daisy-chain. Daisy-chaining multiple DPs allows freeing up I/O pins in the microcontroller and, thus requiring only two pins from the microcontroller i.e., clock and chip select pins, to communicate with multiple slave devices connected in series in the same data bus. It would be difficult to implement dimming in this LED-driver system since it requires 27 dimming control signals. This is because the microcontroller will require 3 individual outputs to control each slave devices.

Figure 4. Daisy Chained Digital-Potentiometers.

The DP receives its dimming control signal directly from a personal computer (PC) via the serial communication ports. Therefore, a communication is established between the PC and the PIC18F4431 dimming controller. In this approach, as soon as DP1 receives its data from the dimming controller, this data is clocked into the DP1’s shift register, as long as its active-low chip select pin remain low [7]. This data is then processed by DP1 and arrives at SO output pin. As the DP’s slave devices are connected in series, the SO of DP1 is connected to SI of DP2, therefore the data is clocked into DP2’s shift register and propagates through DP2’s SO output. This process continues until the data is propagated through the entire daisy-chain until each of the DP’s has received its command signal while the active-low chip select is low.

In this demonstration, a SPI interfaced daisy-chained DPs is used to generate the dimming control signals. In an integrated circuit implementation, an inter-integrated circuit (I2C) interface with each DACs having its own I2C address can be easily implemented.

IV. Experimental Results

A prototype of the proposed LED driver was designed and verified using buck converters operating at 50 kHz. The buck converter was chosen for this application because the output voltage in a buck is lower than its input voltage i.e., the nominal forward voltages of red, green, and blue LEDs are 2.2V, 3.6V, and 3.6V, respectively [7]. The output of the converters was used to drive a 3x3 pixel for LED display panels. The measured efficiency of the each driver is shown in Fig. 3 with respect to input voltages. At an input voltage of 5V, the efficiencies are 91.5%, 95.7%, and 95.7% for the RGB color LEDs. These efficiencies are higher than those reported in [8]. The measured change in output current with respect to input voltage, i.e., the line regulation, is shown in Fig. 4. As can be seen from Fig. 4, the output currents of the red, green, and blue LEDs remain constant at 100mA at input voltages from 5V to 13V. Fig. 5 shows the oscilloscope waveforms of the dimming signal as it changes from 100% to 0% and the voltage at the drain of the current-controlled MOSFET. As shown, the drain voltage changes from 0 mV to 250mV. Figure 6 shows the RGB 9 pixel display in different colors.

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Figure 3 – Efficiency of the red, green, and blue LED drivers

with respect to input voltage variations.

Figure 4 – Load current of red, green, and blue LED drivers with

respect to input voltage variations.

Figure 5. Analog dimming signal and drain voltage of the current

controller.

Figure 6. A 3x3 RGB Pixel.

III. Conclusion

A RGB-driver for driving multiple pixels in LED display panels using a microcontroller is demonstrated in this paper. In this approach, the output voltage of the switching converters and the current in the individual LED are controlled separately, thus increasing the stability of the driver. The use of a digital microcontroller to control the output voltages of the switching converters for each color LEDs leads to reduction in size, weight, and cost of the driver. In this driver, the efficiency is maximized by selecting the minimum drain voltage of the MOSFETs in the current controllers to regulate the duty cycle, and by removing the need for external sensing of the voltage drop across the multiple current controllers. The driver was successfully implemented to show an efficiency of greater than 92% in a 3x3 RGB pixel.

VI. References

[1] Yuequan Hu, and M. M. Jovanovic, “A Novel LED Driver with Adaptive Drive Voltage,” Applied Power Electronics Conference and Exposition, APEC. pp. 565-571. Feb 2008. [2] Yuequan Hu, and M.M. Jovanovic, “LED Driver with Self-Adaptive Drive Voltage,” IEEE Transactions on Power Electronics, Vol. 23, No. 6, pp. 3116 – 3125, Nov 2008. [3] M. Doshi and R. Zane, “Digital architecture for driving large LED arrays with dynamic bus voltage regulation and phase shifted PWM,” IEEE Applied Power Electronics Conference (APEC) Proc., pp. 287-293, 2007. [4] A. Torres, J. Garcia, M. Rico Secades, A. J. Calleja, and J. Ribas, “Advancing towards digital control for high power LED driver,” IEEE Symposium on Industrial Electronics (ISIE), pp. 3053-3056, 2007 [5] M. Day. “LED-driver Considerations,” Analog Application Journal, pp. 14-17, 2004 [6] S. S. Ang and Alejandro Oliva, Power-Switching Converters, Second Edition CRC Press, 2005.

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[7] Y. K. Lo, K. H. Wu, K. J. Pai, and H. J. Chiu, “Design and Implementation of RGB LED Drivers for LCD Backlight Modules,” IEEE International Symposium on Industrial Electronics (ISIE), pp. 584-587, June 2007.

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