Considerations of the Driving Electronics of LED Video Display L. Svilainis Kaunas University of Technology, Department of Signal Processing Studentu str. 50-340, LT-51368, Kaunas, Lithuania,

[email protected]

Abstract. An overview of the electronics used to

Macroblock

drive the light emitting diodes (LEDs) displays is presented. The common problems and the solutions are given. The driver technologies overview indicates the remaining challenges: the high current distribution and the LED dimming range. New driver topology suggested could solve the mentioned shortcomings.

Video Data on disk

Remote source

Display controller

Keywords. LED display technology, PWM

Figure 1. General LED display structure

dimming, gray-scale range, LED driver.

The LED display is probably the largest representative of visual human interfaces. Although capable of displaying many colors, the applications of LED displays are intended for large-screen imaging and high ambient lighting conditions [7, 19]. The large-area video display industry has transitioned completely to LED based technology. Yet, polymer technologies LED [10] screens make a rapid occupation of small-screen imaging market. But the driving electronics principles and problems remain similar for both technologies.

2. The LED video screen structure The LED screens represent the most competitive large-scale display technology for today. The LED video screen is based on the raster-scanning principle. The monochrome or multicolor LEDs form the image pixels. In CRT this pixel control is accomplished by electron beam, deflected by scanning system. In LCD intensity is controlled [23] by shading out the unwanted light portion, image forming is not power-consuming and large number of pixels can be controlled sequentially. In LED displays the pixel control is the major problem [1]. Due to large size of LED displays, a modular construction is used. This allows for flexibility of shapes, formats and transportability. General video display structure is presented in Fig.1.

Host or specialized controller is responsible for decoding, configuration and unified format transmission. Display resolution usually differs from conventional video standards. That is why the format converter is needed. For instance the incoming VGA (640x480) data flow is 23MB/s. Display is typically using the progressive scan. With displays price dropping down some prestigious applications are using an incredible display resolutions [4,20,24,]. The flow required to control the display could get a TB/s. Thus the direct LED control from the host is impossible. The required control data flow is generated by tile controllers. Data distribution within the macroblock general structure is indicated in Fig.2. Data interface

Display Controller

1. Introduction

Host PC

Tile

Tile

Tile

Tile

Tile

Tile

Tile

Tile

Tile

Tile

Tile

Tile

Figure 2. Data distribution inside the macroblock

The interface block is responsible for data reception and further routing to next macroblock. Data dedicated for macroblock is stripped from mainstream and is directed to display controller.

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Proceedings of the ITI 2007 29 Int. Conf. on Information Technology Interfaces, June 25-28, 2007, Cavtat, Croatia

The last kernel in screen design is a tile. One controller is used to control several LED tiles. Tile incorporate small amount of LEDs mounted on single board. Typically it is 8x8 or 8x16 pixels. This corresponds to 192 to 640 LEDs depending on pixel configuration. In order to reduce the control electronics complexity, the amount of controlling wires and the price, display controller should control as much as possible tiles [8].

every individual LED drive by analog source is high. The most common way of LED dimming is a Pulse Width Modulation (PWM) [11]. Since average power of PWM is linearly proportional to pulse duration (refer Fig.3) this also offers a convenient and simple intensity control mechanism. Only the amount of the available duration steps limits the dimming resolution. 100% 67% 27% 20% 13% 7%

3. The pixel The pixel in general is formed by 3 LEDs. In case high display luminance is demanded pixel is formed by more LEDs. Since the luminous intensity efficiency is different for red, green and blue the proportions of same color LEDs also vary. The size and amount of LED define the fillfactor: the ratio of area occupied by pixel LED and the total pixel area assigned for pixel. It is important to keep the fill-factor high [2]. LED light is produced by the phenomenon of electroluminescence. Optical quantities such as luminous intensity, peak and dominant wavelength or chromaticity coordinates, spectral width, deterioration factor or expected lifetime are used to assess the LED [21]. LED directivity properties are expressed as half power angle, a peak emission direction and a far-field pattern (FFP) [22]. LED electrical parameters are represented by forward voltage, forward current, pulsed forward current, reverse current and reverse voltage. For this paper scope just operating current and luminous intensity and color need to be accounted. LED needs to be driven by a DC voltage source in order to generate the illumination. The operating current is 20mA for conventional LEDs and close to 1A for power LEDs. Dominant wavelength is slightly affected by forward current. If LED can be operated at constant current this would ensure the screen color gamut stability.

4. LED brightness control Changing the amount of the current applied through a junction results in variation of intensity of the emitted light. Despite this principle can be used for pixel intensity control this approach so far is not used in LED displays. The reason is twofold: (i) changing the current will affect the emitted light wavelength; (ii) the complexity of

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Figure 3. PWM explanation

Actually the PWM has its own shortcomings. The available LED switching speed and the required refresh frequency limit (i) the number of available steps. The PWM has to be supplied to every individual LED on the screen. The data to LED delivery time limits (ii) the available minimal duration. Even at different pixel intensities there will be a moment when all LEDs are switched on (refer Fig.3: the start of the period) and this will (iii) cause the large current spike (e.g. EMI) at the specific location in time. If PWM refresh rate is not synchronized with the frame data then (iv) intensity spikes will occur. The good test for this phenomenon is the movie with switching black and white strips. There have been a variety of PWM methods suggested for LED control, to name a few these are bit-angle modulation (BAM) [5] pulse frequency modulation and pulse amplitude modulation [17,25]. BAM allows for EMI and control data flow reduction.

5. LED control configurations The LED control configurations can be addressed as static and dynamic drive. Dynamic drive allows for driving circuit reductions so the screen price. The LED lines are driven (lit-up) in sequence [8,16]. Refer to Fig.4 for graphical explanation. The resulting brightness is divided between the scanned lines. If 20mA LED is usually allowed for a 100mA peak current, multiplexing more than 4 lines in sequence will reduce the brightness. Another dynamic drive associated problem is the reduced refresh frequency. Since lines are driven in sequence, the number of driven lines reduces the total refresh.

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N current sources

lightness relationship of vision. Therefore only minor correction is needed when image is output to CRT. When image has to be displayed on the linear response display (the LED video screen) then gamma correction is a must. The recommended response approximation functions are presented in Fig.6. CIE publication 15.2, ITU recommendation 709 and SMPTE standard 240M are presented [18]. Occasionally the Eq.(1) is used as the simplest correction.

DC

100 PWM

Decoder

90 80 Local LED controller

LED bus

CIE

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Lightness,%

Address counter

RAM

Figure 4. Dynamic drive topology

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709 30

Static drive [1] uses individual drivers for every LED (refer Fig.5). With price reduction for LED driver ICs this type of topology is prevailing in all modern designs.

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SMPTE240M

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Normalised Luminance NxN Current sources DC

Figure 6. Recommendations for eye response approximation

PWM PWM PWM

PWM

RAM

PWM

The screen total brightness can get too high if high brightness display (dedicated for outdoor use) is viewed in dim environment. Therefore it is important to have a screen brightness control without loosing the codes needed for gamma correction. Also it is useful to have the ability to vary the gamma in a range of 1 to 4.

LED bus

PWM PWM PWM PWM

Figure 5. Static drive topology

It is essential to note here that tile arrangement is influencing the image format. Since byte-wide control is used the common tile configuration is a multiple of 8 pixels.

6. Gamma correction Another headache in LED screen design is the human vision lightness sensation nonlinearity [18]. Use of luminance-to-lightness relationship of vision corrected signal at the camera end makes maximum perceptual use of the data channel. The luminance Y of the CRT is approximately equal to the applied voltage U, raised to the 2.3-2.6 power. This exponent usually is noted by Greek γ (“gamma”) hence the name for correction function Y =Uγ ,

7. PWM resolution requirements If Eq.(1) is used for internal binary code Cin conversion to CRT-mimicking binary code CCRT with resolution of N bits a rounding error is introduced. Resolution impact on this error for γ=2.5 is presented in Fig.7.

Corrected code CCRT(A.U.)

PWM

Local LED controller

PWM

(1)

Output code resolution 8 bit 10 bit 12 bit 16 bit 19 bit

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For CRT the voltage-to-intensity response is approximately the inverse of the luminance-to-

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Uncorected code Cin

Figure 7. Resolution impact on gamma correction

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Fig.7 indicates that only 19 bit resolution output coding is capable of monotonic variance: every change in input code is changing the same direction change in the output code. Such gamma correction even with high resolution will exhibit some approximation error. The approximation error will be large only at lower intensity values (Fig.7). At large intensities a lot of codes will be thrown away [14]. Even the use of slightly different correction [12] does not completely solve the problem. An interesting verdict: the floating point coding is needed.

8. The flicker Flicker occurs below the integrating capability of the eye. The minimum frequency is named a critical flicker fusion frequency, or CFF [13]. In practice image refresh frequencies of 100Hz are used. In the professional LED screen application, not a human vision is the factor determining the image refresh frequency. The reason is that if LED screen is used for venue imaging it should conform to scene shooting equipment speed. When scene gets very bright, aperture size reduction is not sufficient and exposure time is varied then. In case the LED screen refresh rate is not synchronized to shooting equipment speed, the video camera might capture only one single slice of the PWM. This can create the image patching or even complete disappear at some frame. Therefore the refresh rate needs to be much higher than the requirement for human eye. Usually 400Hz refresh frequency is used.

/('GULYHUV The PWM resolution and the desired refresh frequency define the driver requirements. A large development has been done until the LED control had settled to some driving circuitry standard. Refer to Fig.8 for general LED tile organization drawing. Drv

Out Drv Ltd Latch D Sft Rg Clk

Drv Drv

Pow+

OE

LTD CLK D

SDO

Fig.9. Switch output driver topology

This topology is quite old, use of saturable output switch does not allow for fast switching time. But thanks to saturated mode of the switch this allows for high current commutation. The major disadvantage is that there is a tradeoff between the current accuracy and efficiency. The accuracy is low if LED voltage and resistor voltage drop ratio is low. Usually 2-3V resistor dropout voltage is used. With 2-4V LED dropout voltage this means that more that half of the energy is dissipated in the resistor. The more advanced driver type is the constant current output driver. Usually it is a constant current sink. Pow+

Drv Drv Drv

... ...

IREF OE

OutBuf

InBuf

/OE

Because the data shifting process should not be visible on screen, driver usually contains two levels. One is the shift register, responsible for serial data reception. It has a serial data input node (D) data output (SDO) for cascading and a clock (Clk). The next data path level is a data latch, responsible for holding the previous data. This layer is controlled by separate control line (Ltd). The output stage is using the latch layer data for LED control. The output can be turned off by using output enable signal /OE. The output stage structure divides the drivers into three categories: switch output, constant current output and PWM output. Switch driver is the simplest type: output stage usually is a switch (usually an open drain), connecting the output to the ground. In order to set the LED current, external resistor is added. Refer to Fig.9 for simplified topology drawing.

LTD CLK D

...

Fig.8. LED tile organization

SDO

Fig.10. Constant current output driver topology

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The constant current is provided by individual regulated current sources. Single external resistor sets the sink current. Refer to Fig.10 for simplified constant current output driver topology drawing. Again total outputs control is using signal OE. Constant current technology today is most popular in LED drivers. The current sink output should allow for the faster switching times since the output switch is not saturated. The major advantage of this type of drivers is that usually 0.4-0.7V driver dropout voltage is sufficient. But in such case red LED (2V drop) power supply should be 2.4-2.7V. The conventional power supply efficiency for such output voltage is about 60%. If 3.3V or 5V power supply is used this correspond to 25-40% loss on control element. The internal PWM output drivers represent the upcoming intelligent drivers generation. Pow+

PWM contr

PWM contr

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PWM contr

PWM contr

PWM contr

PWM contr

IREF

LTD CLK D

SDO

Fig.11. Driver with internal PWM

Here, constant current output is combined with individual PWM for every channel. Such a setup allows for tremendous data flow to the driver IC reduction. The current gray level value has to be loaded only once per video frame. The rest of refresh time IC internal PWM is taking care of all the dimming functions. Even the use of constant current output and the internal PWM does not solve the driving efficiency problem. Furthermore, there is a problem associated with power distribution. The LED is essentially low voltage device: depending on color dropout voltage varies 2 to 4V. But the power to be supplied is high. Assuming 640x480 and 3LED per pixel screen with 20mA current per LED, total current is 20kA! The relatively small macroblock 64x48 pixels will demand 180A! Some losses definitely occur when this power has to be distributed. Therefore it is desired to have higher voltage power distribution and have some local DC/DC buck (step down)

converter. But such converter will require the inductor. Moreover if trace length from driver to LED is long it is exhibiting some inductance. Therefore one may think of local step-down converter dedicated only for single LED which incorporates the parasitic inductance, is running at very high frequency (hence call for low inductance) and is using a LED current but not the converter voltage as the feedback. Possible such driver topology is presented in Fig.12. Pow+ LTD CLK D

PWM controller

SDO

DC/DC controller Feedback

Fig.12. Driver using DC/DC converter

It is essential that floating point control can be introduced in such driver. The LED current now can be regulated, giving a different floating point exponent position. The PWM controller can still introduce the PWM modulation but not it is with regulated weight as it is required by gamma correction. Moreover, some fixed gamma correction can already be stored on chip. If gain of feedback amplifier is high, then the voltage drop on current sensing resistor is low. Converter can use a synchronous rectification scheme or recycling diode. Since the current required for LED supply is low so should be the price of switches used. Such driver must preserve the same control interface for backwards compatibility with older generation drivers. Wider application of power LEDs have initiated similar drivers emergence on the market [6,9]. Unfortunately, these drivers require the external analog voltage source for LED current setting and external source for PWM. The other branch coming towards same direction is the idea to use the digital instead of linear control in DC/DC converters [3]. Especially close is the publication [17]: the only difference is that microcontroller is suggested for LED driver control.

&RQFOXVLRQV The use of more complicated PWM modulation techniques like BAM reduces EMI. The use of local PWM allows significant controlling data flow reduction. However, due to reduced LED and driver reaction time wide LED

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dimming range is not easy to implement. Furthermore, majority of control codes are thrown away in order to get suitable gamma correction curve. Floating point control could be the better choice. The existing LED driver technologies have solved majority of problems associated with LED dimming implementation in video displays. The remaining problems are the high current distribution efficiency and the dimming range. The suggested LED driver topology could solve the mentioned shortcomings present in existing technologies.

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on Communications, Circuits and Systems; 2006 Jun 25-28; P.R.China; Beijing; 2006.p. 2193-2196. [11] Mo X, Zhang Y. Consecutive PWM driving video LED display system. Proceedings of International Symposium on Circuits and Systems, IEEE; 1997 Jun 09; Hong Kong; 1997.p. 1437-1439. [12] Mo X, Zhang Y, Wu C. Grayscale model for video LED display. Proceedings of 4th Asian Symposium on Information Display; 1997 Feb 13-14; 1997.p. 117-118. [13] Murray S, Caldwell B. Human Factors in Displays. CRC Press; 2000. [14] Narra P, Zinger D. An effective LED dimming approach, in: proc. 39th IAS Annual Meeting vol.3 (IEEE, 2004) p 16711676 [16] Nguyen F. Challenges in the design of a RGB LED display for indoor applications Synthetic metals 2001; 122(1):215-219. [17] Pachuca P., Borras R. Microcontroller-based LED drivers: topologies and trade-offs. LEDs magazine; Oct 2005. [18] Poynton C. Gamma and Its Disguises, Journal of the Society of Motion Picture and TV Engineers 1993; 102(12): 1099–1108. [19] Putman P. When Old is New Again. Videosystems 2002; 37-43. [20] San Siro stadium installs Odeco LiveAd LED display. LEDs magazine 14 Feb 2006 http://www.ledsmagazine.com/articles/news/ 3/2/19/1 [01/29/2007] [21] Shubert E. Light Emitting Diodes. Cambridge University Press; 2003 [22] Svilainis L, Dumbrava V. LED goniometry: the feasibility study. Measurements 2005; 35(3): 35-40. [23] Takahara K, Yamaguchi T, et al. 16-level gray-scale driver architecture and full-color driving for TFT-LCD. Proceedings of International Display Rerearch Conf, 1991 Oct 15-17; Japan, Atsugi; 1991. p.115-118. [24] Vivid Effect provides eye-catching LED frontage for Austrian bank. LEDs magazine; September 2006. http://ledsmagazine.com/articles/features/3/9 /2/1 [25] Wenyu X, Zhiliang C. A Mixed-Signal Driver Chip for 65K-Color Passive-Matrix OLED. ASICON 2005

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