Taking Advantage of SnapDriveTM to Improve the LED Display Screen Quality

Full-color LED video display screen has become popular in recent years for various applications such as indoor/outdoor commercial advertisement, stadium billboards, traffic message signs…etc. Thanks to the evolution of LED technology which enables lower cost, higher luminance, uniform wave-length and better efficacy LEDs available, the LED display screen makers are able to build high quality display screen with cheaper cost. With the advent of Full-HD video, the need for building higher resolution LED display screen supporting higher grayscale-level and higher refresh rate is continuously driving the LED display makers to upgrade their system specification. The improvement of LED display refresh rate and grayscale-level is associated with several electrical factors, including (1) the channel current output response rate (OE[note1] response rate) of the LED driver, (2) the speed of system clock (DCLK) used to transmit pixel data from control board to the LED driver in a cascaded daisy chain, (3) the scanning methodology of the LED module board which usually is a trade-off between BOM cost and luminance degradation, (4) the transmission throughput to transmit display pixel data from the video control system to the LED display panels.

 
Conventional LED drivers with moderate OE response rate and DCLK can usually provides LED display makers just-enough performance to build a medium-class screen, by trading off display quality to gain system stability at reduced BOM cost and cheaper PCB design. Such LED drivers, however, fall short to meet the ever-growing pursuit for higher resolution, refresh rate and grayscale-level. As such, EnE Technology is rolling out a series of LED drivers with SnapDriveTM [note2] technology to address this demand. The SnapDriveTM technology expedites the LED driver’s OE response rate and DCLK without having suffered the distortion of output current, while at the same time it also alleviates the heat dissipation of LED and hence prolongs the life-time of LED. By taking advantage of the SnapDriveTM drivers, system makers are now able to push the frontier of their design specification to embrace high quality video such like Full-HD.
[Note 1] OEis an input pin of the LED driver IC used to enable/disable the output current driving of LEDs
[Note 2] SnapDriveTMis a series of LED drivers developed by EnE with footprint compatible with conventional (TB62746) drivers
 
Advantage 1: Refresh Rate Improvement
Disregarding the pixel data transmission throughput which involves the number of control boards in parallel and the speed of the transmission media (e.g., 100Mbps or Gbps Ethernet) between the video source and the display screen, the refresh rate of a LED display screen is bounded by several electrical factors:
(1) the OE response rate or equivalently the minimum pulse width of LED channel current If.
[Note] Faster OE response rate enhances the refresh rate
(2) the time (Tacc) required to complete all the channel data update for the drivers in the daisy-chain within a single refresh cycle.
[Note] Shorter Taccenhances the refresh rate.
Tacc is represented by Tacc= 1/DCLK * Cas
Where:
DCLK: the system data clock
Cas: the total number of output channels in a daisy chain, and is calculated as
Cas = (LED driver #) * (channel/per_LED driver)
(3) the scan methodology (e.g., static or 2/4/8/16 scan) used in the LED module board.
[Note] higher scan# degrades the refresh rate
 
For example, given an outdoor LED display screen constituted by 48 pieces of 16-channel driver IC in a single daisy chain, making the total number of channels in a single daisy-chain to be 48*16=768 (i.e., Cas=768). Assuming the panel is static driven (without scanning) with 2-bit/4-step of brightness level adjustment.
 
For DCLK=10MHz, Tacc = 768 * 0.1us = 76.8us
For DCLK=30MHz, Tacc = 768 * 0.33us = 25.6us
 
Considering a conventional LED driver IC with minimum OE response time = 250ns and DCLK=30MHz.
 
Weighted OE pulse cycle is optimized to: 1/64, 1/32, 1/16, 1/8, 1/4, 1/2 ,1 ,2 ,4 ,8, 16, 32
And the resultant refresh rate is:
Tfr= 25.6us * (6 + 63) + 5 * 4us = 1786.4us
Refresh Rate= 1/Tfr = 559.7Hz
 
Where 4us is the typical set-up time between data latch occurs and OE becomes active.
 
For another LED driver (SnapDriveTM) with minimum OE response time=50ns and DCLK=30MHz:
 
Weighted OE pulse cycle is optimized to: 1/512, 1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2 ,1 ,2 ,4
The resultant refresh rate is:
Tfr= 25.6us * (9+7) + 8 * 4us = 441.6us
Refresh Rate= 1/Tfr = 2264.5Hz
 
Comparing conventional LED driver with SnapDriveTM driver, the advantage of SnapDriveTM is apparent: it can improve the refresh rate from 559.7Hz to 2264.5Hz.
Advantage 2: Grayscale-level Enhancement
A conventional LED driver has a typical specification of minimum OE response time=250ns and DCLK= 30MHz. Given the above display system as an example, such driver can only support up to grayscale-level of 8 bits (not including 2-bit/4-step brightness levels), i.e., RGB each has 256 grayscale-level which makes the total 256x256x256=16M colors of the screen. If using such conventional driver to drive grayscale-level up to 14-bit, the refresh rate will become an intolerable of 9.5Hz as calculated below:
 
Weighted OE pulse cycle is: 1/64, 1/32, 1/16, 1/8, 1/4, 1/2,1 ,2 ,4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048
Tfr= 25.6us * (6+4095) + 5 * 4us = 105555.6us
Refresh Rate= 1/Tfr = 9.5Hz
 
The SnapDriveTM driver IC, however, can shorten the OE response time to 25ns and DCLK=30MHz. Given the same example display system above, using SnapDriveTM driver IC can enable up to 14 bits of grayscale levels (not including the 2-bit/4-step brightness levels) with a fairly satisfactory refresh rate of 146.6Hz as calculated below:
Weighted OE pulse cycle is optimized to:1/1024, 1/512, 1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2, 1,2 ,4,8,16,32,64,128
Tfr= 25.6us * (10+255) + 9 * 4us = 6820us
Refresh Rate= 1/Tfr = 146.6Hz
 
 
Improvement on Refresh Rate
@ 14-bit grayscale + 2-bit Brightness
Improvement on Refresh Rate
@ 8-bit grayscale + 2-bit Brightness
 
SnapDriveTM
Conventional
SnapDriveTM
Conventional
Color
4.39x1012
4.39x1012
16.7x106
16.7x106
Refresh Rate
146.6Hz
9.5Hz
2264.5Hz
559.7Hz
Table 1: SnapDriveTM vs Conventional Driver IC Improvement
 
Comparing conventional LED driver with SnapDriveTM driver, again, the advantage of SnapDriveTM is apparent: it can support 14-bit of grayscale-level (exclusive the 2-bit/ 4-step brightness levels) while achieving a satisfactory refresh rate of 146.6Hz. At a moderate grayscale-level of 8 bits, the SnapDriveTM driver significantly improves the refresh rate from 559.7Hz to 2264.5Hz.
 
Output Waveform of SnapDriveTM
Figure 1 demonstrates the average output current of SnapDriveTM driver versus conventional LED driver in various OE pulse duty cycle range from 0%~100% @ T=540ns. The output current curve of SnapDriveTM driver is linear across the range of OE pulse duty cycle from 5% to 100%, implying the output distortion of the current waveform is negligible even when OE pulse width is as short as ~27ns (540ns * 5%). This can be further demonstrated in Figure 3. For conventional driver, the output current curve reveals the driver falls short of current-driving capability in the short OE duty cycle regions and hence the curve is nonlinear. This can also be demonstrated in Figure 2 where the Iout vs OE transient response waveform is heavily distorted.
Figure 1: OE Pulse Width vs Output Current (Iout) Curve
 
Test Condition: Vcc=5V, Iout=38.3mA, RL=47Ω,CL=13pF
Figure 2: Conventional Driver IC Figure 3: SnapDriveTM Driver IC
 
Advantage 3: Reduce Output Current Waveform Distortion
Table 1 compares the conventional driver vs SnapdriveTMdriver in terms of their channel output current transient response as an reaction to the OE signal based on simulation (HSPICE2007) result. The output current distortion will bring negative effect to the LED brightness level and also will cause LED chromaticity deviation and hence it must be kept as small as possible. The SnapDriveTM driver can restrain the distortion to negligible level in order to truly reproduce the pixel color and maintain homogeneous brightness for all grayscale levels.
 
 
Distortion
SnapDriveTMDriver IC
1%
Conventional Driver IC
49%
Table 1: Output Distortion Comparison
Simulation Condition:
Conventional Driver IC specification: Ton =160ns, Tof =70ns
SnapDriveTMDriver IC specification: Ton = 15ns, Tof = 15ns
Vin=5V, Iout=20mA, LED equivalent circuit model: RL=52Ω, CL=10pf
OE pulse width: 250ns
Figure 4: Output Current Distortion
 
LED Heat Dissipation consideration
The brightness of LED is proportional to the forward current (If) flowing through it. By adjusting the pulse width and duty cycle of If, a single LED’s brightness can be adjusted accordingly and in turn the grayscale of a LED pixel formed by RGB LEDs can be manipulated. In this way, full-color LED display screen with millions or billions of colors can be realized.
 
A pulse scheme with multiple pulses and shorter duty cycle is preferable to drive a LED than a one with single pulse and long duty cycle. Even though the turn-on time are equivalent and hence the average current flowing through the LED over time being the same for both schemes (refer to Figure 5), multiple-short pulse scheme prevails the single-long pulse scheme in that it alleviates the temperature rise effect due to the accumulated power dissipation of LED within the same period of time than its counterpart scheme. As LED at higher temperature will exhibit several negative phenomena including chromaticity deviation, brightness degradation and life-time shortening, it is always recommended not to drive the LED with long turn-on time but with multiple short pulses instead. The very nature of SnapdriveTM driver is engineered to support short pulse scheme.
Figure 5: Output current PWM schemes: multiple pulses vs single pulse with the same average current
 
PCB Design Consideration
Although the SnapdriveTMdriver IC enables faster refresh rate and higher grayscale level, there are certain design considerations should be well taken care of in order to prevent the transient voltage spike associated with the parasitic inductance: ΔV= L •di/dt, i.e., as di/dt becomes smaller, the voltage spike becomes higher. The high spike appearing on the output of driver IC can cause damage to the driver IC itself as well as to other components on the PCB. It also complicates EMI issue.
 
ΔV: The voltage spike on the output of the driver IC
L: parasitic inductance of the trace
di/dt: the transient current when output channel switching on-off
 
  1. Use 4-layer PCB instead of 2-layer PCB. The 4-layer PCB should have dedicated power and ground layers staked in the middle-two layers. Also keep all the traces as short as possible.
  2. Add a large capacitor on VLED to ground and on VCC to ground. The capacitor value is recommended to be 1000~1500uF (CP1, CP2) as shown in Figure 7.
  3. Use separated power supplies for VLED and VCC.
  1. Add RC filter circuit at the input of the clock signal to reduce the overshoot/undershoot and EMI. The recommended value is Rt<22Ω and Ct<33pF
Figure 6: LED driver PCB design guideline
 
On the scanning circuit, it is recommended to add Rg and Cg at the output of 74HC138 in order to avoid the voltage spike induced by the parasitic conductor of trace and by the parasitic capacitor across the Gate and Source terminal. The recommended value is Rg<100Ω, Cg<47pF.
Figure 7: The scanning circuit containing 74HC138 and PMOS switch
 
Conclusion
The fast OE response of SnapdriveTMdriver can help to improve the LED display’s refresh rate and grayscale levels without suffering LED current distortion. It further helps to reduce the thermal dissipation of LED being driven with longer time-period pulses. Conventional driver with moderate OE rise/fall time can not render linear output current at high OE rate and, if so, will cause LED chromaticity deviation and brightness degradation. By carefully design the PCB to reduce the voltage spike and EMI issues associated with faster OE response time and using the SnapdriveTMdriver, higher quality LED display screen with longer life time and homogeneous brightness can be achieved.

By Jeff Cheng, Marketing Manager, EnE Technology Inc.
jeff_cheng@ene.com.tw +886-3-6662888 ext. 3260

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