Foreword
AC responses of LED drivers are critical but usually ignored in LED display applications. AC responses affect the major performance of LED display panels, such as grayscales, linearity, EMI, and reliability. Although there is trade-off within these requirements, LED drivers can provide balance. This article will further explain the importance of the AC responses of LED drivers and PCB design techniques to help engineers to design LED panels with good grayscale images.
Minimum OE pulse width vs. Linearity
LED panel manufacturers are asking more on the grayscale improvement to enrich color of image on LED panels which satisfy audiences more. More than 1024 grayscales of each color is now a common requirement for full color LED panels. At the same clock frequency, short OE pulse width and response time (tr / tf) may help achieve more grayscales. Yet, the design of short OE pulse width sacrifices the linearity, which means the proportion between the input data and output brightness. For example, the output voltage waveform of in Fig.1 is shorter than the OE pulse width, and the linearity result is shown in Fig.2. Obviously, the linearity of LED luminance is no longer proportional to the setting of OE pulse width especially when OE pulse width is less than 0.1us. As a result, the linearity is not good for this condition.
There are different definitions of minimum OE pulse width on the market. Some vendors define it as the shortest OE pulse width that output voltage of can respond to, but this definition is only meaningful under good linearity condition.
Overshoot Elimination
The voltage overshoot on the output channels usually damages the LED driver, when an LED driver is turned off. This also influences the reliability of LED panels. The voltage overshoot results from the parasitic inductance between VLED and OUTn (output ports). Fig. 3 and Fig. 4 explain the experiment results of the voltage overshoot. In the experiment, inductors are added to the circuit to simulate the parasitic inductors of the PCB traces. The waveform on node CH1~CH3 in Fig.3 are shown in Fig. 4 respectively. The voltage on the output node (CH3) will reach 26.6V, which is much higher than the breakdown voltage (17V) of an LED driver.
The overshoot voltage can be calculated by the formula below:
V = L x di / dt
where V is the induced surge voltage;
L is the parasitic inductance;
di / dt is the current variation during switching.
There are three approaches to eliminate the voltage overshoot. First approach is to reduce the parasitic inductances. Because the VLED overshoot will also accumulate on the VOUT surge, the traces of power lines and every individual output channel should be as short as possible and the distributed capacitors between VLED and GND should be placed uniformly on the PCB to reduce the overshoot on VLED and VOUT.
The second approach is to switch the driving devices slowly. LED drivers with short response time (tr / tf) are not suggested since it will cause higher voltage surge. Choosing an LED driver with moderate response time (tr / tf) for different applications is sufficient.
The third method is to disperse the switching noise. LED panel manufacturers may choose LED drivers with staggered output delay of each channel to avoid all output channels to switch at the same time. By the staggered output delay between channels, the overshoot can be suppressed.
Conclusion
Carefully choosing LED drivers and designing with good PCB routing as instructed above help LED panel manufacturers improve the grayscales and reliability of LED panels. The latest LED drivers on the market, such as MBI5025 and MBI5039, provide balance to help LED panel manufacturers easily achieve better image quality on LED panels with good system reliability.
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