LED Display Drive Technology Discussion: AC Response
Foreword
The AC response characteristics of LED driver chips are often overlooked, but they are a very important feature. The AC response affects the image quality of the LED display, such as grayscale, linearity, EMI, and reliability. Although these characteristics have a trade-off relationship with each other, a good driver chip should be able to achieve a better balance among these characteristics. This article will explore the importance of AC response and LED driver chip and board design techniques to assist engineers in designing displays with good image quality.
Shortest OE pulse width and linearity
More than 1024 grayscales per color have become the basic specification for LED full-color displays. In order to express richer colors, manufacturers need to be able to display more grayscale driver chips. The shortest pulse width and reaction time (tr / tf) of OE determine the number of gray levels. However, many driver chips often sacrifice linearity in order to shorten the OE pulse width. The so-called linearity is the relationship between input data and output brightness. For example, in Figure 1, the output voltage waveform of OUTn is shorter than the OE pulse width. The linearity relationship is shown in Figure 2. It is obvious that the LED brightness is not proportional to the setting of the OE pulse width, especially when the OE pulse width is less than 0.1us, the linearity is not good at this time.
Figure 1 Relationship between OE pulse width and OUTn output voltage waveform
Figure 2 LED brightness and OE pulse width
There are many different definitions on the market for the shortest OE pulse width. Some chip manufacturers define the time at which the output can react as the shortest OE pulse width, but only such a definition ignores the effect on linearity. Therefore, it is still necessary to actually measure the linearity to ensure that the chip can exhibit sufficient gray scale.
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