Video AD board, large screen, power supply, video encoder need, decoder, line drive, digital signal processor (DSP) has a lot of small subsystem common operation, can provide a video image, but seldom greater insight into a display systems light emitting diode (LED) drive function.
If you watch the video screen, will find that there are hundreds of video panel, again closer to a little, will find that each panel and have the article 16 pixels, each megapixels by three of led strip, respectively is red, green, blue. Each of the anode is LED connected to the output of the LED drive, these hundreds of thousands of LED drive through hundreds of thousands of a positive current LED control, to produce video images.
Electronic design personnel’s challenge is how to use LED drive effective control LED current, most high order LED drive can by many kinds of methods to control system of current, many drive has LED the brightness adjusting functions, such as point correction (DC), pulse width modulation (PWM) dimmer and global illumination control (BC). Although these functions can be played the same basic utility, also is the adjustment LED brightness, but their original purpose is not the same, only know how to proper use of these functions, can provide the best quality video.
Simulation/PWM dimmer reduce brightness LED
Understood the advanced LED drive before function, must first understand two reduce brightness led the method. Respectively is simulated dimmer and PWM dimmer. The simulation just to change through the LED the positive current. If half of the current drivers LED, its brightness has halved. Although unable to reach: the ratio of, LED brightness and positive current present linear relationship between roughly.
PWM adjustable just to be opening and closing the current, to change the current pulse width, and the amplitude. Brightness LED through open the percentage of time that LED to control or load cycle. The naked eye can only feel about 60 Hz the following frequency response, with more than 60 Hz if the rate of opening and closing LED. Naked eye can only feel average LED brightness. Most of the display USES more than 100 Hz PWM dimmer. For fixed frequency PWM dimmer, load cycle will lead to reducing LED brightness is reduced. Chart 1 shows that the 20 mA (mA) LED the light to 25% brightness. Simulation and move light PWM dimmer comparison between, the two LED brightness are the same.
Flexible LED Strip Lights manufacturing processing tolerance is not easy to control, for high quality of the display, the two main characteristics are led the luminous intensity and frequency spectrum purity. However, the characteristics of different LED on very different, even the same LED manufacturing bath also are such. In the ideal case, use the same processor LED drives to current open all red LED, can achieve the same brightness. In fact, however, with the same current drivers LED, brightness will a huge difference. General LED on the market in the same positive parts current drivers, will present the 2-1 brightness difference.
If not deal with such differences, LED panel will have very good between the pixels luminance match, and some fixed can effectively deal with luminous intensity differences. Point correction is to point to will led display the pixels or points in the density of the amended or normalized. Through the simulation of individual LED adjust current can be reached such effect. Panel manufacturing, after the completion of the original factory must first point correction. Test equipment measurement LED density, and product set in the point above the IC correction coefficient, adjusts the brightness LED.
General high density led actuators can be single resistance of each output settings LED current limit. Point correction using simulation dimmer, will adjust the current limit for the current percentage degree. Some fixed the adjusted a count of too much, and eventually led the brightness of the agreement. Six sites can provide correction from 0 Ma rset resistance to define the current limit of sixty-four different current level, and can gradually to the scale of 1.59% positive current adjustment; 7 sites modifications can gradually to the scale of 0.79% positive current adjustment. Figure 2 shows quantity production after some fixed before and after the panel brightness.
For some fixed before, all LED has the same positive current, therefore cause brightness difference 2-1. For some correction, LED drive in different current drivers can all LED, that LED to the same brightness. It is worth noting that. Point correction must be able to adjust the brightness for less than the panel will be the most dark LED value. In any panel of a large display of a production environment, must will be all LED to the brightness of the set is equal to or lower than the system is the most dark LED brightness, so the data table should provide led the brightness of the lower limit specified range.
Point correction can also be used to adjust the brightness of the display. At noon, the sunny outdoor use full brightness, but the same brightness for indoor appears at night or too bright. Will all point to a revised lower percentage of the values of the previous level, also can reduce overall panel brightness. If the system using a correction of the IC, but not use point correction function, they will be all point to a revised set of the upper limit value of the half, can achieve 50% brightness. After seven sites for the revision of the LED drive, will be all the points a revised from one hundred and twenty-seven down to sixty-three, can achieve 50% brightness. The following equation can be set to use some of the positive current modified LED:
Among them, is the Imax rset set the output current limit, the point is the output DCx x a revised, n is point correction of digits. For the above example, Imax equal to 40 ma, DC is equal to sixty-three, all LED currently set to 40 Ma is 63/127 = 19.84 Ma.
If the system is already using a fixed function to achieve consistent panel brightness, can still use some fixed adjust light. Will point to a revised set the initial value of the half, can achieve 50% dimming effect. 7 sites fixed LED drive as an example. If LED require one hundred and eight points in a revised, will be set up as the current consensus panel brightness of correct value, will be the new point of a revised set to 54, adjust adjust for 50% of brightness.
Using fixed adjustable light fixed panel brightness drawback is. The luminance match of the available a number of effect would be decreased.
Brightness control precision matching brightness
In order to maintain certain luminance match, at the same time, the whole panel of the light, high order LED to provide the individual BC drive buffer to achieve integral move light panel outside of the individual point correction utility. And some correction, brightness control is also more light through simulation. It’s the difference between them is that point correction can be adjusted individual LED, brightness and control can be adjusted at the same time more LED. Point correction and brightness control the number of buffer and configuration with IC is concerned.
For example, drive eight stars in groups of three LED a group of 24 channel LED used LED drive contains a point correction 24 buffer (each each used for individual LED), including the three brightness control buffer, each one for each individual LED group. Another 16 output LED actuators can be driving sixteen of the same color LED, including 16 internal point correction buffer, and contains a brightness controller buffer, can be adjusted at the same time all sixteen output. Oxonian correction and brightness control functions help accurate control LED luminance match, at the same time, the whole panel can be dimming. Contains some correction and brightness in the control of the IC, LED current is made of two kinds of buffer values common definition: one is the Imax Rset set the output current limit, the point is the output DCx x a revised, n is point correction of digits, BC is global brightness values, m is the brightness fixed number.
The low light PWM brightness maintain colour
Precise control of high definition video brightness is just one of the links, the second part is the right color matching. Old Money monitor USES the light to set the color simulation mixed LED brightness and color quality to have a negative effect. Figure 3 shows positive current changes and LED the relationship between color changes. Figure 3 Real green LED in this particular definition LED the brightness (20 ma) color. If you want to use the 25% dimmer simulation brightness, need 5 ma positive current. This enables chromatographic from 525 nanometers (nm) into 531 nanometers, need to be present for the display of true color is concerned. This is unacceptable levels.
PWM dimmer and can be called gray-scale PWM dimmer, can eliminate LED move light color changes. This kind of light energy to the high definition video low light, maintaining the correct LED colour. No matter why LED brightness, PWM dimmer can make through the current LED consistent, in order to eliminate LED color changes. The color displays each picture element is composed by three LED, respectively, in red, green, blue. By simultaneously, red, green, and blue LED the pulse and mixed, pixels can appear as many as sixty-eight billion seven hundred million kinds of colour. The following example will illustrate the PWM light.
For the sake of simplicity, this example use only the PWM the three persons in the light. Three bits equal to 23 = 8 a shadow chromaticity, so each LED can be set to 0 ~ 7 PWM gray-scale degree. Each video box to start. All are all LED goes out. In the first of the frequency PWM rose edge, all LED will light up, only to 0 grey-scale values set LED not on the light. Each PWM frequency cycle begins. IC will increase gray-scale counter. Each LED will continue to light up, until the PWM gray-scale counter led the PWM value more than set so far.
Figure 4 shows the program, which presents the simplified three PWM dimmer controller waveform and block diagram (figure 5). Will, red, green, and blue LED the grey-scale values for seven, four were set and a, can be on the picture presents orange megapixels. Set to four grey-scale values of green LED will in the first PWM frequency cycle of edge light up to rise, and after four complete frequency PWM continued to cycle on the light. The three PWM dimmer example for every pixels can produce five hundred and twelve (23 23 23) kind of colour. For 16 LED drive, it can produce two hundred and eighty-one million (216 x 216 x 216) kind of colour.
Use proper collocation point correction, brightness control and PWM dimmer, big screen image of more perfect. So perfect design, to make the masses to big screen image of the response, and in order to achieve megapixels, luminance match between the display brightness lighting conditions, mixed with environment colour with perfect image of concern and efforts, then quietly hidden in the brilliant fluorescence behind the scenes.