The following simple application can be used to align LED brightness while keeping superior color quality. This application uses pulse width modulation (PWM) dimming formula. This application can be used for single-line scrolling displays, transportation road signs, simple scoreboards, ever-expanding advertising markets covering convenience gas stations, shops, stadiums, and stores. Following pictures shows color shift stimulated by analog dimming, analog dimming vs. PWM dimming, orange pixel generated with 3-bit PWM dimming, and inrush current control.
The high-quality, full color video demands hundreds or thousand of brightness levels between 0% and 100%. To improve video quality, we can applies analog dimming by altering the LEDs forward current. It is simple but it shifts color spectrum that is unaccepted in displays that require a true color representation. To figure out this problem we can use this application. It uses TLC5940 to improve video quality and enhanced color shading by adjust the brightness levels.
PWM dimming brings down brightness by modulating the LED’s forward current between 0% and 100%. It controls the LEDs brightness by adjusting the relative ratios of the off time and on time. The advantage of this formula are the forward current is always constant, and pulsing the current provides accurate brightness control while preserving the color purity. But it also has disadvantage, it is the maximum number of distinct brightness level is limited.
For example, if we would like to produce an orange pixel on the screen, we just program the grayscale value of the blue, green, and red LEDs to 1, 4, and 7 as shown in third picture. The colors that can be acquired by TLC5940 12-bit PWM dimming are 68.7 billion colors. The fourth figure shows the comparison between LED supply current without controlled inrush current and LED supply current with controlled inrush current. Here is the electrical schematic:
(Source : Texas Instruments)