Backlight Dimming Algorithms
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In previous videos, we have talked about the basic components of LCD display modules and specifically about the LED backlight driver. Today, we'll focus on different ways to control the brightness of LEDs through your backlight driver. There are two basic ways of controlling the brightness of an LED-- a purely analog approach through current control and a digital approach by controlling a PWM duty cycle.
A pure analog approach involves linearly changing the currents through the LEDs. Current control is simple to manage and highly efficient for delivering high power. However, it does not provide the best resolution. At low brightness levels, it is difficult to provide very small values of current without experiencing fluctuations. When current controlled, the non-linearity of LEDs limits the minimum brightness. These are highly undesirable factors of linear brightness control.
A PWM approach involves running the LEDs at the maximum desired current by turning them on and off fast enough using a PWM signal to make them appear dimmer. PWM control enables very low minimum brightness levels, well below 1%, especially as the resolution of the PWM signal is increased. Since the current delivered to LEDs is constant even at low brightness, there is no shift in color temperature. However, due to their very nature, PWM signals suffer losses at high duty cycles. In the context of an LED driver, high brightness levels, which would have the highest use case, would have very poor power efficiency and generate a lot of switching noise. Low efficiency and high switching noise can be disastrous for your system.
So what could a designer do to achieve optimized functionality where they don't lose brightness resolution at low levels and don't have to compromise system performance at high brightness levels? This brings us to a control method we call hybrid dimming. Hybrid dimming combines the best benefits of both analog and PWM to me. Let's consider an example use case where you want to drive a string of LEDs at 100 milliamps of peak current. We will define a transition point of 25% peak brightness, or 25 milliamps. Let's assume your system can support a nominal 10-bit PWM signal.
With hybrid dimming, you would break your control algorithm into two segments-- 0% to 25% and 25% to 100%. For brightness levels from 0% to 25%, you would keep the current constant at 25 milliamps, which would be the peak current for this segment, and modulate the duty cycle of your 10-bit PWM signal to control brightness. This gives you 1,024 brightness levels within the first 25%.
As you get to 25% brightness and hit 100% PWM duty cycle, you would linearly increase the current for higher brightness until you get to your peak display brightness of 100 milliamps. By implementing hybrid dimming, you obtained the precise brightness resolution from 0 to 25 milliamps through PWM control and the high efficiency, low noise operation from 25 milliamps to 100 milliamps through current control. The best of both worlds.
A lot of TI's LED backlight drivers already integrate this functionality. Simply tell the device your max current and your desired transition point. The driver will analyze the incoming brightness signal and automatically drive the current syncs through the hybrid dimming algorithm to give you an optimized design. For more information, check out ti.com/backlight_training. Thanks for watching Display University.
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