Feedback regulation for power optimization
As has been shown above, a feedback path from the LED driver to the SMPS sets the drain voltage to the minimum required value. The output current sink can be implemented either with a simple, defined current output driver and an external capacitor (see Figure 9, left-hand diagram) or with a digital control circuit which sets attack/release times and controls the current output with a digital-to-analog converter (IDAC) (see Figure 8, right- hand diagram).
Figure 8: Two different methods for implementing a feedback loop to the SMPS
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Both of these solutions offer good efficiency, work with every type of SMPS with voltage feedback, and can be implemented by attaching feedback lines from more than one driver to the same SMPS, as is required in mixed- architecture systems.
However, the second, digital implementation provides some special advantages. As well as not requiring an output capacitor, the digital circuit also gives the designer the freedom to define the feedback system’s attack and decay times. By selecting a fast attack time combined with decay latency and relatively slow decay, the display’s performance can be improved. This benefit is particularly noticeable in scenes that require brightness to change rapidly. In this case, a fast attack time eliminates perceptible brightness artifacts as the screen changes from dark to full brightness. The analog solution (from figure 8) dims the LEDs’ output gradually during a short dark frame, resulting in a visible delay in achieving full brightness for the next bright frame.
This is a noticeable distraction for TV viewers because films and other video content create large dynamics from one frame to another. Such artifacts can be eliminated with digital regulation circuits by inserting latencies of several hundred milliseconds into the decay instruction. Thus, when bright frames are interrupted by a short sequence of dark frames, the second bright frame starts at full brightness because the driver has automatically delayed the voltage ramp-down. Digital feedback algorithms implementing decay latency can be found in products from ams.
Another useful feature integrated in LED driver ICs is a fast Serial Peripheral Interface (SPI). In direct backlit TVs, the LEDs are arranged in a large number of relatively short strings, so that small areas of the panel can be dimmed to save energy. Typically, such arrangements contain 256 channels in a matrix of 16x16 fields, each individually configured through pulse width modulation (PWM). But generating 256 PWM signals with variable PWM width and delay is a hugely intensive processing task, even for the fastest microcontroller.
These backlighting systems therefore use local PWM generators integrated into the LED driver ICs. This enables brightness to be configured with simple SPI data transfers. In an architecture with multiple driver ICs (e.g. 256 channels with 16 channels per IC, and 16 ICs), the LED channels can be configured by daisy-chaining SPI signals and transferring the data that are used in a VSYNC frame to the frame before.
In this arrangement, data transfer over an SPI can reach speeds of 20 Mb/s, or 50 kb/frame at a 400 Hz frame rate. This is fast enough to change dimming of each field in sync with the actual frame. So ideal local dimming can be achieved with minimum overhead on the microcontroller.