Those huge LED billboards and advertising signs at ballparks and on the sides of buildings traditionally run their many LED drivers from 5-V buses. Equally traditionally, they have been terrible heat generators, partly from losses in the controllers themselves, and partly from simple conductive losses in each rail, due to the enormous amounts of current needed to support all the triple LEDs that constitute the LED video arrays.
This interesting fact came up because I asked Tony Armstrong, director of product marketing at Linear Technology, about one of the specifications on Linear’s LT3746 (see the figure). Developed expressly for next-generation scoreboards and signs, the chip integrates a 32-channel, 20-mA LED driver with a 55-V buck dc-dc controller.
The LT3746’s datasheet (see http://cds.linear.com/docs/Datasheet/3746f.pdf) discusses “individual 6-bit dot correction current adjustment and 12-bit grayscale PWM (pulse-width modulation) dimming.” But in examining the document, I was immediately drawn to the 57-V maximum rating for VIN. I asked if that was the top of the normal voltage range for a nominal 48-V power supply.
Armstrong agreed and said it was indeed the primary feature that led Linear to design the driver. He said that losses from low conversion efficiency and reliability problems created by excess heat were two of the biggest headaches faced by scoreboard and sign makers. Once system efficiency had been addressed by allowing the use of a much higher bus voltage, other features were added because of requirements related to evening out brightness differences among all the LED pixels in the scoreboard.
Globally, the LT3746 converts a higher input voltage to a single lower LED bus voltage, supplying 32 parallel LED strings with an adaptive-tracking-plus-pre-charging technique. Locally, the part regulates and modulates the current of each string to an independent dot correction and grayscale PWM dimming setting sent by a serial data interface.
The dot-correction and grayscale adjustment compensate for individual pixel LEDs that are unnaturally bright, compared to the rest of the display. The datasheet provides a detailed explanation, as it does for a patent-pending tracking-plus-pre-charging technique that improves both system efficiency and transient response.
These benefits were all necessary, but not sufficient, to build into the new driver. But the seed concept was as simple as reducing I2R losses by boosting the bust voltage and using a buck technology.