LED Driver Design Choices - Page 1
Forum section: electronics / projects

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: February 27, 2019, 02:20:22 pm
content: Assume I have 3-4 strings of series LEDs, and a DC source that has voltage above the highest Vf string.Why would I use a dedicated MCU to create the constant current LED driver versus a dedicated constant current buck IC from TI or other vendors? Easier to deal with LED shimmer, color temperature fluctuations over heat and time, etc.?

author: ajb
author_rank: Super Contributor
timestamp: February 27, 2019, 07:53:27 pm
content: Probably when this happens it's usually because an MCU is required anyway to handle other control functions, so if it can serve as the switching controller as well then it saves on having to have another IC. It also allows the control scheme to be arbitrarily sophisticated (within the MCU's processing capacity, anyway), and things like thermal fold-back or other operating regime limits are easier, you can do system health monitoring, etc.Controlling color temperature is only possible with more than one set of LEDs, but yeah, you can do things like that with an MCU. There are LED light bulbs that use multiple sets of LEDs to change the output spectrum as they're dimmed to emulate the red shift of incandescent bulbs. And if you need to dim the LEDs, then an MCU may give you better low-end dimming by adjusting to a different PWM scheme at the low end versus the high end of the dimming curve, or even doing hybrid analog/PWM dimming.So I guess the answer is either 'because the MCU is already there' or 'because plain switching controller can't do ____'.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 27, 2019, 08:50:16 pm
content: MCU-defined switch mode converters are great when:* You need a special case of converter, no IC available at all, or expensive* You need a surprisingly common case of a converter, but the IC designers have utterly failed to produce a sensible part, hence you need to Design-it-Yourself. For example, you need current limitingwithout entering an error stateeven though it's such a trivial and usual feature!* You need the MCU there anyway, and using an MCU capable of being the converter as well costs less than using an additinal IC* You have already designed a few, so you know you can do it, and don't want to take the risk that the commercial IC is revealed Broken-by-Design (does happen) - i.e., you trust yourself more than, for example, TI. If you don't know what you are doing, this is called "not invented here" syndrome. If you know, you are an expert. The former tends to lead to the second. Do it and you learn.* You don't know exactly what you need, so the flexibility through modifying software is appealing for you.Of course, in majority of cases, you don't need them. In my latest design, I have 9 standard switch mode converters using ICs, and one MCU-based, because only this one is such that there are no relevant IC options available (dual-phase synchronous buck 500W 48V to 25V CC-CV battery charger, in this case).What comes to LED drivers, the market is literally saturated with the control ICs; LED lighting has been a big market for over a decade now. Yet I'm 100% positive many of these fail in their intended job, and those that do work always leave some gaps and corner cases where you need to create your own.