Using an inverter w/ an LED driver to control a high power CC LED driver? - Page 1
Forum section: electronics / projects

author: Starlord
author_rank: Frequent Contributor
timestamp: September 08, 2015, 08:47:39 am
content: Okay, so here's what I'm trying to do...I want to drive some high power LEDs. To do this, I have selected the following constant current LED driver:http://www.diodes.com/_files/datasheets/BCR420UW6_BCR421UW6.pdfI also want to be able to adjust the brightness of these LEDs. But I don't want to PWM them directly with my microcontroller pins. I want to control them at a distance using I2C.I looked around for an I2C I/O expanders which could do PWM and I didn't find much.The only thing I found that might work (because it defaults to low, not high when it powers up) was this, but they're $2 each, there's only a few thousand available so there's a danger of them going out of stock, and the chip's rather large since it has 16 outputs: (I only need like 5 or 6 for the high powered LEDs)http://www.nxp.com/documents/data_sheet/PCA9685.pdfBut I had an idea. I have some other 5mm LEDs in the same location I also want to animate along with the high power LEDs. And I only need to drive like 7 of those. So maybe I could use the same 16 LED constant current driver I will be using elsewhere to drive the 7 5mm leds and provide a PWM signal to the 6 high powered led drivers?http://www.ti.com/lit/ds/symlink/tlc59116.pdfThere's just one problem though. On power up, this driver will go high. That would turn on my high power LED drivers. I could turn them off right away, but I don't want those to flash at all when I apply power initially.Then I thought, what if I use an inverter? And I found this inverter array with six outputs, which can push and pull 12ma:http://www.ti.com/lit/ds/symlink/sn74lv14a.pdfSo my question is, do you think this will work? My main concerns here are:1. Whether the inverter can supply sufficient current to the transistor. I assume the answer is yes.2. Whether the inverter can handle the frequency at which the LED driver will be doing PWM. (I think this is 25Mhz?)3. Whether the transistor in the high power LED driver can handle the frequency at which the LED driver will be doing PWM at that 12mA drive current.

author: ajb
author_rank: Super Contributor
timestamp: September 08, 2015, 02:41:20 pm
content: Quote from: Starlord on September 08, 2015, 08:47:39 am1. Page 6 of the LED driver datasheet gives you an Ien vs Ven chart, it takes bugger all current at 5V.2. The datasheet for the IO expander says 97kHz, which any logic family you can buy will be able to handle no problem.3. The second paragraph of the LED driver datsheet says:QuoteYou can probably get around that last problem by using an IO expander with either a programmable frequency (not sure what options you have there), or higher resolution PWM (higher resolution means the PWM clock gets divided further, meaning a lower PWM frequency), or an external clock.

author: Starlord
author_rank: Frequent Contributor
timestamp: September 09, 2015, 03:34:04 am
content: Ah, damn I missed that bit about the 10KHz. :/ (Well I probably saw it at some point but I've been working on this for months.)Well that's not good. I don't think that IO expander/LED driver has adjustable PWM. But I'll look into it I guess.