Lumissil IS31FL3235A LED Driver problem - red and greens don't fully turn off - Page 1
Forum section: electronics / projects

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 12:24:31 am
content: I'm grasping at straws, thought I'd do a few searches on the forums. Didn't find much.I'm breathing life into a new board I designed. It has the Lumissil part IS31FL3235A on it. I've got 100% control over the part - I can control LED pwm etc. The problem: the green and red leds won't turn fully off. They just get dim when I turn them off. Blue and white are fine, which I suspect is because of their greater Vf. I've probed around the board and I'm not sure what's going on with this part. I replaced it on the board with no change, so either I have some odd code bug (though can't see how that could be) or a hardware issue.The "dim" leds are not being PWM'ed when off. When LED is on, PWM duty is as expected.Thanks in advance.

author: free_electron
author_rank: Super Contributor
timestamp: June 11, 2022, 12:49:41 am
content: whats the setting for the SL registers on those led's ?red and green does not need a whole lot of current to turn on...

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 01:00:20 am
content: Imax. With the 3.3k resistor dangling off the R_ext pin, that Imax is 23mA.I measured the current over the series resistor on one of those leds and it was about 0.3mA.

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 01:03:17 am
content: Another datapoint: For a test, I removed all related code except: raise the enable pin (sdb) to high, then set the shutdown register to 1 (which enables the part). I did not write any of the other registers (pwm, current and on/off, etc. ).All the red/green leds turn on, dim.

author: james_s
author_rank: Super Contributor
timestamp: June 11, 2022, 01:06:21 am
content: Are all the LEDs of a given color the same brightness? I'm grasping at straws here but is it possible there's some electrical leakage like from flux or something on the PCB? Modern LEDs glow visibly at freakishly low currents, the kitchen light at my friend's place has the switch (incorrectly) on the neutral currently and the LED fixture glows noticeably when turned off.

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 01:31:46 am
content: Correct, all the same colors are lit at the same brightness.I tried changing the series resistor on one of the row of green leds, and it's brighter than the others. See arrow.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2022, 04:03:02 am
content: What voltage do you measure at say OUT3 or OUT4 vs OUT5 or OUT6? Is the LED drive +5V higher than the VCCat pin 31? If so, even by a little bit, can you put a diode like a 1N4001 in series with the +5V LED drive?

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 04:16:28 am
content: With all the LEDs in off mode (but some are dim of course), the voltage at the out pins varies between 2.7 and 3.0. It's a consistent voltage for each LED color, but different between colors (due to the varying Vf's I'd assume).The 5V supply is the same on those 5V pins.I can test your diode idea by replacing one of the series leds with a 0603 diode. I suspect it will work, since the blue and white leds are working correctly and have higher Vf. Still, why would this be? Datasheet for the part is not as clear as some on how the innards of the part work. I'd think there would be a simple nfet to turn off the led.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2022, 04:57:38 am
content: Quote from: passedpawn on June 11, 2022, 04:16:28 amI agree, and if you are sure you are correctly writing the D0 (OUT) bit to zero (from Table 5 of the datasheet) then I really don't know what the issue is. But I've no actual experience with this chip, so maybe it is something obvious that is getting overlooked.

author: free_electron
author_rank: Super Contributor
timestamp: June 11, 2022, 05:46:05 am
content: 0.3mA is enough to make an 0603 led glow ... especially red and green ones. driving those guys with 23mA is waaaaay too much. they are full brightness with 1 to 2 mA. i would drop that led current. and program SL for those channels to 1/4 setting

author: David_AVD
author_rank: Super Contributor
timestamp: June 11, 2022, 05:57:40 am
content: That is odd. The specs say 0.3uA max leakage on the outputs so that shouldn't be it.Do you have the required caps for the 5V supply close to the chip?

author: thinkfat
author_rank: Unknown
timestamp: June 11, 2022, 07:11:32 am
content: All the Lumissil drivers typically have an anti-ghosting function. See if you can enable it. That might fix your problem.EDIT: I just noticed you're not using a matrix driver. "Ghosting" therefore cannot be the reason for what you experience. I'm sorry for giving a wrong clue.

author: thinkfat
author_rank: Unknown
timestamp: June 11, 2022, 08:49:40 am
content: Okay, just from the description and the schematic I cannot see if you're doing anything wrong. Maybe we have to start looking for non-obvious things.First of all: Are you sure that the outputs are not driven? They should not, obviously, but have you checked with an oscilloscope? Lacking that - if the PWM frequency is set to 3kHz, maybe you can probe the outputs with a soundcard input?The leakage current of the outputs is specified as 0.2µA max. If the chip is hot, this might make it leak more. How's the thermal design?One more thing to check: Is there any voltage difference between the chip VCC and LED power input?

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 01:54:48 pm
content: Solved.Turns out it was (I think) a soldering issue. I probed around the chip (not so easy due to pkg) and measured 3V on pin 31/VCC - should have been 5V. There indeed was 5V on the cap right next to it. Not sure what was going on there, but I swiped some solder paste across those pads and it works perfectly now. I had put the part on twice and apparently had same problem both times, gah!Which means, the part was likely getting power through the diode output pins, which would have been about 3V.Thanks everyone for the helpful comments, and sorry for wasting your time!

author: thinkfat
author_rank: Unknown
timestamp: June 11, 2022, 02:26:38 pm
content: That was something I experienced myself. Those chips are really tiny, only 4mm x 4mm and with 0.4mm pad pitch. Also, to be thermally stable you have to connect them to a huge copper surface, with lots of vias through the center pad. They are a pain to solder by hand, and even with an oven it's not trivial, you have to follow the profile closely. I remember that I reflowed one chip 5 times until all pads connected properly.Well done, good to see you sorted it out.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2022, 03:01:10 pm
content: Quote from: passedpawn on June 11, 2022, 01:54:48 pmNot a waste at all, especially compared to some other ongoing threads.I don't know the specific internals of this device, but many devices with output arrays have 'freewheeling' diodes that allow any current spikes from turnoff to pass through to VCC. This means they can backfeed as you observed.

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 05:15:52 pm
content: Quote from: bdunham7 on June 11, 2022, 03:01:10 pmRight, protection diodes. Couple of interesting anecdotes regarding those:I met a dude who would power processors this way, through the protection diodes. Those diodes are limited to some small current, but it was enough to power the thing. The purpose was he only had 2 wires going to the board, and that needed to carry pwr, gnd, and data. Don't recall exactly how it worked, but you can imagine.Also, I worked with an RF module that had uart signals into it. To absolutely reduce the off-mode power, I removed power to the module when not in use. But, the device continued running by sucking power off the uart signals, presumably through those protection diodes. I had to add code to change those pins when I went into low power mode.

author: passedpawn
author_rank: Contributor
timestamp: June 11, 2022, 05:18:10 pm
content: Quote from: thinkfat on June 11, 2022, 02:26:38 pmYep. I've got a sweet Saleae protocol analyzer - great for verifying that what you THINK the code is doing, is actually what's getting to the target. I don't know how I lived without that thing for so long.The footprint for this thing was a little nutty. I got it right, but it wasn't trivial.

author: james_s
author_rank: Super Contributor
timestamp: June 11, 2022, 05:54:59 pm
content: Glad you were able to get it working, I didn't suspect a soldering problem because you saw the exact same symptom both times, I'd have thought you'd be likely to get different bad connections. I don't consider it a waste of time, I like these threads that end in a solved mystery.