LT3755 LED buck driver troubleshooting (low, unsteady current, too high voltage) - Page 1
Forum section: electronics / projects

author: exscape
author_rank: Contributor
timestamp: December 01, 2020, 05:27:21 pm
content: About two years ago I built (with some help from this forum!) a custom Wi-Fi controlled LED driver/lamp which I've been using as my main light source in my bedroom since then -- it's been working great! Until recently.I've noticed that it flickers for the past few weeks or so, and I finally got around to start troubleshooting today.I'm not sure how much how info to share. The full PCB is fairly large and complex and I'm not sure how helpful it is to share images of it, but I certainly could if it may help.I could also share Altium files in case anyone is interested in looking at them.The important part of the circuit diagram is attached below.WHITE_DIM is a 0-1.3 V signal driven by a DAC and a voltage divider. Not shown are the DAC, the RTC circuit and the RGB dimmer/driver which should be entirely unrelated to this (and seems to work perfectly).LT3755 datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/LT3755-3755-1-3755-2.pdfThe LEDs are two Cree XHP70.2 6 volt LEDs in series. Datasheet:https://www.cree.com/led-components/media/documents/ds-XHP702.pdfI wrote down voltage and current readings for the LEDs in this circuit just two months ago for unrelated reasons. They looked like this for different settings/presets I've created:Preset: LED voltage, LED currentEvening: 10.41 V, 91 mAMid: 10.82 V, 391 mABright: 12.71 V, 2872 mAMax: 14.59 V, 4468 mAHowever, the measurements I just recorded look like this!Evening: 11.03 V, 91 mAMid: 13.09 V, 395 mABright: 18.56 V, 1545 mA, noticeably flickersMax: 18.54 V, 1545 mA, noticeably flickersSo voltages are up everywhere, and the current is limited and looks very unstable. It's hard to measure accurately on my entry-level Rigol scope (DS1102E) since the signal is only about 34 mV (over 22 mOhm). The measurements above are taken with a multimeter.It's absolutely worth pointing out that I noticed that the cooling fan had failed about two months ago, which led to the LEDs running extremely hot for an unknown time -- probably weeks or months. There was a fly stuck in the fan!The heatsink was about 85-90 degrees C on either "Bright" or "Max" (can't recall), and it's around 45 C when the very low-airflow fan is on.However, I don't think this failure occurred at that time, but rather a month or more after the cooling issue had been resolved. I didn't notice any issues at the time, and the "before" current measurements I listed above were taken a week AFTER I had resolved the fan failure (back in mid-October).So onto the main question:Is this a likely failure mode if the LEDs have been overheated for too long, or is it more likely this is unrelated and that (some component in) the driver is at fault? I don't have any other LEDs to test with, nor do I have an adjustable power supply, so I can't even test them that way.My thinking is that since the driver is a current source, it increases the output voltage until it reaches the expected voltage (100 mV for "Max") over its sense resistor, which never happens. Instead we get almost VOUT = VIN (VIN is about 19.6 V at load) which may cause regulation issues that would explain the flickering.Does that sound likely enough to order new LEDs? Any other troubleshooting suggestions to narrow the possible causes down?I have a decent multimeter and the Rigol DS1102E, but not a whole lot more TBH. No lab power supply as mentioned, and I don't know anyone nearby that is better equipped.

author: exscape
author_rank: Contributor
timestamp: December 04, 2020, 09:41:21 pm
content: Turns out the answer is "yes"! I ordered two new LEDs (XHP70.2, 4000 K (instead of 3000 K), 1485-1635 lumen each: XHP70B-00-0000-0D0HM440H) and went through the rather involved process of disassembly, desoldering, soldering and reassembly. Took me about 3.5-4 hours, but it worked!New readings for voltage, current and power:Evening 10.39 V / 94 mA, 0.98 WMid 10.73 V / 395 mA, 4.24 WBright 11.99 V / 2877 mA, 34.5 WMax 12.52 V / 4458 mA, 55.8 WAlmost a 10 W reduction for "max" which is nice, though I rarely use it since the difference vs "Bright" isn't that large.