Fast Load transient LED driver for lightning simulation - Page 1
Forum section: electronics / projects

author: makebit
author_rank: Unknown
timestamp: October 11, 2025, 03:35:15 pm
content: Hi,We are designing a LED driver SMPS that will drivewhite LEDs in a highly transient manner such that weimitate lightning.So we have just made the feedback loop really fast by making the ITH pin capacitorvery small as shown.Do you agree this is the best way to service these transients most quickly and effectively?LTspice sim and PNG attached

author: MariuszD
author_rank: Regular Contributor
timestamp: October 11, 2025, 08:21:16 pm
content: Quote from: makebit on October 11, 2025, 03:35:15 pmThe loop is unstable, and the settling time is not short at all, for a system operating at 180kHz. Are you sure this was designed correctly?Quote from: makebit on October 11, 2025, 03:35:15 pmThe fastest regulator for LED is a linear current regulator, such a regulator can be powered by any switching power supply with a large capacitor.I ran the simulation, but I don't know what your intention was. Should the circuit work as a constant voltage source, or should it limit the current?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2025, 11:28:08 pm
content: Quote from: makebit on October 11, 2025, 03:35:15 pmNot really.Simpler is to drive the LEDs from your pulsed 4A current sink, and use a large energy storage cap to average the pulses.Then you have a simpler, standard power supply charge the cap with some inrush / max current control active.The power supply never sees the fast pulses, it just tries to keep the cap charged.

author: makebit
author_rank: Unknown
timestamp: October 12, 2025, 02:13:44 pm
content: Thanks, i realise that from an academic point of view it is a poor quality power supply design.But what it is seen to be doing on that simulation is exactly what we want it to do.So we are wondering is there any point in changing it?We need a fast response to the load transients, and thats what we get.Is there any reason why we should change it?We also need a low component count aswell...and preferably no big capacitors, or as few as possible.We dont see any harm that can be done by what's running in that simulation?...even though it is "academically" so poor.I originally had an external comparator aswell......which ran the LTC1871 in like a hysteresis mode.....but the boss told me to ditch the comparator so i did...and to be honest....what can be seen in the sim of the top post does work perfectly fine.I told my boss that it is academically poor, and he just said "so what, it does the job."

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 12, 2025, 02:31:23 pm
content: As MariuszD pointed out, the controller isn't stable. Should at least correct that, should be doable by just changing component values.And it's not even capable of sustaining a 5V output with such high load currents. Though it's not clear if this is a problem since the actual driver for the LEDs is not shown, and there's no clear specs on bandwidth or settling time or anything.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 12, 2025, 10:24:02 pm
content: Quote from: makebit on October 12, 2025, 02:13:44 pmDid anyone think to check the idle current ?Quote from: makebit on October 12, 2025, 02:13:44 pmIt's electrically poor too, but hey, it is his call. Maybe your customers do not care about battery size or operational times.

author: inse
author_rank: Super Contributor
timestamp: October 13, 2025, 05:15:39 am
content: When the design fulfills its requirements and you are not willing to change it, why did you ask in the first place?

author: MariuszD
author_rank: Regular Contributor
timestamp: October 13, 2025, 07:09:55 am
content: Quote from: makebit on October 12, 2025, 02:13:44 pmSmall oscillations in an unstable loop can become large oscillations with just a slight change in the coil or capacitor parameters.Quote from: makebit on October 12, 2025, 02:13:44 pmI didn't feel like designing seriously, but by changing a few capacitors, I improved the settling time from 400us to 70us. I can achieve a shorter settling time and a stable loop, by using a smaller output capacitor and larger compensation capacitors.If I were doing this as part of my job, I wouldn't risk releasing it to production without completing the design.Quote from: makebit on October 12, 2025, 02:13:44 pmThere's nothing wrong with a slow pulse response. But instability is a real threat.Quote from: makebit on October 12, 2025, 02:13:44 pmExplain to the boss what instability is.You asked which solution is the fastest. You got the answer that it's not fast. It's also not correct because the loop oscillates.If you provided specific requirements for how fast your system needs to be, the discussion wouldn't be "academic."

author: Zero999
author_rank: Super Contributor
timestamp: October 13, 2025, 08:02:46 am
content: Well 4A at 5V doesn't sound like much.lightning is very bright. Is just just fo visual effect or for a propper test?I've done a simiar thing in the past, for a photographic application and used much highr power pulses.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2025, 10:36:00 am
content: Caution.It is believed that makebit==Treez/Faringdon/ocset/zenerbjt etc etc

author: Terry Bites
author_rank: Super Contributor
timestamp: October 13, 2025, 10:51:30 am
content: Very interesting.https://www.semanticscholar.org/paper/High-Speed-LED-Driver-for-ns-Pulse-Switching-of-Halbritter-Jager/10f2f700b856e6751c25bada5435057f6174d333/figure/1