Using RDSON to current limit LEDs - Page 1
Forum section: electronics / projects

author: Bradlez
author_rank: Contributor
timestamp: July 21, 2021, 05:19:39 am
content: I've developed a bit of a possibly ridiculous idea to use the RDSON of a MOSFET to current limit an LED. Normally to switch an LED, you would need a resistor in series to satisfy the forward current of the LED. As my LED choice is high current (around 100mA) and I'm running under a 3.3V power supply, I've calculated my required resistor to be about 3 ohms. Normally this means I would just simply put the LED in series with the resistor and a MOSFET to control it. However, is it possible to use a MOSFET with a fairly poor RDSON (around 3 ohms) and completely omit the extra resistor? I've checked all the curves including the LED forward voltage curve at different currents, the RDSON curve at different temperatures and voltages and found them to all be within the acceptable driving range for my LEDs. How sure can I be when trusting these curves? Will they be consistent with real life and will they degrade significantly over time? The point of this is to reduce my BOM cost on the PCB and the need to solder an extra resistor.I'm not sure if I can really call this an open-loop circuit design as I am saturating the MOSFET.

author: mariush
author_rank: Super Contributor
timestamp: July 21, 2021, 06:13:48 am
content: You could probably find one, but a npn transistor would probably be easier to set up (though you'd still need a resistor to limit the current through the base of the transistor)I don't get how a mosfet would be cheaper than a resistor. 3.3 ohm is an E value so such resistors would be super cheap and common and would also allow for some room for the led ... you can run it at 90..95 mA and still have a very bright led and it will probably have a longer life.There's also linear drivers that are like diodes, you simply get one preset at 100mA and put it in series with the led, and it will limit current to 100mA. But these usually have a higher voltage drop, usually around 0.5v or more.

author: brabus
author_rank: Frequent Contributor
timestamp: July 21, 2021, 06:44:43 am
content: The datasheet tolerance and temperature coefficient make RDson a pretty shitty resistor. I would never rely on the declared value: the circuit may work now and fry the LED in winter.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2021, 06:57:06 am
content: There is quite some scattering in the forward voltage of the LEDs. The LEDs itself may or may not have a significant internal series resistor. So with some LEDs it may actually work. With other the negative TC of the LED voltage may cause trouble: when the LED gets warm the current would go up and causer thermal run-away.A BJT with a limited base current may actually be the easy solution, though it is also not ideal: the gain usually goes up with temperature. So the current is not very stable.

author: Bradlez
author_rank: Contributor
timestamp: July 21, 2021, 07:10:46 am
content: Quote from: brabus on July 21, 2021, 06:44:43 amI was worried about this and did think about it. My LED does have a massive range of current tolerance (65mA all the way to 240mA) which can be seen in the JB3030-3V datasheethttps://cree-led.com/media/documents/data-sheet-JSeries-3030.pdfand the MOSFET I chose was NX3008PBKWhttps://assets.nexperia.com/documents/data-sheet/NX3008PBKW.pdf. To the other guy, I guess it doesn't really affect BOM cost that much but I am pretty tight on space on my PCB.

author: Bradlez
author_rank: Contributor
timestamp: July 21, 2021, 07:13:31 am
content: Quote from: mariush on July 21, 2021, 06:13:48 amI actually considered those diode looking things. However the dropout voltage was too high for me to use them (I'm using a 3.3V supply to drive 2.9V LEDs). I wish there was a 3 legged one that had a VDD, VSS, and LED pin so they wouldn't have to enforce such dropout limitation. I probably should have mentioned I was tight on PCB space and didn't want to solder an extra resistor per LED haha.

author: Berni
author_rank: Super Contributor
timestamp: July 21, 2021, 07:26:14 am
content: Don't rely on that RDSOn for that since it changes a lot with temperature. Also with such low resistances the characteristic curve of the LED will have more effect on the current than the resistor. This characteristic curve of a LED is the reason why you can often just stick a blue LED across a 3.3V supply rail and it works fine. The forward voltage of the LED climbs enough as the current increase it to limit it to some point.So for your particular case it probably doesn't matter if you use a resistor or not. Since such a low resistance won't do much. I tend to just put something along the lines of 20 to 200 Ohms in front of blue leds on 3V3, since often these blue leds are way too bright anyway, so i tend to run them far below the rated max current, this brings the forward voltage of the leds somewhere closer to 3V, so the resistor ends up with a reasonably high value.

author: magic
author_rank: Super Contributor
timestamp: July 21, 2021, 10:18:08 am
content: A current mirror thingy? I'm pretty sure you can get SOT23-6 duals with guaranteed Vbe offset less than a few mV.The three bands at 100mA ±15mA are the three values ofdt(die temperature mismatch) at decent Vce. The three slightly different lines near 100mA are for 300mV at the collector. 75mA is for 200mV at the collector.R2 reduces sensitivity to thermal mismatch but increases dropout voltage. Have fun.Someone somewhere could be making this as an IC with no external resistors and much improved permormance

author: Siwastaja
author_rank: Super Contributor
timestamp: July 21, 2021, 11:11:56 am
content: The only chances to get it work reliably is to properly thermally couple the LED and the MOSFET. This way, as higher current flows, LED heats up, heats the MOSFET, which significantly increases Rds_on, regulating current. It's a hack but for the sake of interest, could be workable.

author: Bradlez
author_rank: Contributor
timestamp: July 21, 2021, 11:40:04 am
content: Quote from: Siwastaja on July 21, 2021, 11:11:56 amI actually also did think about this but I didn't know if it made much difference since the only way I can currently couple them is by connecting them together, which is already how it's designed. The PCB is already out for manufacturing even before I could make this post (I was short on time) so there's not much I can do. I just wanted to check if I made a fatal mistake. I probably should've chose a low RDSON MOSFET and put an extra resistor to save all the hassle. The LEDs are dirt cheap anyways so I'm not too worried if they blow up.

author: Bradlez
author_rank: Contributor
timestamp: July 21, 2021, 11:54:46 am
content: Quote from: magic on July 21, 2021, 10:18:08 amNow that was one IC I really was looking for and I did find some current source ICs but they were just current mirrors and you still had to do the rest yourself. I recently just found a MAXIM IC that seem to work really neatly and do multiple LEDs at once but that could only do up to 60mA. There's another Maxim IC that does much higher currents but the IC takes up way more space. Maxim chips are also unreasonably expensive in general.

author: magic
author_rank: Super Contributor
timestamp: July 21, 2021, 01:23:04 pm
content: Quote from: Bradlez on July 21, 2021, 11:54:46 amI'm curious, do you remember any part numbers of those current mirror ICs?The sort of linear LED drivers that I have seen before use the simplest diode-diode-resistor-transistor current source with >0.7V dropout voltage. They are unusable for high Vf LEDs on 3.3V supply.

author: Bradlez
author_rank: Contributor
timestamp: July 21, 2021, 03:41:17 pm
content: http://ww1.microchip.com/downloads/en/DeviceDoc/CL520_CL525-Fixed-Constant-Current-Linear-LED-Driver-Data-Sheet-20005805A.pdfSO CLOSE. If this was high sided and could draw more current, this would've been perfect.

author: Bradlez
author_rank: Contributor
timestamp: July 21, 2021, 03:44:34 pm
content: Quote from: magic on July 21, 2021, 01:23:04 pmhttps://www.infineon.com/dgdl/Infineon-BCR402-DataSheet-v02_02-EN.pdf?fileId=5546d4626102d35a01617524e8f40619

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 21, 2021, 04:04:45 pm
content: Quote from: brabus on July 21, 2021, 06:44:43 amYes. Although it's absolutely possible, RDSon of MOSFETs have wide dispersion and are very sensitive to temperature. So definitely not a robust solution.The most common way of circumventing this is to add a feedback loop as a means of regulation. And you end up with a current sink. That's a couple additional components.

author: magic
author_rank: Super Contributor
timestamp: July 21, 2021, 04:17:37 pm
content: Quote from: Bradlez on July 21, 2021, 03:41:17 pmQuote from: Bradlez on July 21, 2021, 03:44:34 pmLOL, no.Both of those chips are exactly the kind of rubbish you will find everywhere. With Microchip you even have it spelled out right on the first page:Quote

author: Gribo
author_rank: Frequent Contributor
timestamp: July 21, 2021, 07:21:42 pm
content: There is the AL5809 from Diodes Inc, it can't get any simpler than that.

author: magic
author_rank: Super Contributor
timestamp: July 21, 2021, 08:00:01 pm
content: QuoteAnother unsuitable for 3.3V supplies, thenUnless somebody invented a current regulator capable of running with VCC=200mV. That would be impressive and by all means

author: Berni
author_rank: Super Contributor
timestamp: July 22, 2021, 05:15:57 am
content: Yeah LED drivers with this little dropout are rare.But you could certainly build your own constant current sink that does this. Get a good low RDSon mosfet, give it a shunt to GND of say 0.1 Ohm, then drive the MOSFETs gate with a opamp (low input offset, rail to rail in/out) according to the voltage across the shunt. This solution is quite expensive and high part count for just simply driving a LED. But it will fit the required 200mV dropout easily while delivering a rock solid stable current.Just a question of how stable you really need to have the LED current.Then again if you have a 3V3 rail that usually means you also have a higher voltage rail somewhere else. Usually low voltage rails like 3V3 are regulated locally from some higher voltage rail that comes in. 5V tends to be about as lowest as you commonly find sent across cables for power.