Driving an optocoupler LED at different supply voltages

[Giaime]

with only 100mV to work with, 1.8V LDO

It seems the best solution to me so far, just lower the supply voltage to the minimum expected and calculate the resistor accordingly.

[langwadt]

with only 100mV to work with, 1.8V LDO

It seems the best solution to me so far, just lower the supply voltage to the minimum expected and calculate the resistor accordingly.

yeh, but come to think of it, it might be far too slow ..

[Zero999]

Do you see on what part of text I have reacted? Do you get it?

Not really. You've presented a bipolar solution that doesn't really help the OP (far too large voltage drop). Then you've referred to "unobtainium" JFETs, as if it is important to present a current within +/-1% to an optocoupler. After that you lost me.

The JFET solution is not precise, but will work down to almost 0 V voltage drop.

I'm not convinced. The JFET constant current based sources I've found require a few volts for devices with a current limit of 4mA.
http://njsemi.com/datasheets/J500%20-%20J511.pdf
https://my.centralsemi.com/get_document.php?cmp=1&mergetype=pd&mergepath=pd&pdf_id=1N5283-5314.PDF
http://www.mccsemi.com/up_pdf/CLD20B(DO-214AA).pdf
https://atcsemitec.co.uk/wp-content/uploads/2018/08/Semitec-CRDs-webpage.pdf

[Benta]

In my experience, the JFET works as a current source down to appr. 1...2 V drain-source (depends on current). Below that it tends to go resistive, which is not really an issue in this application.

[Kasper]

Generally I'd say current regualtor is ideal for driving LEDs but if you want the smallest and easiest solution, this could be one the rare occassions where zener voltage regulator is good.

Look up: EEVblog #908 - Zener Diodes

[Kalvin]

Use a small boost-converter with Vin 1.8V ... 4.5V and Vout 5V.

Something like this:
https://www.banggood.com/0_9-5V-To-5V-DC-DC-Step-Up-Power-Module-Boost-Converter-Board-1_5V-1_8V-2_5V-3V-3_3V-3_7V-4_2V-To-5V-p-1209841.html

[Yansi]

I am becoming lost. What would you use the boost converter for? I thought, there was not power supply available on the LED side? 

[spec]

UPDATE: I see Kalvin at reply #30 beat me too it

I have a project that involves a serial communication interface, that is isolated with a 6N136 optocoupler, at a relatively high speed, but well below the maximum of 1Mbaud for this optocoupler. The biggest problem is the varying supply voltage, between 1,8V and 4,5V. In that case it is impossible to choose a proper resistor for the LED, because the current will be either too low for low voltages or too high for high voltages. I experimentally determined the minimum LED current at around 4mA.

Several problems overlap in such a case, because the LED is driven at a relatively high frequency, and the minimum voltage is close to the forward drop of the LED (the Lite-on version of the 6N136 features a maximum Vf of 1.7V).

Does anyone have a simple idea of how to drive such an LED with a current that fits between the 4mA and 25mA with varying voltage? Some simple current source maybe?

Hi fenclu

I would suggest a simple low power switch mode PSU chip to generate, say 5V constant from your 1V8 to 4V5 varying supply, and then drive the LED from that supply via a resistor.

https://datasheets.maximintegrated.com/en/ds/MAX17220-MAX17225.pdf

https://www.digikey.co.uk/products/en/integrated-circuits-ics/pmic-voltage-regulators-dc-dc-switching-regulators/739?k=MAX17220&k=&pkeyword=MAX17220&sv=0&sf=0&FV=ffe002e3&quantity=1&ColumnSort=0&page=1&nstock=1&pageSize=25

PS: Have you had a look at the 6N137 family of optocouplers?

[Zero999]

I don't know if OPA with 1.8V min. Vdd exists, digikey listed from +-1V. May be you can convert supply voltage to known +2.5V - 5V and drive led as usual,  than search boost converter

Thing is, it's a bit complicated. This design is for a tiny module with an MCU on it, that is supposed to measure the battery voltage and send the data over an isolated data line. These modules will be made in big panels, so any additional cost is not desirable. Preferably i wanted to use discrete components.

I had an idea, see the attatched photo. The MCU would switch one side of a paralleled resistor based on the voltage it reads. However, I am afraid that when the R2 is not needed, the MCU would pull the output high, essentially disabling the LED.

• I would disagree about that circuit being complicated. Your requirements of 100mV drop-out, over a power supply voltage range of 1.8V to 4.5V aren't straightforward. It can be simplified by omitting the driver transistor, which probably isn't needed for only 4mA.
• The circuit you've suggested seems sensible, but the  MCU's IO pin will have to be set to an input or high impedance mode to avoid shorting the LED.

[Kalvin]

Alternative to the boost-converter idea: If your optocoupler could be driven off from a 1.8V minus 100 mV or so, you could use an RRIO op amp as a constant voltage source (read: 1.8V voltage regulator) to provide constant 1.8V for the LED with a proper series resistor. If your LED driver output is open drain or open collector type, you could drive the LED directly by the output pin. Just select the RRIO op amp which is able to provide enough current for LED even at the 1.8V. You can use a simple diode as voltage reference if the temperature drift is not an issue, and you do not have any other convenient voltage reference available.  CMOS outputs and MOSFETs can sink quite well down to 0V, but bipolar transistors will have 100mV - 200mV saturation voltage across collector and emitter.

[TrickyNekro]

How about using a 1.2V reference and a control transistor (either BJT or MOSFET) for the LED. Easy enough.

You get the stability of the reference and the the fast switching of the transistor. Most optocouplers their LED is IR based so dropout is at around 0.8V - 1V.
You can almost get away with using a BJT for switching. The collector - emitter saturation is around 0.2V. If you want to be sure you get to use a MOSFET like 2N7002.

And there are dirty cheap references out there, you need it mostly for regulation anyhow.

Edit: And there are parts that can work from 1.8V to 5V or maybe higher. Also you look for a voltage output reference not a shunt reference.

[SiliconWizard]

In my experience, the JFET works as a current source down to appr. 1...2 V drain-source (depends on current). Below that it tends to go resistive, which is not really an issue in this application.

@Zero999 and Benta: indeed. Using a JFET was my first thought as well, but then I quickly realized this would not work properly in the op's use case.

Beyond the variation of the current between JFETs of the same model, which may not be a big problem here (you'd just have to settle for a minimum current, the circuit would work fine for higher currents), you have to consider the characteristic curve of a JFET (Ids vs. Vds) @Vgs = 0 to see that below a certain Vds (which would typically be somewhere between 0.5V and 3V depending on the JFET), the current would drop a lot, defeating the purpose of driving the optocoupler's LED at a somewhat constant current regardless of the supply voltage. You could try selecting a JFET for a minimum acceptable current for the LED at Vds = 0.1V, but good luck finding a proper JFET for this while still limiting the max current to an acceptable level for the LED for the max. Vds (thus max. working voltage).

As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky...

[TrickyNekro]

As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky...

Do you have to use the 6N135/6? Cause I can find many parts that are much cheaper and have a lower forward voltage.
Example:
https://eu.mouser.com/Optoelectronics/Optocouplers-Photocouplers/_/N-6x5ji?Ns=Pricing%7c0&No=25&Rl=6x5jiZgjdhqbZ1yuoc3aZ1yuo3qbSGT

And... a more specific example:
https://eu.mouser.com/ProductDetail/Vishay-Semiconductors/VOH1016AB-T?qs=sGAEpiMZZMtd3yBnp8bAgLhQurE%252baQR29IOeeMf%252bGH1H6u1Y7D9ZWg%3d%3d

I am sure if you search a little bit you can find better parts than 6N135/6. These are quite mature parts, that alone makes them expensive.

[fenclu]

As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky...

Do you have to use the 6N135/6? Cause I can find many parts that are much cheaper and have a lower forward voltage.
Example:
https://eu.mouser.com/Optoelectronics/Optocouplers-Photocouplers/_/N-6x5ji?Ns=Pricing%7c0&No=25&Rl=6x5jiZgjdhqbZ1yuoc3aZ1yuo3qbSGT

And... a more specific example:
https://eu.mouser.com/ProductDetail/Vishay-Semiconductors/VOH1016AB-T?qs=sGAEpiMZZMtd3yBnp8bAgLhQurE%252baQR29IOeeMf%252bGH1H6u1Y7D9ZWg%3d%3d

I am sure if you search a little bit you can find better parts than 6N135/6. These are quite mature parts, that alone makes them expensive.

This one needs a supply voltage of at least 3V.

If you have spare digital pins on the uCPU, than pre-set 4-5 pins with different resistors value to a LED. Only one pin set as output at a time, others are inputs. Depends on the power source voltage selection of the appropriate resistor is made.

I think that MasterT's solution is the closest to what I had in mind. I will have to try that in practice and let you know how it went. Thank you everybody for your ideas  .

[Zero999]

Here's an implementation of MasterT's solution.

[voltsandjolts]

Maybe microcontroller can measure the supply voltage then use PWM to set LED drive current, along with on-off keying for the comms. Probably lowest BOM cost.

[TrickyNekro]

As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky...

Do you have to use the 6N135/6? Cause I can find many parts that are much cheaper and have a lower forward voltage.
Example:
https://eu.mouser.com/Optoelectronics/Optocouplers-Photocouplers/_/N-6x5ji?Ns=Pricing%7c0&No=25&Rl=6x5jiZgjdhqbZ1yuoc3aZ1yuo3qbSGT

And... a more specific example:
https://eu.mouser.com/ProductDetail/Vishay-Semiconductors/VOH1016AB-T?qs=sGAEpiMZZMtd3yBnp8bAgLhQurE%252baQR29IOeeMf%252bGH1H6u1Y7D9ZWg%3d%3d

I am sure if you search a little bit you can find better parts than 6N135/6. These are quite mature parts, that alone makes them expensive.

This one needs a supply voltage of at least 3V.

Hmmm yes, the isolated side, because you got a amp and gate there, there are others that use only a transistor.
Are you going to run both sides with the same power supply? I thought your problem was the LED!
How fast do you want it to be? Sorry, If I missed an earlier comment or something but something is not adding up.
Cause if you need an optocoupler with a low voltage switch LED there are solutions, as far as I remember you needed it for comms.

Edit: I´ve read your first comment again, the 3V is for the gate side not the LED side. Also if you want it for serial, your best bet is anyhow going with a gated design cause you can be sure that it works.
Or you want to have bidirectional comms? As far as I understood it, it´s for the LED side.

A gated design is also the best for both varying driving current and simplicity. Remember KISSing is always your safest bet.

[TrickyNekro]

And in case you did not understand it! Here! Page 3 of Datasheet, you look at the input characteristics!
Your microcontroller is going to be at the input side. As far as you told us anyhow.
Maximum Vf = 1.4V! You can run it down to 300uA in case it is a battery operated project.
Unless you are stuck with a bunch of 6N135/136 for whatever reason, there are many other optocouplers that are far superior for your use case, ready out of the box!