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Driving an optocoupler LED at different supply voltages

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fenclu:
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?

MasterT:

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

fenclu:
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.

MasterT:
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.

schmitt trigger:
What MasterT said. The trick here is to disable the unused ports configuring them as Hi-Z inputs, such that they don't contribute or draw any current.

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