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Driving an optocoupler LED at different supply voltages

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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?  :o

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


--- Quote from: fenclu on January 16, 2019, 01:41:41 pm ---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?

--- End quote ---
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:

--- Quote from: MasterT on January 16, 2019, 02:11:13 pm ---
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

--- End quote ---

--- Quote from: fenclu on January 16, 2019, 02:17:53 pm ---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.

--- End quote ---

* 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.

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