# Sizing the input current of a isolated standalone MOSFET gate driver for a stable output

After a suggestion from my previous question I'm planning to use this Photovoltaic isolated MOSFET gate driver to trigger a trigger input of this DAQ. It seems these devices do not require an auxiliary power supply. Below V1 is a 33Hz 0-5V pulse train input to the gate driver. Rtr is the input impedance of the trigger input of the DAQ where I guess it is at least 10Meg:

Digital input limitations of the DAQ is given as follows:

And here is the gate driver input current output voltage curve:

How can I size the input current hence R1 so that the trigger receives guarantee always between 1V to 5V input? The plot doesn't have much detail for 5V output.

The datasheet shows that the forward LED voltage drop is 1.4V at 5mA. In my case from Figure 3. it will be around 1.1V assuming I need 1mA input current.

So for 1mA R1 = (5V - 1.1V) / 1mA then R1 = 4k. But if I set the input current to 1mA; between 0 to 5mA Figure 1. curve is very steep which doesn't look stable. How can I size the resistor and current in that case for a more stable output around between 3V to 5V? A voltage divider at the output? I have no experience with this device.

edit:

• I don't think you want a stable output, other than a strong logic "0" or "1", correct? Use the circuit as is. – analogsystemsrf Dec 4 '18 at 8:38
• yes but my fear is, what if the input current 1mA varies a bit and results an output voltage of 6V. that would exceed 5.5V max rating. and if i set the input current too low like 0.5mA now a little variation might set the output voltage less than 0.8V making a false logic zero. the curve is very steep there. to make it stable the input current must be much higher but then the output exceeds 7V. that's my dilemma. – atmnt Dec 4 '18 at 8:45
• or should I use a 5V zener at the output with a series resistor? – atmnt Dec 4 '18 at 8:56
• Are you positive that the input impedance is 10M or is just a guess? The input is specified as TTL which is far under 10M. en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic . Use a voltmeter , if the input is TTL you should have some voltage on it. – Dorian Dec 4 '18 at 10:13
• Dorian youn are right I didnt think of it, and actually I also have to add that I now realized I need to send this to six TTL input in parallel after the MOSFET driver, that will even make the resultant input impedance 6 times less. In that case what current you recommend for a stable high logic without exceedin max rating 5.5V. Or can I use a 5V zener? – atmnt Dec 4 '18 at 10:19