What's the maximum current at every output of this driver?
You are attempting to use the part outside of its specified range. The Voltage vs Current characteristic is a curve, but the manufacture has only given you 2 data points, what happens outside this range is not guaranteed. 

If you are a hobbyist, just figure it out by trial and error. If you are building a product to be mass produced, you should avoid using parts outside their specified range.



Driver output: 2.0 V @ 20 mA

Optocoupler input: 1.8 V max drop @ 10 mA

The differential is only 0.2 V, not good.

The optocoupler normal current input is 10 mA

R = 0.2 V / 0.01 A = 20 ohms

Two is the most you can drive without making assumptions about what happens at higher loads. Each should have its own resistor.

These calculations are only for the minimum case, the maximum voltage/current may take you outside of the recommended optocoupler current. You are fighting an uphill battle, reconsider your design.







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