I have a two wire communication channel, with a data line and a ground line, with the data line being pulled high via a resistor on the receiving end. I am supposed to send information on this line by connecting the data line to the ground. There are also other devices on the bus which may be transmitting at other times, so the data line is not always high. The data line normally has 5V, but I don't have a separate VCC output line.
I am allowed to leak 30 µA to the ground when I'm not sending and I need to sink atleast 15 mA to the ground when I am sending. Communicating on the line should be done by some isolated circuit.
A simple phototransistor output opto-coupler fits the bill perfectly. One combined with a suitably selected base resistor will reach 1-2 µS switching times.
However, there are a few things I do not like in this solution:
If I need to sink 15 mA and still stay fast, I need to use a lot of current to achieve that: opto couplers with a high CTR tend to be slower and requiring a high CTR is problematic if one wants to have equipment that will still work in 10 years without problems.
All the parts need to be selected just right to meet the specifications. If an input or output voltage is suddenly different (because a manufacturer does not adhere to specifications), it is likely that some resistor is too large or too small. It is hard to get wide enough margins for everything to deal with such events.
The switching time is adequate, but I'd love to get a solution that would be really fast. Getting under 1 µs would be great, 100 ns would be splendid!
So, I am wondering if there is some combination of simple components that would allow me to achieve faster, lower supply current and a more robust solution.
All the faster opto isolators tend to require the VCC input or simply do not work with open collector outputs. There are some with fast switching times and supply power requirements in the µA range, so I have been toying with the idea of just pulling VCC from the data line and keeping a capacitor to tide over the times some other device is pulling the line low. This would be kind of like 1-wire bus. However, such couplers would need to be coupled with a transistor or FET to achieve the needed sink current and I am guessing that might negate all my speed advantage and make the design difficult.
I also toyed with the idea of using ADuM5201, which provides isolated power as well as a high speed data link. That combined with a suitable FET might do the trick. However, such chips are somewhat complex beasts, requiring bypass capacitors, extended pads as heat sinks, large supply current at start up, and EM emission considerations.
So, I am wondering if anyone has a cool solution, maybe something with modern components as I believe most of the advanced opto coupler stuff was done when many things were a bit more primitive.
Thank you in advance.