Dear all!
First I want to say that THVD8000 is such a great gift for us!
As our team was developing a solution for polarity-free power over data bus, using standard RS485 transceiver with external circuitry doing exactly OOK modulation/demodulation.
Then comes the THVD8000!! Like a miracle!
We'd like to deploy THVD8000 in many application, replacing traditional RS485 bus and SLC-like bus.
However, we have some question concerning the maximal number of nodes on the bus.
I am aware of the following documents:
Datasheet : https://www.ti.com/lit/gpn/thvd8000
Design Guide: https://www.ti.com/lit/pdf/slla496
And EVM User Guide : https://www.ti.com/lit/pdf/sllu140
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According to the DS or UG (also as standard RS485), suppose we are using 120Ohm termination, then the remaining bus loading have to be greater than 375Ohm.
Refer to Figure 4 in DS, or simply by calculation, if we choose the carrier frequency to be 1MHz, and the required number of nodes are 200, then the inductor would be 12mH!
( or more precisely, 375Ohm * 200 / (2PI*1MHz) = 11.9mH )
This size of inductor seems hopeless, taking for example
Bourns RLB-series inductor : https://www.bourns.com/docs/Product-Datasheets/rlb.pdf
RLB1314-103KL has 10mH, but the SRF is only 0.53MHz, the its inductance would reduced dramatically when operating in 1MHz.
Same problem goes if we maximize the carrier frequency to 5MHz:
Even though the inductor size can be shrink to 2.4mH, but it seems even more difficult to get an inductor with that inductance and have sufficient SRF.
Our goal is to reach the maximal number of nodes up to 150!
I am thinking...
What if we mitigate bus loading requirement on inductor by eliminating the terminate resistors.
Say we leave the bus un-terminated, then in 1MHz carrier frequency, 200 units, the inductance would be 54Ohm * 200 / (2PI*1MHz) =1.7mH.
Similarly, 340uH for 5MHz carrier frequency with 200 units. This inductance looks promising when choosing inductors, right?
But here comes the problem of signal integrity, mainly the reflection issues.
I've never designed a un-terminated RS485, it's kind of something unpredictable for me.
However, as the carrier frequency is very very fast comparing to a single data bit, is it feasible as long as the reflection dies out within a small amount of a bit time.
Do I understand this correctly?
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To recap my question:
1. If using termination, and number of units is up to 150. Is it simply not practical to choose a huge inductor to accomplish power over RS485?
2. If NOT using termination, is there anything we can do about the reflection issues? Simply reduce the data rate, say 1:100 or 1:1000?
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Thank you guys for reading this post, and thank you again for all the great products offering to the world!
Zt.
