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THVD8000: Power over RS485

sajul babu
Prodigy 85 points
Part Number: THVD8000

Hi ,

  I am planning to use THVD800 for my new power over RS485 design, I need help with following things

1. Need a Pspice for TI  model for THVD8000 part.

2. Is there a Maximum DC voltage  current rating for A & B lines using this application.

3. How to calculate the Inductor value when there is a possibility to connect multiple devices on the bus (for example design the product to have 3 slaves connected to one master and circuit works fine with 1,2or 3 devices connected  while still rated for say 80 mA load per device), do we have an application note around this.

4. How to do wiring between master and slaves (star/bus).

5. Can you assign any FAE for my case so that I can get the circuit reviewed 

I have already done proof of concept with a point to point Master-slave config at data baud rate of 9600 and carrier frq of 115200 which works well for 24V DC supplying 80mA over a 100ft cable.

  • 0
    TI__Genius 15345 points

    Hello,


    An engineer has been assigned to this thread and will respond to you shortly. Please allow some time for the Holiday season.

    Best,

    Chris

  • 0 in reply to sajul babu
    TI__Guru 74252 points

    Sajul,

    Thanks for your continued patience on this, please expect a response in the new year.

    Regards,

    Eric Hackett

  • 0
    TI__Guru 52172 points

    Hi Sajul,

    I apologize for the delay while I was out of office.

    1. We don't have a full model - but we do have a simplified PSpice library (can import into any spice software that except HSpice I believe) . 

     THVD8000.lib

    Some notes:

    1a) Its a 5V only model at this time

    1b) it doesn't model propagation delays or mode switch delays (there is still some inherent delay in the model - but it is most likely shorter than reality) 

    1c) it doesn't model Duty Cycle Distortion, modulation frequency variance, or spread spectrum clocking

    1d) It does model device functionality,  electrical specifications of device,  and rise/fall times of inputs and outputs in a typical condition use case within +/-5% for most specifications (common mode voltage may go to around +/-8% at extreme conditions)  - it does not include max/min quantities at this time. 

    2. IF you asking specifically for data nodes A/B lines - yes there is and its small. Recommended max current is +/-60mA  (it can go higher but we don't directly spec a max current into the device). But the input impedance for A and B is a minimum of 96K - so even a small current can cause a large voltage spike on the pins causing damage. So using a surge or tvs diode (depending on current needs) is strongly recommended for power over RS-485.

    If you are asking about the shared bus (power and data) or the power node - this current is limited by the inductor that couples power onto the the shared node - where the inductor value is limited by the common mode loading standard (common mode load needs to be >= 375 Ohms including coupling inductance) . If the current you need is too large for any practical inductor (size or cost become too much) then a high voltage/low impedance approach needs to be taken - https://www.ti.com.cn/cn/lit/an/slla590/slla590.pdf?ts=1672857642272 - please note that this design says AC outlet but a similar approach was used with a 600V DC line with an equivalent of 1 Ohm load. Coupling happens via a transformer and high voltage capacitor with an additional line driver to accommodate the lower load impedance.  

    3. You calculate the inductor value based off of the largest number of nodes. So if you have 1, 2, or 3 nodes on the system you design for 3 nodes as if you design for 2 nodes you will violate the common mode loading standard but having more inductance than needed is okay. This is explained well in our THVD8000 design guide here:https://www.ti.com/lit/an/slla496a/slla496a.pdf?ts=1672857956709&ref_url=https%253A%252F%252Fwww.google.com%252F

    4. Daisy Chain is preferred or a spine with junction boxes is second. It is possible to do a star configuration with the THVD8000 - but it is a bit more complicated - I have added an app note if customer prefers star network: https://www.ti.com/lit/an/slla573/slla573.pdf?ts=1672858044306&ref_url=https%253A%252F%252Fwww.google.com%252F

    5. I am probably currently the best person to help you with the THVD8000 as I am the AE on the part and we typically have a bit more part level understanding as we each individually cover less parts than FAE's do, so we have more time to focus on our devices. So I would be more than happy to help review your circuit. I can review it here on the forum or I can send you my email and we can review offline if you'd feel more comfortable doing that. 

    Best,

    Parker Dodson