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THVD8000: inductor total equivalent impedance question

Tess Chen
Intellectual 2675 points

Part Number: THVD8000
Other Parts Discussed in Thread: THS6222

Hi Team,

The parallel aggregate impedance should be selected so that the total equivalent impedance at the carrier frequency is Z ≥ 375 Ω. This assumes RS-485 loading with 60 Ω termination. If no termination is used in the application, then the total equivalent impedance at the carrier frequency could be reduced to Z ≥ 60 Ω.

Question:

Why the equivalent impedance should bigger than  375ohm with 60 ohm termination RL? How does the 375ohmn comes out? Thanks.

  • 0
    TI__Guru 54042 points

    Hi Tess,

    In the RS-485 standard it defines the bus to support 32 "unit loads" which in practice are other transceivers. The 32 units loads are in parallel with a minimum input impedance of 12K as defined by the RS-485 standard (some devices can surpass the min of 12K but that is the absolute minimum for RS-485 compliant devices) - 32 12K impedances in parallel equals out to 375 Ohms - which means that RS-485 compliant drivers can have a minimum impedance to ground of 375 Ohms on the A and B lines. Since the THVD8000 equations assume the inductors are AC grounded the THVD8000 driver sees the inductances as to "Ground". That is where the 375 Ohm number comes from. Its a slight approximation due to it neglecting input impedance of the THVD8000 - but with only a couple of nodes they can largely be ignored - however if more THVD8000 nodes exist in the system the more you need to account for its contributions as well. If you go below 375 Ohms you risk attenuating the communication signal or possibly overstressing the driver. 

    There are ways to drive communication over low impedance power lines (without the 375 Ohm requirement) but that would include the addition of a line driver like the THS6222 to source the extra current. We have an application note that should be published onto TI.com within the next 2-3 weeks with information on these types of implementations if the 375 Ohm limit is  too large for your application. 

    Please let me know if you have any other questions and if you want I can send you the the link when the new application report comes out!

    Best,

    Parker Dodson