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THVD4431: Consommation THVD4431

Rob
Prodigy 60 points
Part Number: THVD4431
Other Parts Discussed in Thread: ISOW7742

Tool/software:

Hello,

What maximum current does the THVD4431 consume at 5V (worst case)?

I saw 75 mA at 5V. Correct?

I would like to power the THVD4431 with the DC/DC output of the ISOW7742 component (isolated circuit).

Thanks in advance,

Rob

  • 0
    Guru 265440 points

    75 mA sounds about correct (for RS-485 with full termination), but I do not find it in the datasheet. Where did you see it?

    The ISOW7742 can supply 110 mA, so this should work.

  • +1
    TI__Mastermind 45987 points

    Hi Rob,

    So the 75mA that you see - I am assuming you are referring to figure 5-14 where at max RS-485 data rate at 5V supply its right around 75mA - is not a worst case graph and in complete transparency that is the result of 1 IC at room temp - so variations are possible. Its a good approximation of a typical use case at room temperature but it shouldn't be read as "guaranteed". 

    With that being said there are a few concerns I have with this supply. 

    1. During RS-485 bus short fault  - the current limit is at +/-250mA  - so that could cause the power line to crash and depending on system setup it could lead to damage downstream. This is the main concern because that 250mA can be sourced from supply. 

    2. The graph doesn't account for shifts in temperature so 75mA is not the worst case you can see during normal operation. 

    3. The graph is not representative of a fully loaded bus - i.e. there is no common mode impedance in the test - only the differential so its not testing the current that is needed to feed the common mode impedances as seen in real system (i.e. the current the goes into the receivers essentially and not the current over the termination resistors). In most cases this is much smaller than the differential current - but its not zero and can have an impact on the power supply if there isn't much room to work with. 

    During normal operation I am not too concerned as there is a decent amount of margin above the 75mA to account for temperature variations and bus loading - but during fault conditions on the bus it could lead to negative impacts at a system level as it could cause the power supply to crash if the current requested is too much for supply to source - that is my major concern. 

    I'd honestly try to pick a device that will be able to supply more current (this is just general to RS-485 devices in general as the 250mA is part of the standard) because I'd be worried about fault scenario performance in your desired setup - if you have any questions please let me know and I will see what I can do!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 60 points

    Yes, it is in Figure 5-14.

    Ok, thanks for the explanations.
    I will choose a power supply capable of providing this current (RS-485 bus short fault).
    A 1.5W power supply should be sufficient.

  • +1 in reply to Rob
    TI__Mastermind 45987 points

    Hi Rob,

    1.5W power supply should be plenty and I don't have a concern there. 

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson