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THVD1410: What is the driver output current when unpowered, IO(off), and the driver output current when disabled

Erich Beyer
Prodigy 20 points
Part Number: THVD1410

What is the driver output current when unpowered, IO(off), and the driver output current when disabled?

I could only find the receiver output current when disabled in the data sheet.

  • +1
    TI__Mastermind 46697 points

    Hi Erich,

    The specification you are looking for is Ib - or the bus input current. Its listed in the receiver section - but this still applies to your question.

    Ib is measured with VCC = 0v to 5.5V with DE = 0V - so the device is off or on and the driver is disabled - since A and B act as driver and reception pins that is the spec that will be closest.  We measure at -7V and 12V (and for this part also -15 and 15V) in these cases - but this spec is more or less linear (or can pretty safely assumed to be) so you can take a linear interpolation to get an estimate at an input voltage. When I run a linear interpolation of the data in teh datasheet I get two equations:

    1. Linear Appx. Typical values based on input voltage: Ib = 6u * Vi - 30u with an R^2 value = 0.9941 

    2. Linear Appx. Max/Min Values based on input voltage Ib = 10u * Vi - 30u with R^2 value = 0.9923

    While these equations are not perfect and don't fully capture every potential value - that should guide you on the input/output current when DE = 0 and VCC = 0V - as a note the bus will approach 0 when it is idle or off  - so there should be very little current. 

    That being said you can just simplify the whole thing and use the minimum input impedance (which is #nodes supported * 375 - its not perfect but it is a super common approximation for RS-485 devices) - so for a 256 node device - that is 96K input impedance  - as any current in/out of the A/B pins when DE is off and VCC is off or on is going to be dependent  on that implied impedance (most applications will see > 96K - but that should be close to the minimum value).

    There really isn't direct leakage spec'd for many RS-485 drivers as the input bias current includes that in its specification and you really can't separate the two - as if DE is active than the impedance is very low as its actively driving and you want low output impedance. 

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

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 20 points

    Thank you so much for the help. The bus input current will work perfectly for my design goals. Thank you for performing that regression analysis! I will probably use that more complex model for my design. 

    For the alternative model that you propose based on minimum input impedance, where did you find the definition

    Parker Dodson said:
    #nodes supported * 375

    ?

  • 0 in reply to Erich Beyer
    Prodigy 20 points

    This appears to come from the unit load characteristics of the TIA-485 specification 

  • +1 in reply to Erich Beyer
    TI__Mastermind 46697 points

    HI Erich,

    Sorry for the delay!

    Completely understand wanting to create a more complex model for a more accurate estimation. 

    Yes the 12K comes from eh TIA-485 specification - standard RS-485 defines a unit load as shown below:

    A 12K load line will be compliant over the entire range (-7V to 12V) - in reality its a bit more complicated as this impedance can behave slightly non-linearly  so there is some margin around the 12K on some of the devices (unless they explicitly state a minimum input impedance) - however most devices using the 12K as the standard is a very common short-cut that is generally a good design value to work with.

    Best,

    Parker Dodson