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SN65HVS880: input thermal caused by using a pull up resistor, looking for alternative pull up solution

Jin-Suk Choi
Intellectual 1450 points
Part Number: SN65HVS880

Hi~

One of our customer is interesting in SN65HVS880 for their industrial control board.

 

On using 24V level input, default input status should be high status(default pull up), so they used 4.7k internal pull up on 24V high level bias for SN65HVS880 input.

 

But pull up resistor has high thermal when input becomes low, and it looks due to high power dissipation as I think.(24^2/4.7k=122mW.)

 

Just to reduce bias current, suggested to increase pull up resistor value up to 10kohm(and then, guess channel current would 2.4mA, and power will reduce around half,) but their buyer channel current spec is typically 5mA, so can’t reduce(4.7k pull up makes around 5.1mA bias current).

 

Do you have any idea to make default high status of SN65HVS880 without pull up resistor circuit? If they have no thermal issue, we can have more chance to apply SN65HVS880 at customer site.

 

Let me have your idea.

Thanks.

  • 0
    Guru 243080 points
    Does that "channel current spec" say that the digital input *must* source 5 mA, or that the external device can sink up to 5 mA?
  • 0
    TI__Intellectual 2900 points
    Hello Jin-Suk,

    In the schematic you show, RLIM = 24.9KOhms. This translates to IP0 internal sinking current is 1.25V * 72 /24.9KOhms = 3.6144mA, when IP0 pin is logic high.


    Assume the "default input high" requirement does not exist:
    I(IP0) = +3.6144mA, when voltage on screw is 24V.
    I(IP0) = +0mA, when voltage on screw is 0V.


    Back to your case of "default input high", the IP0 pulled up to 24V through a 4.7KOhm pull-up resistor:
    when voltage on screw is 0V, roughly speaking,


    V (IP0) = 24V * (1.2kOhms) / (1.2KOhms + 4.7KOms ) = 4.88V. But actually, the V(IP0) will be lower than 4.88V because some portion of the current from 24V voltage source will flow into the IP0 pin, while the rest of current will flow out to the screw.



    Base on the above analysis, I have two questions that need your help to understand.

    1. when you mention this customer's buyer has a "5mA typical channel current spec", is the 5mA specified at screw or specified at the IP0 pin? What's the current direction? Is it 5mA current flowing from an external sensor to the screw, or is it 5mA current from this board to the external sensor?

    2. you also mentioned "default state of IP0 should be high". How would you define the "default" state? Is default state when the screw is flowing and not connected to any external sensors?



    Thanks
  • 0 in reply to Fan.Wang
    TI__Intellectual 1450 points
    Thanks, Wang. Below is my answer on your question.

    1. when you mention this customer's buyer has a "5mA typical channel current spec", is the 5mA specified at screw or specified at the IP0 pin? -> screw, What's the current direction? Is it 5mA current flowing from an external sensor to the screw or is it 5mA current from this board to the external sensor? later is right.


    2. you also mentioned "default state of IP0 should be high". How would you define the "default" state? Is default state when the screw is flowing and not connected to any external sensors?
    ->yes, default mean no connection.(floating)
  • 0 in reply to Clemens Ladisch
    TI__Intellectual 1450 points
    Hi~ Ladisch. sourcing 5mA is right, Thanks.
  • 0 in reply to Jin-Suk Choi
    TI__Intellectual 2900 points
    Hi Jin-Suk,

    Thanks for clarifying the questions we have. To translate it to circuit requirement, what this customer is looking for is an intelligent non-ideal 5mA current source and the souce impdence of this current souce is insignificant to avoid heat. To be more specific, customer wants to have:

    1. when input screw is floating
    the IP0 is pull voltage high enough (not neccesarily to be 24V as long as it's logic high). And the current flowing into IP0 is 3.6mA. Because IP0 internal circuit is a 5mA current sink pulled down.

    2. when screw is not floating
    2.1. input voltage is high
    3.6mA current will flow into IP0 as current sink.
    2.2. input voltage is low
    5.0mA current will flow out of the screw as current source.

    If everything above is correct, a single resistor type of current source with 24V pull up is not able to meet what you need. I will get back to you by email and introduce our new device which can potentially handle your case easily.