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

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.

2 Replies

  • 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?
  • 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

    Fan Wang