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TPS4H000-Q1: TPS4H000-Q1: open-load detection pull-up resistor

Federico Battaglin
Intellectual 545 points
Part Number: TPS4H000-Q1

Hello,
in a new project, I'm using the chip TPS4H000-Q1 to drive 3 led lamps with power 3,8W each.
In the demo, in the circuit there is the possibility to check the open-load condition using an external pull-up, as described in the data sheet (SLVSD73A, at section8.3.6.2.2) and I used 18k resistor, but there is a significant current that flows thru the load and so the led lamps show a small light and this is not good.
How I can solve this issue, without removing this test? Is it possible to increase the resistor value, even if the maximum recommended is 20k?

I await for your reply
Thank You and Best Regards
Eng. Federico Battaglin

  • 0
    TI__Mastermind 28220 points

    Hello,

    For open load detection in off state, the voltage across the FET should be less than 2.6V. This voltage is also across the pull up resistor Rpu (Rpu and FET are in parallel). The off state sinking current is I(ol,off) = 75uA max. The voltage drop, across the Rpu in off state, is Rpu x  I(ol,off). This voltage should be less than 2.6V. This gives max Rpu = 2.6V/75uA = 34.6K.

    If the LED still show a small light with 34K resistor, a switch in series with the Rpu is recommended.

    Regards

     

  • 0 in reply to Mahmoud Harmouch
    Intellectual 545 points
    Dear Mahmoud,
    in first, thank You for your reply.

    But:
    1) You wrote 34.6k as result for the formula for Rpu. But in the data sheet the recommended value is 20k. But I don't find the limit of that resistor, for example the maximum value of Rpu. I use 18k. But which is the best limit?= 34.6k?
    2) Yes, I thought to use a small external mosfet to connect or disconnect the Rup for all outputs, but this increases the BOM and requires an additional pin of MCU. The good solution was to use one of actual N.C. pin to connect all Rpu and internally this pin is connected to Vs when the diagnostic is enabled... Which is your opinion about this solution?
  • 0 in reply to Federico Battaglin
    TI__Mastermind 28220 points
    1) The 20K in the datasheet is a typical value and works in any application. If the supply voltage is higher than 13.5V, sinking current can be higher and results in higher voltage drop across the resistor. The 34K is good for 12V application.
    2) It is a good idea but requires internal design changes
  • 0 in reply to Mahmoud Harmouch
    Intellectual 545 points
    1) is 34k is good for 12V system, I can think 20k will be good for lower system voltage... otherwise the current is more.
    2) yes, I know... but...why is not thought before? I'm not a genius, but...in my opinion, it's a basic function...

    In any way, thank You for your useful reply!
  • 0 in reply to Federico Battaglin
    TI__Mastermind 28220 points
    Hello Federico,
    the I(ol,off) sinking current is higher a higher voltages. So lower value pull up resistor is required to maintain Vds(off) < 2.6V. I hope this answer your question.
    Regards