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TPS4H000-Q1: load disconnection test in off state

Federico Battaglin
Intellectual 545 points
Part Number: TPS4H000-Q1

Hello,

in my project I use TPS4H000-Q1 version B and I drive the led lamp at 12V.

I have to add the test of load disconnection in off state.

Following the indication in data sheet SLVSD73A, at chapter 8.3.6.2.2, I tried to use a pull-up of 18k-39kOhm, but the leds are in on, even if the light is very small.

I tried to increase it but over a specific value, I can not recognize the disconnection in off state.

So there's no solution following the data sheet indication.

My idea is to power the pull-up resistors not at Vs voltage, but less, for example 3.3V, value too much low to power on the leds.

Is it possible to do it? I didn't try because I don't have the application board now.

I await for your opinion.

Best Regards

Federico Battaglin

  • 0
    TI__Mastermind 24740 points
    Hello,

    Pulling up the resitor to lower voltage such as 3.3V, does not work. In order to report the fault In off state, the output voltage should be close to VS pin voltage. For making sure the open load, in off state, is detected, the voltage difference Vs - VOUT = V(ol,off) should be around 1.6V. If the pull up resistor is tied to 3.3V, Vs - VOUT is around 12V - 3.3V = 8.7V and it is higher than the specified V(ol,off).
    Just a thought, you may change to LED to different one less sensitive to to low current.
    Regards
  • 0 in reply to Mahmoud Harmouch
    Intellectual 545 points

    Dear Mahmoud,

    thank You for your reply.

    The lamp is a standard lamp for street use and I can not change it. The link to the product data sheet is this:

    Because that chip is automotive qualified where the led lamp is very common, I think it must be a solution for this test.

    Which other circuitry solution is possible to found?

    Thank You

    Federico

  • 0 in reply to Federico Battaglin
    TI__Mastermind 24740 points

    Hello Federico,

    The issue is related to the sensitivity of the LED lamp. I understood from your previous message that a current of 12V/39K = 300uA is enough to keep the light on. I believe lowering the lamp current to below certain value may fix the issue. The solution for that is to add a low impedance (PWM on/off) in parallel to the LED bulb in off state. This is an external circuit solution requiring 2 small transistors, 2 resistors and capacitor. The circuit is controlled by FAULT pin.

    This is what I can think off as top level solution. Please let me know if it is acceptable so we can check if it works.

    Regards     

  • 0 in reply to Mahmoud Harmouch
    Intellectual 545 points
    Ciao Mahmoud,
    1) if I take lower the pull-up current, the led is always a bit in on, but the load disconnection test fails, because the pull-up is over a specific limit. And so, this way is not good.
    2) about your solution, if I understand well, the transistor connects the resistor in parallel to led lamp trying to put the led in off. This is enough clear, even if I have a big leakage current, when led is in off. This solution, that I just tried with only resistor in parallel to led lamp is not good because the application is powered by battery. But, question, who drives the transistor? Is the input (in not logic) or the fault pin? Sorry, but it's not clear...

    In any case, TPS4H is not completed solution to drive output with diagnostic... because the diagnostic is not full.
    If this chip is for automotive market, which is the solution to have the fully supervised output?
  • 0 in reply to Federico Battaglin
    TI__Mastermind 24740 points

    Hi Federico,

    please check attached drawing. R1, R2 and C determine the PWM frequency for averaging the current into the LED lamp. The FAULT will be a PWM signal as well. When T1 is ON, R3 pulls OUT lower and the current in the LED lamp as well. I thought of this work arround as sheap solution. Please let me know if it looks OK. I only support the high side switches. We might have different part in the power switches product line. I will forward the message to the apps engineer.

    Regards

  • 0 in reply to Mahmoud Harmouch
    TI__Mastermind 24740 points
    Hello,

    There is no part in the product line that can report open load fault with no pull up resistor. This thread will be closed tomorrow 7/20/18 if there is no more question.

    Regards
  • 0 in reply to Mahmoud Harmouch
    Intellectual 545 points
    Dear Mohmoud,
    in first thank You for your drawing and for your explanation: it's very clear.
    But is not so practical, because I have 4 channels used for 4 led lamps, and so I increase the complexity of circuit.
    I thought the open load test on led lamp was possible because the chip is for automotive market and in this market is important to advise problems on lamps (short circuit, open load), as happens on my car.
    Question: normally, in the automotive drawing, which is the solution to drive the led lamp? Is it similar to your drawing or is it used a different chip? For example low-side?

    Thank You again
    Federico
  • 0 in reply to Federico Battaglin
    TI__Mastermind 24740 points
    Hello Federico,

    I understand the concern. I believe only high side switches have full detections for abnormal situation. The high side switches do not have an issue with other loads in automotive environment. All high side switches in the market needs pull up resistor for open load detection in off state. I believe the only solution for now is external and needs external parts. I hope this answer your question. Please let me if you still have question or I can close the thread.

    Regards
  • 0 in reply to Mahmoud Harmouch
    Intellectual 545 points

    Dear Mohmoud,

    thank You for your answers!

    Best regards

    Federico