This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS92692: Behavior at power-on with LED shorted

Part Number: TPS92692

Hello

There is a question about the behavior at the time of LED short circuit in the boost LED driver circuit like Figure 36.

If VIN is turned on with the LED short-circuited, The gate voltage of Qdim was close to VIN.
However, there seems to be a voltage difference between the source and gate for a moment,
The FET turned on momentarily, and the voltage was appearing on the drain side.

Therefore, when Qdim is turned on, VIN will be over-current drawn,
and it will stop due to over-current protection on the VIN side.

I want to pull up the gate of Qdim to the source side to prevent this.
Is there any problem in particular?

Best regards
Hara

  • Hi Hara,

    In order to sense a short we have to turn on the Qdim FET so the current can flow in the current sense resistor.  That's the only way we can sense a short is knowing the current.  If you pull it up then we do not sense the current.

    Thanks Tuan

  • Hi Tuan

    Thank you for your reply.

    If you turn on VIN with the LED short-circuited, what kind of operation is supposed to take?
    The input of the DIM terminal is in the L state.

    Since Qdim is off, current does not flow even if there is a short circuit in the future,
    I thought that the output short-circuit protection of TPS92692 would operate because the overcurrent flows for the first time at the moment when the DIM terminal becomes H.

    However, when actually operating, overcurrent flows at the moment when VIN is turned on,
    The overcurrent protection of the VIN power supply was working.
    Do you know what caused it?

    Best regards
    Atsushi

  • Hi Atsushi,

    I have a hard time following your question.  Please share any scope captures along with comments of what you are talking about. 

    Thanks Tuan

  • Hi Tuan

    I'm sorry my explanation is hard to understand.

    First of all, I wantto know what the correct behavior is supposed to be,
    Please tell me what the operation will be like in the case of the following conditions in the circuit of Figure 36.

    [Q1]
    With LED+ and LED- shorted, with IADJ pin and DIM/PWM pin input at L
    Would you please tell me what kind of operation will occur when the power is supplied to VIN?
    [Q2]
    Would you please tell me what the operation is like when the power is turned on to VIN with COUT short-circuited?
    In that case, it is recognized that the IC cannot perform protection operation. Is it correct?

    Best Regards
    Atsushi

  • Hi Atsushi,

    Q1:  With LED+ and LED- shorted and IADJ and DIM/PWM at 0V the Dimming FET will be off and no switching is taking place

    Q2:  When Vin is first applied Dimming FET is off and when DIM/PWM input and internal voltages reaches appropriate level like UVLO then dimming FET will turn on and if the output is shorted it will sense that it's an over-current condition and turn off the dimming FET.

    This is all the details we will provide on this.  If you need anymore you can obtain our evaluation board and test it.

    Thanks Tuan

  • Hi Tuan

    Thank you for reply.

    I understood the answer of Q1.
    But I have additional questions about Q2. 

    If COUT is short-circuited in the circuit as shown in Fig. 36,
    isn't protection possible even if Qdim is turned off?

    Best Regards
    Atsushi

  • Hi Atsushi,

    If Cout is shorted we do not protect for that...We only protect for LED output short.with the Dimming FET.

    Thanks Tuan

  • Hi Tuan,

    I asked about Q2, which I asked before.

    Which of the overcurrent fault threshold or the undervoltage fault threshold does overcurrent protection work?

    When I actually test in this state, it seems that protection is effective before the SS pin voltage rises.

    The data sheet shows that "The undervoltage fault detection circuit is internally disable based on the SS pin voltage and internal PWM.
    status.",
    So is it that overcurrent fault worked, not undervoltage fault?

    Does the overcurrent fault work when the SS pin voltage is low?

    Best regars.
    Atsushi

  • Hello,

    I can answer application of the TPS92692 but the question you are asking are about internal design of the part and that is proprietary and I will not be able to answer your question.

    Thanks Tuan

  • Hi Tuan.

    The last question was about TPS92692.

    Then please tell me only the following points.
    Does the overcurrent fault work when the SS pin voltage is low?

    Best Regards
    Atsushi

  • Hi Atsushi,

    We will point to the data sheet in section 8.3.15 on page 21 for how the part does output short-circuit detection.  This is all we can mention about this.

    Thanks Tuan

  • Hi Tuan.

    I have a question because I don't understand it after looking at the data sheet in section 8.3.15.

    Is it possible to interpret that the overcurrent fault can operate regardless of the state of the SS pin voltage?

    Best Regards
    Atsushi

  • Hi Atsushi,

    No you cannot interpret it that way.  This is the only answer I can give you this subject and I will close this thread now.

    Thanks Tuan