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Original question:

LM5175: OVP spec of LM5175 and LM5176

LM5176-Q1: Functional safety considerations for OVP

Alan O
Mastermind 32575 points
Part Number: LM5176-Q1
Other Parts Discussed in Thread: LM5176, TLV1805-Q1, TLV431

Hello,

My customer is planning to use LM5176-Q1 in an automotive subsystem that requires functional safety support. The use case is 12V battery input and 15V output. We shared the Pin FMEA and FMD documents.  While the customer knows LM5176 supports internal output OVP feature, he is considering an external OVP circuit as safety mechanism, in case the LM5176 PWM controller fails (e.g. erratic PWM duty cycle output may result in unexpected over-voltage).

Could you please help to analyze this potential failure scenario (erratic duty cycle) and mitigation options to prevent OV on the output of LM5176?  Is there a way to estimate the output over-voltage that could be expected in this scenario?

One idea for an external OVP circuit is to add a high-voltage comparator (like TLV1805-Q1) to detect the OV condition on the output of LM5176 and use TLV1805-Q1 to drive a P-MOSFET pass element to disconnect the load during OV condition.  Example: Overvoltage protection using TLV1805-Q1 (link).

Thanks,
Alan

  • 0
    TI__Guru* 78100 points

    Hi Alan,

    Thank you for posting.  The LM5176 internal OVP is implemented by monitoring the FB voltage.  If the FB pin fails, the IC can lose the OVP function.  So the idea of employing an external OVP circuit is a good one.  Then you will have duplicate OVP function to cover single point failure. 

    For the external OVP, I would recommend the TLV431, and use it to monitor the Vout and let its Anode to pull down the EN?UVLO pin.  The TLV431 has internal reference voltage so the implementation becomes simple an straightforward.

    Thanks,

    Youhao Xi, Applications Engineering