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TLIN1431-Q1: HSS pin can withstand short to GND/battery?

Ben King
Expert 5996 points
Part Number: TLIN1431-Q1

Hi LIN Team,

My customer is planning to use the HSS (pin 18) on TLIN1431 to power an external LED module.  However, the application would require the HSS output to be protected against damage when HSS is shorted to Vbat or GND.   Also, HSS being shorted to Vbat or GND should not cause the LIN transceiver to stop working.

Can you please help me to answer this inquiry?  

Vbat is typically 12V automotive battery, with max voltage assumed being ~36V during load dump.  

With the abs max voltage rating on HSS pin of 48V AND  requirement to be >0.3V above Vsup, I'd imagine that the HSS pin can withstand short to Vbat under most conditions, except where Vbat voltage is not present on Vsup - as this would result in HSS being >0.3V above Vsup...  What happens in this scenario?  Is an internal protection diode biased that can cause thermal damage to the IC?

Also, is there any chance the LIN transceiver operation will stop when HSS is shorted to GND or battery voltage?

Thanks,

Ben

  • 0
    TI__Mastermind 26806 points

    Hi Ben,

    Thanks for the questions here. First, let me make a small clarification:

    Ben King said:
    With the abs max voltage rating on HSS pin of 48V AND  requirement to be >0.3V above Vsup

    The HSS pin voltage cannot exceed VSUP by more than 0.3 V. I think maybe you meant to put a < sign instead of a > sign.

    Now, regarding your question, short to battery should normally be fine, provided it is no more than 0.3 V above VSUP, as you mentioned:

    Ben King said:
    I'd imagine that the HSS pin can withstand short to Vbat under most conditions, except where Vbat voltage is not present on Vsup - as this would result in HSS being >0.3V above Vsup

    Exceeding the absolute maximum ratings of the device could cause permanent device damage (see the note at the bottom of the "Absolute Maximum Ratings" table in the data sheet).

    The main concern here is regarding a short to GND. While it is technically within the absolute maximum voltage rating for the HSS pin, the concern is thermal dissipation. The HSS does have an overcurrent limit characteristic (see IOC(HSS)), but this is a lot of power dissipation. Assuming absolute worst-case situation, we can estimate:

    P = IV = IOC(HSS),MAX * (VSUP - GND) = (300 mA)*(12 V) = 3.6 W

    This is a lot of power to dissipate in the device, and thermal considerations would be in play (see the "Thermal Characteristics" table). Maximum TJ is 150°C. Theoretically during 36-V load dump, this would be 3 times as high.

    The expectation is that this device will be Functional Safety Quality-Managed upon full release, which will provide many documents (see details here) that describe failure cases. This situation (short to GND on HSS) would be in the pin FMA.

    And for this question:

    Ben King said:
    Also, is there any chance the LIN transceiver operation will stop when HSS is shorted to GND or battery voltage?

    Exceeding the max junction temperature for the high side switch and LIMP high side switch cause the switches to be turned off until junction temperature falls below the thermal shutdown temperature threshold.

    Best,

    Danny

  • 0 in reply to Danny Bacic
    TI__Expert 5996 points

    Hi Danny,

    Thank you for the detailed explanation!   In the event of short to GND, would the power dissipation really be very high to cause issue?  I'd imagine that in a short to GND scenario the current would be high enough to immediately the trip the OC threshold, making the high power dissipation a short duration... is this not the case or am I not considering something?

    The customer was hoping to use the integrated HSS to replace a competitor switch, but our initial response here makes them believe it would not work due to the issue with short to GND. 
    I just want to be certain this is a possible issue, as this limits the use cases for the integrated HSS and makes the TLIN1431 slightly less desirable.

    Additional details from the customer's response:

    "I am looking for an alternative to BD1HD500HFN high side switch, I was hoping to replace it with the HSS in the TLIN1431 but it looks like this will not work.

    Footprint and pinout can be any, load should not be more than 100mA. It has to be protected against short to B+/GND."

    Thanks,

    Ben

  • 0 in reply to Ben King
    TI__Mastermind 26806 points

    Ben,

    No worries, perhaps I was a bit too pessimistic in my answer above. You are correct, from a use-case perspective, overcurrent detection (and subsequent shut-off) should prevent this from becoming an issue in application. It would be very unlikely for this ultimately to result in thermal shutdown since this mechanism is present.

    This should satisfy the concern about short to B+/GND from my perspective, but please let me know if I can help clarify any further!

    Best,

    Danny

  • 0 in reply to Danny Bacic
    TI__Expert 5996 points

    Hi Danny,

    I discussed this some with the customer and they agree that the HSS switch current limit likely provides sufficient protection against short to GND  -  at least assuming it should if it is called a high-side switch.  

    In order to give more confidence, I'm trying to determine what exactly occurs when the current limit is tripped on the HSS.    Can you help with this explanation?     

    The datasheet says "When a over current is detected, there is a filter time, tOCFLTR, to determine if over current is valid. If valid there is a shut off time, tOCOFF, time for the HSS to shut off"

    Filter time is 16us typ and shut off time is 300us max.  

    Does this mean there is a 16us delay to shut off and then it shuts off for 300us before retrying?   Or does it mean it waits 16us + 300us before shutting off?
    And what happens after the overcurrent - does it enter some hiccup mode to automatically retry? Either the datasheet doesn't describe it or I'm missing that section.   

    Also, would it be possible for you or someone on the team to test this in the lab -  to have HSS passing through 12V and short to GND to observe the behavior?    

    Thanks,

    Ben

  • 0 in reply to Ben King
    TI__Mastermind 26806 points

    Ben,

    To your first question, you're correct, that's when it turns off. But I need to confirm when it would try again, and what that would look like.

    I can try and take a look at this in the lab, but given current workloads I'm not immediately sure how soon I'd be able to get this done.

    In the meantime, let me look a bit more into our design info to see if I can get you an answer from there.

    Best,

    Danny

  • 0 in reply to Danny Bacic
    TI__Mastermind 26806 points

    Hi Ben,

    Still working on finalizing a deliverable here, but I wanted to follow up to address what we discussed over email and Webex. Basically, the gist is that the HSS will turn off and will not automatically retry unless PWM is active.

    Do you know how the HSS is being interfaced/implemented? i.e. Is the transceiver in pin mode or SPI mode? Is PWM being used on the HSS? This would help me more quickly tailor an answer to your use case while we finalize language to add to the RTM data sheet.

    Best,

    Danny

  • 0 in reply to Danny Bacic
    TI__Expert 5996 points

    Hi Danny,

    Some responses form the customer:

    1. Pin control.
    2. This switch will be used to drive LEDs in another module and will use PWM to control dimming, although in certain conditions (battery voltage < 9V), PWM duty cycle will be 100%.

                   Do I understand correctly that that the PWM signal is causing the switch to reactivate? If yes then I think that could still use the HSS.

     The main question though is will the HSS shutdown cause the LIN function to shut down? if so then we can’t use the HSS switch.

    Thanks,

    Ben

  • 0 in reply to Ben King
    TI__Mastermind 26806 points

    Ben,

    Thanks for your patience here. We are adding clarifying language to the final data sheet document that will be available upon RTM.

    For now, here is an overview based upon the situation you've described in your customer's use case:

    To clarify, the PWM programmability of the HSS is only available in SPI control. Since you're in pin control, I am interpreting your response #2 to mean that the HSS is being used to power a downstream LED driver module, which itself generates a PWM.

    In pin control, when the HSS is controlled with HSSC, the over-current condition on HSS would turn off the high-side switch. After this, a toggle on HSSC is needed to reactivate the high-side switch.

    Ben King said:
    Do I understand correctly that that the PWM signal is causing the switch to reactivate? If yes then I think that could still use the HSS.

    However, if #1 was intended to say SPI control and the device is using PWM, then this internal "toggling" behavior will reactivate the HSS itself. In other words, the low part of the PWM pulse will automatically re-enable the HSS.

    Ben King said:
    The main question though is will the HSS shutdown cause the LIN function to shut down? if so then we can’t use the HSS switch.

    No, HSS shutdown does not impact LIN function. The over-current feature is basically just a gate to the HSS, but it does not amount to a state change of the TLIN1431-Q1.

    Hopefully this helps clarify things!

    Best,

    Danny

  • 0 in reply to Danny Bacic
    TI__Expert 5996 points

    Hi Danny,

    Thanks for the details.   For my own clarification, can you help address the following questions?

    -The HSSC pin can just be viewed as an enable pin for the HSS right?   No other functionality - just high level = enabled switch and low level = disable switch?   

    -Also, what happens if PWM signal is applied to the HSSC pin?  Datasheet only says it is controlled by HSSC, but that isn't particularly clear if it can accept an external PWM signal in the same 200Hz-400Hz that the switch can support.   I don't know if the customer plans to implement it this way, but just want to know if it would be a bad idea.   

    Best regards,

    Ben

  • 0 in reply to Ben King
    TI__Mastermind 26806 points

    Hi Ben,

    Seems like we are on the same page. Slight smile

    Ben King said:
    The HSSC pin can just be viewed as an enable pin for the HSS right?

    Yes, the HSSC controls the HSS as you mentioned while in Standby, Normal, or Fast modes.

    Ben King said:
    Also, what happens if PWM signal is applied to the HSSC pin?

    The HSSC pin has a max input frequency of 400 Hz, indicated by fSW in the electrical characteristics.

    Best,

    Danny