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TPS7A94: Start up with negative bias on OUT

Hirotsugu Kobayashi
Mastermind 8485 points
Part Number: TPS7A94
Other Parts Discussed in Thread: TPS7A85A, TPS7A85

Hi team,

Can the TPS7A94 start up with negative bias on OUT? 

Best Regards,

  • 0
    TI__Mastermind 37925 points

    Kobayashi-san,

    How negative do you mean? I think there's an ESD cell that will not allow for a negative voltage greater than about a diode drop.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Mastermind 8485 points

    Nick-san,

    It can be biased to a negative voltage through the load in positive and negative supply applications.

    The TPS7A85A improves on this point compared to the TPS7A85.

    Is the TPS7A94 designed for dual power supply applications?

    Best Regards,

  • 0 in reply to Hirotsugu Kobayashi
    TI__Mastermind 37925 points

    Hi Kobayashi-san,

    I'll ask the design team and get back to you on this. 

    Thanks,

    Nick

  • 0 in reply to Nick Butts
    Mastermind 8485 points

    Nick-san,

    Do you have any updates?

  • 0 in reply to Hirotsugu Kobayashi
    TI__Mastermind 37925 points

    Hi Kobayashi-san,

    Sorry, I forgot to get back to you. 

    I talked with the designers. Since there's no integrated reverse polarity protection, it is limited in what it can handle at the output. Whether it can handle the reverse voltage in your application will depend on how strong the pull-down through the load is, or maybe more simply, what its off-impedance is. If the pull-down is strong enough that the ESD diodes are forward biased (which will happen if the abs max rating of -0.3V is exceeded), there is a risk for damage. The ESD diodes can reliably sink on the order of around 1mA, so that puts a minimum requirement on the off-impedance of the load at R > V_N / 1mA, where V_N is your negative supply. 

    If this cannot be guaranteed, you can also add a resistor to ground at the output of the TPS7A94 to help bleed off any current that is coming through the load. Of course this will add more Iq to your system, but at least it would alleviate the risk of damage. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Mastermind 8485 points

    Hi Nick-san,

    Thank you for your answer.

    I understand that there is no reverse voltage protection and that exceeding the maximum ratings can damage the internal ESD protection diodes.
    So I usually use a Schottky diode on the OUT pin to protect against negative bias.

    However, some LDOs cannot start with a negative bias, like the TPS7A83.
    Could you please check if the TPS7A94 can start even if it is negatively biased?

    Best Regards,

  • 0 in reply to Hirotsugu Kobayashi
    TI__Mastermind 37925 points

    Hi Kobayashi-san,

    I'll have to get some bench measurements because this isn't normally characterized. I'll also have to outsource to a colleague because I'm working from home because of an injury. Please allow 2-3 business days to provide testing results. 

    Thanks,

    Nick

  • +1 in reply to Hirotsugu Kobayashi
    TI__Mastermind 37925 points

    Hi Kobayashi-san,

    Here are the bench results from my colleague. As you can see, the device starts up normally when there is a reverse voltage on the output. One of these scope shots shows the negative bias exceeding the VOUT abs max and still starting. Of course we cannot recommend doing this because there is no guarantee that all devices will not fail when operating outside abs max, but I wanted to include it to help build confidence. As I mentioned in a previous comment, this is contingent on maintaining all operating conditions within the specified range (i.e. VOUT abs max) for this to be reliable. To guarantee this, the load impedance while it is off needs to be studied - the load impedance should be sufficiently high to limit the current through the ESD cell to 1mA. 

    TPS7A94_startup_bench.zip

    Regards,

    Nick