• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 65 subscribers
  • Views 1407 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

UA78xx - Ground Pin Floating, TAB Connected

Kristof Berg
Prodigy 180 points

I'm looking at an application where UA7824CKTTR has been cross replaced for a regulator from another manufacturer.

Due to the existing footprint the ground pin, PIN 2 is floating, however the TAB is connected to ground, per the datasheet's pin functions list.

In this application I'm seeing some parts failing, could this be related?

  • 0
    TI__Guru 71516 points

    Hi Kristof,

      Thanks for using E2E, although I'm a bit confused by your inquiry.  You are asking if the competitor's device is failing due to pin2 floating, is that correct?  Or did you replace the competitor's device with the UA7824, and the TI UA7824 is failing?  If it's the latter case, how are these devices failing?  And is it immediate with power-on, or over time?

    Regards,

    ~Leonard 

     

  • 0 in reply to LeonardEllis
    Prodigy 180 points

    The failing part is TI"s uA7824. 

    The part it replaced didn't have a PIN 2, it was electrically connected through the ground tab. 

    Since the footprint didn't have a PIN 2 but this was redundant to the function of TAB it was decided this was ok to leave pin 2 floating on the TI part. 

    This is an inductive load running various motors. 

  • 0 in reply to Kristof Berg
    TI__Guru 71516 points

    Thanks for the clarification Kristof.  I'm getting the LDO experts involved, and we will probably need to see the schematic, at least the LDO portion. Also, how are these failing, and is it a catastrophic failure (such as short to GND of VOUT)?  Please also note in the datasheet, page 3, pin 2 is called out as the COMMON connection, no mention of the tab, which I believe is just for thermal purposes.

    ~Leonard  

  • 0 in reply to LeonardEllis
    TI__Mastermind 24485 points

    Hello,

    Yes the tab and PIN2 are electrically shorted, so there should be no issues with PIN 2 not connected as long as the tab is electrically connected.
    What kind of issues are you seeing with the device?
    It is not likely to be related to a floating PIN2.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Prodigy 180 points

    Schematic is attached. Please understand IC10, component L7824CD2T has been switched out to a TI UA7824.

    Take note of the input capacitors h-bridge configuration. Any input would be greatly appreciated.

    780387-2_sch2.pdf

  • 0 in reply to Kristof Berg
    TI__Guru 55940 points
    Hi Kristof,

    Do you have any scope shots of the LDO IN and OUT?

    From your schematic nets it appears that your input is 24 V unregulated and you are attempting to regulate it with uA7824 to 24 V. Due to the nature of linear regulators, a minimum headroom (dropout) is required between Vin and Vout for regulation. Please note that the minimum Vin for uA7824 is 27 V.

    Very Respectfully,
    Ryan