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SN74ACT244 - Short circuit between Vcc and GND

YUKI NAKASHIMA
Prodigy 200 points
Other Parts Discussed in Thread: SN74ACT244, SN74LV244AT, SN74LVC244APart Number: SN74ACT244

Question type: Development and troubleshooting

Support topic: /Product/Development and troubleshooting/

Description of the problem: Short circuit appears between Vcc and GND in SN74ACT244DW after 5V power supply turn off.
Short circuit resistance is about 15k ohm. with no supply
Other input and output are large impedance with no supply.
What is this cause? ESD damerge?
Please give me your expertise.

  • 0
    TI__Guru 62060 points
    Hi User ,

    When the power supply is turned off , are the input and outputs are active ?
    This device doesnt have Ioff protection which ensures that the I/Os can have signals when Vcc is powered down without adverse leakages to Vcc through the ESD diodes.
    It looks like that the I/Os were active when Vcc =0V and blew up the ESD diodes by heavily fwd biasing them.
    You could consider SN74LVC244A which are octal buffers with Ioff protection available in tiny X1QFN package. If you need TTL compatible inputs , then you could consider sn74lv244AT which have Ioff protection.
    You can send out the scope shots and the schematics to check if there are anything else to look for.
  • 0 in reply to ShreyasRao
    Prodigy 200 points
    Thank you for your reply and expertise.

    I think I/Os were not active. Because, Output was no connect and Input was controlled by your product, TMS320F28XXX.
    TMS320F28XXX core and I/O is supplied by your product, TPS series, Regulator. TPS regulator uses +5V power supply.
    +5V power supply is used as Vcc source for SN74ACT244DW and monitored by your product, TLC serise, voltage supervisor.
    TLC voltage supervisor reset TMS320F28XXX and enable/disenable SN74ACT244DW.
    From the above, I think,
    When +5V power supply turn off, TMS320F28XXX is reset before +5V power supply being fully downed.
    And, SN74ACT244DW input is not active for TMS320F28XXX reset.
    Additionally, TMS320F28XXX I/Os is driven by TPS regulator with +5V power supply.
    So, heavily fwd biasing between Vcc and input of SN74ACT244DW is not generated.

    But, at the same time, TMS320F28XXX was destroyed.
    Is this affecting?
    Also, the cause for TMS320F28XXX being destroyed is unknow.

    Thanks,