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SN74AVC8T245: VCCA current consumption while OE is disabled

Gil Orbach54
Prodigy 40 points
Part Number: SN74AVC8T245

Hi,

We have encountered several cases where the current consumption through VCCA is 1mA to 2mA.

This behavior appear on about 5% of our boards (out of 10k).  

Chip is marked: WE245  87K G4  DDJE

Additional information:

  1. Current was measured on VCCA pin.
  2. Verified that OE and DIR are pulled up, all port A pins are input no pull on the MCU side. Port B and VCCB are off.
  3. Protection diodes values are ok compared to a good chip.
  4. Tried to set and reset port A pins (instead of setting them as inputs), the current consumption reduced to 0.6mA.
  5. Tried to move the chip to a good board, the 1mA issue moved to the new board.
  6. Assembled good chip on the failed board, the 1mA was resolved on the failed board.

We are not sure what can cause to this leakage and would like to know what else can be checked and what actions we can take to correct this issue.

Thanks,

Gil

  • 0
    Guru 316240 points

    The OE pin disables only the outputs, not the inputs, so if the VCCA side is powered, any inputs must be driven to a valid voltage level.

    You can either configure the A port as output (without VCCB, the pins will be high impedance), or add pull-up or pull-down resistors to any floating signal lines.

  • 0 in reply to Clemens Ladisch
    TI__Guru 61570 points

    Also refer to the FAQ on Vcc isolation/ hi-Z mode on translators.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    The datasheet indicates that "When OE is set to high, both A and B are in the high-impedance state." (page 15).

    Although setting all inputs to reset indeed resolve the current issue through VCCA, we would like to know if there's any way to reduce the current (on units with high current on VCCA) while there are floating pins as in the schematics above (DIR is fixed to high).

    In addition, are units with good current consumption may become unstable with high current later? or we can assume that good units will remain good even some I/Os on port A are floating.

    Thanks,
    Gil

  • 0 in reply to Gil Orbach54
    Guru 316240 points

    CMOS inputs always have a high impedance. "OE" means "output enable"; if it affected also the inputs, it would have been called "EN".

    An active input with a floating voltage will have high current consumption. The only way to prevent this is to make it an output, or to prevent the voltage from floating with pull-up/-down resistors or a bus-hold circuit.

    As long as you stay inside the absolute maximum ratings, there will be no permanent damage.