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SN74AUC74: SN74AUC74 draws high current when inputs biased

Donna Barton1
Prodigy 20 points
Part Number: SN74AUC74

I'm using an SN74AUC74 and SN74AUC86 in combination for a phase detector (see below). The chip driving the SN74AUC74 has differential outputs swinging approximately 1.25V. I've used a biased balun to convert the differential output to single-ended input. However, the biasing appears to be causing a latch-up in the SN74AUC74 since the current draw from the device is >30mA. Presumably, I need to add a resistor or diode, or ensure the device starts up in preset/clear. What do you recommend?

  • 0
    Guru 275800 points

    Logic signals are supposed to be at or near the rails. If not, both input transistors are activated, and you get cross currents.

    And while the schematic has too low a resolution to be readable, I suspect that the capacitors on the inputs make the signal edges slower than the allowed 20 ns/V.

    See [FAQ] How does a slow or floating input affect a CMOS device?

    To handle analog signals, put a comparator in front of the logic gate.

  • 0
    TI__Mastermind 44620 points

    Hi Donna,

    Could I get some scope shots of the clock inputs and d inputs of the SN74AUC74 device?

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Clemens,

    Thanks for the reply. The caps on the clock inputs are actually not placed at this time.

    This topology was used in a previous system with an SN74LVC74 instead of the SN74AUC74. The SN74LVC74 does not draw more than 5mA, even in standby (i.e no clock input). Why is the SN74AUC74 different?

  • 0 in reply to Donna Barton1
    Guru 275800 points

    Other logic families specify a value for ΔICC, but not AUC. This might indicate that handling of non-digital signals was never a design consideration for AUC. Furthermore, support of low supply voltages requires that the transistors have a very low threshold voltage, so this would explain the higher cross currents.

    I suspect that these single-ended signals were outside the recommended operating conditions (VIL, VIH, Δt/Δv) even for the LVC devices.

    I repeat the recommendation to use comparators. (These might be able to replace the baluns.)