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SN74AVC4T774: Partial power off current consumption

Ievgenii Meshcheriakov79
Prodigy 70 points
Part Number: SN74AVC4T774

Hi,

I'm using SN74AVC4T774-RGY in a design that can have 0V on Vccb during normal operation. My understanding of the datasheet is that partial power down is supported, and the maximum current consumption in this case should be in tens of microamperes. Unfortunately when I measure it I get much large numbers.

My test setup is: DIR1=DIR3=Vcca, DIR2=DIR4=~OE=GND, A1=A3=B2=B4=GND, 100nF bypass capacitors on Vcca and Vccb. All output pins open. No resistors were used.

In case when Vcca=Vccb=3.3V the current consumption is < 1uA

In partial power down:

  Vcca = 3.3V, Vccb = 0V current consumption is 5.4 mA

  Vccb = 3.3V, Vcca = 0V current consumption is 4.8 mA

Is such high power consumption expected from this device? And is there any way to improve it?

Regards,

Ievgenii Meshcheriakov

  • 0
    TI__Mastermind 27835 points
    Hello Ievgenii,

    No, that definitely is not normal. Are you sure that VCCB is at GND and not floating slightly above ground? If VCCB is any value above ground the Ioff feature cannot be guaranteed.
  • 0 in reply to aozer
    Prodigy 70 points
    Yes, Vccb is connected to ground with a jumper wire (ca. 10cm). I verified that there is no detectable current flowing there (< 1uA).
  • 0 in reply to Ievgenii Meshcheriakov79
    TI__Mastermind 27835 points
    Hello Ievgenii,

    Are you seeing this current draw on the input pin when a logic high is applied? I have seen this happen before on a different device, where the supply is at 0V and a voltage is applied to the input, the current draw can be relatively high at that input port.

    I do not have any SN74AVC4T774 on hand but have just ordered some now. They should be here in a few days and I can test this setup in the lab and see if I have similar results.
  • 0 in reply to aozer
    Prodigy 70 points

    The power consumption is not changing significantly if I change logic levels on A3 and A1 if Vcca=3.3V and Vccb=0V (and B4=B2=0V). There is no current that I can measure flowing in or out of signal pins in this case. Even applying 3.3V to B2 and B4 does not change power consumption and there is no current in or out of those pins too.

  • 0 in reply to Ievgenii Meshcheriakov79
    TI__Mastermind 27835 points
    What pins are you seeing the 5.4mA and 4.8mA on?
  • 0 in reply to Ievgenii Meshcheriakov79
    TI__Mastermind 27835 points

    Hello Ievgenii,

    I tested the setup in the lab, I think I know what is causing your high current consumption. In order to prevent the large current consumption you are getting, all I/O pins must be at GND when either supply is shut off. This is because when either supply is shut off, all I/O pins will be in a high impedance state and can float to unexpected voltage levels. The pins that were initially configured as outputs and did not need to be at GND are no longer configured as outputs when either supply is at GND, so they must be grounded as well.

    With all I/O pins at GND, I was seeing .012 uA at VCCA with VCCB at GND and .009 uA at VCCB when VCCA was at GND. With any I/O pins floating, I was seeing several mA of current if either supply was shut off. Try grounding all of your I/O pins and see if this improves your power consumption.

  • 0 in reply to aozer
    Prodigy 70 points
    Thanks, that works for me.