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Original question:

SN74AVC4T245: VCC isolation

SN74AVC4T245: Excess current draw during high impedance state

Mark Olivier
Prodigy 40 points
Part Number: SN74AVC4T245
Other Parts Discussed in Thread: SN74AXC4T245

This is regarding adding pull up resistors for the input pins to ensure a known state is defined.  This is to prevent excess current draw during the High impedance state.

Do you define the input pins for A/B as when set by the 1DIR /2DIR pins.  i.e. if DIR is set high.  direction is A to B, So A is the input pin, not B.  I just want to clarify if I require pull ups on the output side as well?

The second question I have is the sizing of the pull up resistor required.  We have a battery run device, so current consumption is critical.  I was hoping to use at least 100k, but after reviewing the calculation regarding switching characteristics (3.9ns max for 1.8V), also depend up trace capacitance, etc. Using Ct as 5pF, I calculate max pull up resistor of 1.9k approximately.

I was told we do not require pull up resistors, but after evaluating the part on an eval board (and seeing similar issues on this thread) , I was able to verify similar results (excess current draw during high impedance mode, VCCA line is powered down as well). I found installing pull ups on the inputs/and output does seem to solve the issue.  I am currently determining if we require pull ups on the outputs.  I am using 100k pull ups for this evaluation.

Thanks,

Mark

  • 0
    TI__Expert 6680 points

    Hi Mark,

    Your first question can be answered from section 8.1 of the Datasheet:

    The SN74AVC4T245 is a 4-bit, dual-supply noninverting bidirectional voltage level translation device. Ax pins and control pins (1DIR, 2DIR,1OE, and 2OE) are supported by VCCA, and Bx pins are supported by VCCB. The A port is able to accept I/O voltages ranging from 1.2 V to 3.6 V, while the B port can accept I/O voltages from 1.2 V to 3.6 V. A high on DIR allows data transmission from Ax to Bx and a low on DIR allows data transmission from Bx to Ax when OE is set to low. When OE is set to high, both Ax and Bx pins are in the high-impedance state.

    Second, you are correct, this part does not require pull up resistors. Are you planning on leaving the inputs floating? This part doesn't support floating inputs, and will require a weak pull up or pull down to logic high or low respectively.

    Alternatively, you could use the SN74AXC4T245 instead of this part, which has the weak pullups built in. You can find this part here.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 40 points

    Hi Chad,

    Thank you for the feedback.

    In our current design we do not have any pull up resistors installed because we were told they are not required.  But it is because of that we are observing higher than normal current draw.  In our design, the current draw should be around 90uA, but we observe around 150-400uA.  Worse yet, if you add cleaning alcohol to the leads (1-3mA).  I realize the issue is due to floating inputs, because when I installed a week pull up resistor (100k) to all the inputs the problem goes away.

    The problem is the sizing of the pull up resistor.  10k to 100k pull up is great but it seem to violate the rise time of 3.9 ns Max.  I would have to use around a 1.9k resistor.. This means more current draw during activation period..

    I did take a look at the alternate part you suggested, but I was not able to find in the data sheet regarding having week pull ups internally.  I will keep looking..

    Thanks,

    Mark

  • 0 in reply to Mark Olivier
    Guru 316690 points

    The input buffers of the SN74AVC4T245 are always active (the DIR and OE pins disable only the output drivers), so any I/O pins that are not currently an active output must get a valid voltage level by external circuitry, i.e., another device, or pull-up/-down resistors.

    The 3.9 ns is not a requirement, but a device characteristic. For your purposes, a 100 kΩ resistor is perfectly fine.

    Footnote (3) in section 6.3 of the SN74AXC4T245 datasheet implies that there are no pullups:

    All unused inputs of the device must be held at VCC or GND to ensure proper device operation. Refer to the TI application report, Implications of Slow or Floating CMOS Inputs.

  • 0 in reply to Mark Olivier
    TI__Expert 6680 points

    Mark,

    First, a correction: I mentioned that the SN74AXC4T245 has internal pullups; I was mistaken, they are actually dynamic pulldowns. Also, you're correct, the datasheet for the SN74AXC4T245 indeed does not list internal dynamic pulldowns as part of the spec, but they are there. You are more than welcome to order some samples and test them out to see if they fix your current draw issues.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 40 points

    Hi Chad,

    Thanks for the info.  Yes I have ordered samples.

    Thank you again,

    Mark

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    Hi Clemens,

    Thank you for the information.

    Best Regards,

    Mark