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max currents for VCCA and VCCB of SN74LVC8T245

Mark Liang
Intellectual 740 points
Other Parts Discussed in Thread: SN74LVC8T245

Hello,

Could you please let us know what are the max currents for VCCA and VCCB, when VCCA=2.5V, VCCB=5V, for SN74LVC8T245?  Thanks.

Best regards,

Mark

  • 0
    TI__Intellectual 2530 points
    Hi Mark

    I will look into the characterization report to see if we have data at these supply voltages. I expect 20mA, but I would like to see if I can assure that with data.

    Thanks
  • 0 in reply to Jennifer Joseph
    Intellectual 740 points
    Hi Jennifer,

    20 mA for each of them seems low, as I have measured them in the lab with one channel working:
    VCCA (2.5V): power up: 12 mA, operation: 6 mA; VCCB (5V): power up: 72 mA, operation: 52 mA

    There are eight channels, so I would think that it may be in around 100 mA for VCCA (2.5V), 600 mA for VCCB (5V), unless the device in the lab is broken.

    Best regards,
    Mark
  • 0 in reply to Mark Liang
    TI__Intellectual 2530 points
    Hi Mark

    When requested for max currents, we need to assure/specify it over the Temp range -40C to 85C and over process and wafer lot corners.

    As you can see, the max IOH/IOL for Vcco at 5.5V is 32mA, but you measured 52mA on the bench on one part for Vccb :5V.

    Can you confirm what your output is (Vcco). Is the direction always from A to B, from 2.5V to 5V?

    Thanks
  • 0 in reply to Jennifer Joseph
    Intellectual 740 points

    Hi Jennifer,

    The Vcco is 5V, and the data is always from A to B, i.e. from 2.5V to 5V.

    Best regards,

    Mark

  • 0 in reply to Mark Liang
    TI__Genius 16430 points
    Mark,

    It sounds like you're trying to figure out what the maximum current through the power rails can be if all of the ports are loaded, correct?

    ICCA and ICCB are spec'd in the datasheet, but with the stipulation that there are no loads on the outputs (Io = 0 A).

    Your worst-case current consumption will be at the point where all outputs are at maximum recommended load (for VCCB = 5V then IOH max is 32 mA). We will need to bound your input constraints to Vi = VCCA, because you will get more leakage as Vi < 0.7 * VCCA.

    THen at this point, it is simple math:
    ICCB = Icc_static + IOH for each port

    So ICCB = 32 uA + 32 mA * 8 = ~256 mA.


    As you mentioned, when you have 1 port loaded, you are experiencing current consumption above the maximum defined in the datasheet. I would suspect that the cause of this is because you likely have your unused 7 channels left floating. The datasheet states that you should ground all unused channels (inputs & outputs) in order to keep current consumption at a minimum.
  • 0 in reply to Jonathan Valdez
    Intellectual 740 points
    Jonathan,

    Thank you very much for the reply.

    Where do you get the value for Icc_static (32 uA) from, as I does not find it from the datasheet?

    Best regards,
    Mark
  • 0 in reply to Mark Liang
    TI__Genius 16430 points
    Mark,

    My mistake. After looking at the datasheet, I see that ICCB is listed as 15 uA.
  • 0 in reply to Jonathan Valdez
    TI__Expert 3186 points
    Dear Team,
    We would like to use the same part to push BJT base with about 50mA for each output.
    We plan to combine two output bits.
    On the datasheet the absolute maximum for Vccb is "100mA for each VCCa,VCCb".

    What is the maximum current we can draw from each outB and in total from all the outB pins ?

    BR,
    Nir
  • 0 in reply to Nir Flysher
    TI__Mastermind 23875 points
    Hello Nir,

    The maximum current the SN74LVC8T245 can provide at each output is about 32mA at 5V. You can also tie two inputs/outputs together to increase the maximum output drive. However, the total combined current of all of the output pins should not exceed the maximum current rating for the corresponding supply pin (in your case VCCB).