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Original question:

SN74LVC16244A-Q1: How can to calculate the maximum current?

SN74LVC16244A-Q1: SN74LVC16244A-Q1

Randy Mitchell
Intellectual 330 points
Part Number: SN74LVC16244A-Q1
Other Parts Discussed in Thread: SN74LVC16244A, SN74LVCH16T245

General question: In trying to do a power budget, if a buffer is capable of driving +/-24 ma on its outputs and it is a 16 bit device, that says 16 channels x 24 ma = 384 ma.  But if the current output is limited to 100 ma per each Vcca, Vccb and GND.

In the case of the 74LVC16244A, there are two Vcca power pins and two Vccb pins.  Since you can only drive one direction at a time, does this mean that you can only get 100 ma total drive current for all 16 output pins?  100 ma / 16 outputs = 6.25 ma?

And from a power budget stand point, I should allot 100 ma for the device, not the 384 ma?

Thanks for your input.

Randy

  • 0
    TI__Guru 57390 points
    Hi Randy,

    The sn74LVC16244A device guarantees 24mA per output.
    This device has 8 ground pins and 4 Vcc supply , so you are still within the abs max rating of the device.
  • 0 in reply to ShreyasRao
    Intellectual 330 points

    ShreyasRao said:
    Hi Randy,

    The sn74LVC16244A device guarantees 24mA per output.
    This device has 8 ground pins and 4 Vcc supply , so you are still within the abs max rating of the device.

    Thank you for your reply.

    OK, I asked a very bad question.  I was just trying to use the forums and picked upon the current draw for a sn74LVCH16T245 device.

    But the same question applies for the sn74LVCH16T245.  This part has 16 output pins on the A side and 16 pins on the B side rated at +/-24 mA and also has 2 Vcca pins and 2 Vccb pins.

    But the continuous "Continuous current through each VCCA, VCCB, and GND" is spec'ed to be +/-100mA.  16 output x 24mA per output is 384mA.  Is the "continuous current through each Vcc" 100mA for the whole device or 100mA x 2 as there are two Vcc pins on the A side and B side.

    If I'm doing a power budget, do I use 100ma, 200mA or 384mA?

    Thanks again,

    Randy

  • 0 in reply to Randy Mitchell
    TI__Guru 57390 points

    Hi Randy,

    You can safely operate the device with +/-24mA on each of the 16output ports.

    The distributed gnd pins and Vcc pins can handle the rated current.

    Do you have the application this being used or is it only for power budget calculations?

  • 0 in reply to ShreyasRao
    Intellectual 330 points

    Hi ShreyasRao,

    At this point it is going to be used for power budget calculations when driving address / data lines to memory and pal logic.  I'm trying to come up with a reasonable current draw for a power budget.  Clearly it is not going to require 24mA to drive those lines.

  • 0 in reply to Randy Mitchell
    TI__Guru* 96681 points

    Hey Randy,

    In most logic applications (ie communication between CMOS devices), power is much more dependent on capacitive loading and frequency of operation than it is on DC current.  The dynamic power consumption is given by:

    P_dynamic ~= P_transient + P_capacitive-load

    with:

    P_transient = C_pd * V_CC^2 * freq * N_sw

    P_capacitive-load = C_L * V_CC^2 * freq * N_sw

    There's more info in this application report: CMOS Power Consumption and Cpd Calculation

  • 0 in reply to Emrys Maier
    Intellectual 330 points

    Thank you Emrys Maier!  That completely answered my question...