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74HC02 Output Resistance

Stephen Bennett
Prodigy 80 points


Hi Team,

I'm in the process of performing some worst case analysis involving a 74HC02 and need to know some additional output characteristics that I haven't been able to determine.  I've taken a look at various documents including Input and Output Characteristics of Digital Integrated Circuits at 5-V Supply Voltage, but nothing quite seems to have what I need.

For my analysis I need to know the minimum and maximum output resistances, both sourcing and sinking current.  I believe I've been able to determine the minimum resistance to be 25Ω but the maximum output resistance is more difficult to determine from datasheet specifications.  Any assistance with these values would be much appreciated.

Thanks,
Stephen

  • 0
    TI__Guru* 97046 points

    The output impedance of the part will be related directly to the selected Vcc and the output current via Ohm's Law.

    R = V/I

    In order to get a minimum resistance, we use the largest available I value and the smallest available V value.  That means Vcc = 2 V and Io = 25 mA, R = 80 ohms. (Note that 25 mA is the absolute max output current of the part, I don't know if it can really hit that or not, and if it did, it would be on the brink of failure -- ie 25.1 mA is outside the rated max)

    Since you could theoretically have no current output, it could be infinite, but we know that isn't going to be true.  You can get a range by plugging in the max current and voltage values:  Vcc = 6 V, Io = 25 mA, so R = 240 ohms.

    So, in summary, R_out_min will range from 80 to 240 ohms based on Vcc, and R_out_max is infinite.

    The typical value for R_out will be 5.8 V / 5.2 mA ~= 1115 ohms (from the VOH information on the datasheet).

    I hope this helps.

  • 0 in reply to Emrys Maier
    Prodigy 80 points

    Thanks for the quick reply.

    Looking into this a bit more (found HCMOS Design Considerations), wouldn't R_high_min actually be the drop across the output resistance, so:
    (VCC - VOH) / IOH = (6V - 5.8V) / 5.2mA ~ 38.5Ω

    Similarly:
    R_low_min = VOL / IOL = 0.15V / 5.2mA ~ 29Ω

    Then, an upper bound on Rout might be something like:
    (VCC - VOH_min) / IOH = (2V - 1.9V) / 20µA = 5kΩ
    or
    VOL_max / IOL = 0.1V / 20µA = 5kΩ

    Thanks for your assistance.

  • 0 in reply to Stephen Bennett
    TI__Guru* 97046 points
    Those are realistic values.