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SN74HCS273: Directly drive a FET

Atsushi Hirai
Expert 4180 points
Part Number: SN74HCS273
Other Parts Discussed in Thread: UCC27517

Hi team,

My customer want to drive the gate of a FET by each output of SN74HCS273 directly. Their supply voltage is 3.3V.

The load(gate) capacitance is much higher than 50pF discribed in the datasheet for example 500pF~1000pF but, their operation frequency is very slow as 1kHz.

From dynamic power consumption point of view, it may be no problem because the operation frequency is low as 1kHz.

Pd = (Cpd * f * Vcc^2) + (Cl * f * Vcc^2 * 8bit) = 87.33uW

Is it okay that they connect a FET at each output of HCS273? Is there any other concern?

Best Regards,

  • +1
    Guru 281220 points

    A large capacitor initially acts like a short, so you can easily exceed the absolute maximum rating of 35 mA. (This limit must not be exceeded, not even for a short time.)

    If you can tolerate slower switching, add a current-limiting resistor of at least 68 Ω. If you need to switch the load quickly, add a gate driver (e.g., UCC27517, which would also allow a higher gate voltage).

  • +1
    TI__Mastermind 19620 points

    Hi Atsushi,

    The load capacitance will be the highest concern here. The amount of current being drawn from the pin is more important than the dynamic power consumption. If the customer exceeds the abs max values for the output current even briefly it's possible the device will be damaged.

    Otherwise there aren't any major concerns. I would recommend putting some sort of serial damping resistor on the output to limit the amount of current coming out of the device.

    Best,

    Malcolm

  • 0 in reply to Malcolm Lyn
    TI__Expert 4180 points

    Lyn-san,

    The absolute maximum rating is "Continuous".

    Is it possible that you propose an allowable current rating for a short pulse from your experience or design rule or so?

    Best Regards,

  • +1 in reply to Atsushi Hirai
    TI__Mastermind 19620 points

    Atsushi-san,

    Unfortunately, there's nothing I can propose outside of the datasheet. The +/- 35mA is a good metric. It depends on the length of the pulse. 

    Although TI cannot guarantee anything, if the pulse is only a few nanoseconds long it probably will not damage the part at low frequency.

    Best,

    Malcolm

  • +1 in reply to Malcolm Lyn
    Guru 281220 points

    The limit is not specified as a time interval but as a capacitance. The output can handle capacitive loads of up to 50 pF without a series resistor.