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SN74AHCT08: Peak output current

Takashi Yamaguchi
Genius 11090 points
Part Number: SN74AHCT08

Hi team,

My customer is evaluating SN74AHCT08. Could you help with the following question?

Customer said that an output current flows 150mA for 0.3usec on the output of this device in their system. Is that acceptable? The d/s doesn't describe about the peak current, so customer would like to know how much peak current does this device can handle.

Regards,

Yamaguchi

  • 0
    TI__Genius 16725 points

    Hi Yamaguchi,

    Are you saying that the SN74AHCT08 is outputting 150mA of current? This is too much current and can cause damage to the device. If you look at the IOH and IOL specs those are the maximum currents the device should output. There is also a limit of 25mA continuous output current in the abs max table of the datasheet.

    Are the inputs of the device slow or noisy? Are any of the inputs floating? These are some common reasons for large current flow in the device.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Genius 11090 points

    Hi Karan-san,

    Thank you for the answer.

    Sorry, the 150mA is typo. The correct number is 120mA for 0.3usec.  Does the SN74AHCT08 have peak current specification?

    It seems that the customer is connecting the SN74AHCT08 to a MOSFET gate through a gate resistor.  I would recommend to customer to add gate driver IC between the SN74AHCT08 and MOSFET.

    Regards,

    Yamaguchi

  • 0 in reply to Takashi Yamaguchi
    Guru 243220 points

    Logic devices are tiny and do not have any useful amount of thermal mass. For most practical purposes, the continuous current rating (25 mA) is the peak current rating.

    Higher currents are allowed only for extremely short times, i.e., when driving loads as small as 50 pF. Apparently, that MOSFET has a larger input capacitance; 300 ns would be much too long.

    It is possible to use a logic device (or even multiple CMOS outputs in parallel) as a gate driver, with a gate resistor that limits the current to 25 mA (or N × 25 mA). But if the switching time becomes too long, you have to use a stronger gate driver.

  • 0 in reply to Clemens Ladisch
    TI__Genius 11090 points

    Hi team,

    Thank you for the answer. I understand.

    Regards,

    Yamaguchi