• State Answer Suggested
  • Locked Locked
  • Replies 5 replies
  • Answers 3 answers
  • Subscribers 21 subscribers
  • Views 130 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74AHCT125: load capacitance = 100pF

Shinji Tsuji
Genius 15329 points

Part Number: SN74AHCT125

Hi team,

In my PCB condition, SN74AHCT125-Q1 need to drive 100pF load capacitance, but I found a description below in the datasheet. 

Could you please advise what is the concern/risk and if it is feasible to drive 100pF load?

"The SN74AHCT125-Q1 can drive a load with a total capacitance less than or equal to 50pF while still meeting all of the data sheet specifications. Larger capacitive loads can be applied; however, it is not recommended to exceed 50pF."

 

regards,

  • 0
    Guru 325260 points

    The absolute maximum rating for the output current is 25 mA. A capacitor acts like a short, so that limit can be exceeded. You should insert a current-limiting resistor of at least 75 Ω (this will also slow down the output edges).

  • 0 in reply to Clemens Ladisch
    TI__Genius 15329 points

    Hi Clemens-san,

    Thank you. 

    Could you please elaborate on the graph above?

    regards,

  • 0 in reply to Shinji Tsuji
    Guru 325260 points

    The graph shows the typical relationship betwen the output voltage and output current. During normal operation, only the range between 0 V and 5 V and between −8 mA and 8 mA matters. Below −0.7 V and above 5.8 V, a large current flows through the ESD protection diodes.

    When an output that tries to drive high is shorted to 0 V, the output current is about 55 mA. When an output that tries to drive low is shorted to 5 V, the output current is about −60 mA.

  • 0 in reply to Clemens Ladisch
    TI__Genius 15329 points

    Hi Clemens-san,

    Absolute max rating of the Io is defined as continuous current (+/-25mA max).

    Charging current to output load should be inrush transient, not continuous current. 

    How long is this "continuous current" period? 

    I would like to know if ns or us or even ms order can be considered "transient" and be allowed. 

    regards,

  • 0 in reply to Shinji Tsuji
    Guru 325260 points

    There is no separate limit for transient currents; you have to always use the continuous current limit. (But the current transients from capacitive loads of up to 50 pF are allowed.)