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SN74AHCT123A: Diode on RC Pin

TI-CSC
Genius 10420 points
Part Number: SN74AHCT123A
Other Parts Discussed in Thread: SN74LS123

Hello E2E Experts,

Good day.

On the LS123 datasheet, there is a protection died as seen in the image below.

On SN74AHCT123A datasheet page 7, there is no diode as seen in the snippet below.

Question is: 

1. Do I need to include the diode on SN74AHCT123A, for reliable operation? If yes, What diode parameters do you suggest?

2. What are the effects of having or not having protection diodes on RC pins?

3. Above what size capacitor is it needed for reliable AHCT123A designs?

Thank you in advance for your support.

Regards,

CSC

Regards,

CSC
  • 0
    TI__Guru* 96681 points

    Hi CSC,

    TI-CSC said:
    1. Do I need to include the diode on SN74AHCT123A, for reliable operation? If yes, What diode parameters do you suggest?

    The SN74LS123 datasheet is not referenced or used for other devices. Only the SN74AHCT123A datasheet applies to the SN74AHCT123A devices.

    An external diode can be added if you see a need in your system.

    TI-CSC said:
    2. What are the effects of having or not having protection diodes on RC pins?

    Diodes conduct current when forward biased. The device has internal diodes with maximum current ratings as shown:

    If more than 20mA is expected to be discharged through the diode due to the power supply ramp rate during power-off, then an external diode can be used to protect the internal diodes.

    TI-CSC said:
    3. Above what size capacitor is it needed for reliable AHCT123A designs?

    The SN74AHCT123A datasheet provides these recommended combinations of R and C: