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CD74HC221: Transition time

Ryuuichi machida
Guru 11505 points
Part Number: CD74HC221
Other Parts Discussed in Thread: SN74LV221A

Hello,

I would like you to confirm that CD74HC221 will receive damage or there is no problem for reliablity in following case.

--
* User input following signal to 9 pin("2An").

 Tr(rising time) : 60msec (0V to 5V)
 Tf(falling time) : 1msec (5V to 0V)
 (Vcc = 5V condition)

In this case, is there any issue for reliabity ?
--

According to following URL, there is description about such slow transition time.

http://www.ti.com/lit/wp/slla364a/slla364a.pdf

In this case, shoot-though current will occur and this will cause damage to device.
However, it seems that this document is described for "SN74" parts.
So,

1. Could you please confirm whether you can say same thing for "CD74" parts as well ?
2. If so, is there any data for shoot-though current for "HC" parts of "CD74" parts ?

Best Regards,

  • 0
    TI__Genius 16725 points
    Hello,

    The SN is a prefix we add to devices to denote that the device was developed and manufactured by TI. The CD prefix denotes that the part was developed by Harris Semiconductor and is now manufactured by TI. The shoot through current is an issue that occurs for push pull CMOS inputs.
    The transition rates listed are too slow and will cause damage to the device as mentioned in the appnote you attached.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    TI__Genius 16725 points

    Hi,

    If you need to have these slower inputs, then I recommend using the SN74LV221A. It has the same pinout, and comes with Schmitt trigger on all inputs. This allows for slow inputs on the device without any damage.

    Thanks!
    -Karan

  • 0 in reply to Karan Kotadia
    Guru 11505 points

    Hello Karan-san,

    Thank you for your reply.

    1.

    The shoot through current is an issue that occurs for push pull CMOS inputs.The transition rates listed are too slow and will cause damage to the device as mentioned in the appnote you attached.

    Does it mean that both "CD" parts and "SN" parts do NOT have resistor or something which can limit current such shoot through current ?

    (The longer the transition time, the more shoot through current will increase.)

    2.

    Thank you for sending alternative device. I will suggest this schmitt trigger type device.

    Best Regards,

     

  • 0 in reply to Ryuuichi machida
    TI__Genius 16725 points

    Hi,

    Here is what happens when the input input voltage is at the threshold voltage. As seen in the image, both the FETS turn on and the current shoots through from Vcc to GND. The current peaks at this threshold and so if the device has a slow input, the current is at this peak for a long period of time causing damage to the device. Now if you add a resister at Vcc, then not only do you drop the Vcc voltage required to operate the device you get losses and larger power consumption.

    Thanks!

    -Karan