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HONEYWELL QUESTION REGARDING 74HC595 ESD CLAMP DIODE RESPONSE TIME

WhiteHorse
Genius 4430 points

 Honeywell has the following question regarding AC logic driving HC logic.  Please see below.

LGC, Email Technical Support, www.ti.com, 74HC595

HC logic has protection diodes to 5V/GND.

When we do IBIS simulation, we see these clamping current and thereby limiting ringing, but on our real hardware, we have more ringing.

We are driving the HC logic from AC logic.

We infer that the IBIS model is showing clamping that the actual chip is not providing, at least not at the frequencies excited by the AC logic drive (about 400 MHz/ 2.5nS ringing period).

Questions:

1. can you say anything about the relative response times of the clamp diodes on AC logic vs HC logic (we believe that our devices are recent production).

 

Regards,

John

602-625-9406

jwiemeyer@arrow.com

  • 0
    TI__Mastermind 46480 points

    Hi John

      I am not sure about difference in response time of the diodes.

    The AC parts are high drive and will have fast edges that may cause ringing. The HC parts are low drive with slower edges whic should not ring much.

    ther are sloutions for ringing. one being use a part with slower edges, another you can use series resistors to slow the edge.

    If you could give me the part numbers i could make recomendations for less noisey parts that would be drop in.

    When you say 400Mhz are you talking about the noise or the singal you are trying to send through the device?

     

     

  • 0 in reply to Chris Cockrill
    Genius 4430 points

    HI Chris,

    I agree that AC logic has very fast transition times which is likkely the source of the issue the customer is having.  It appears that the customer is expecting the ESD diodes to absorbe the ringing.  I don't think think they are capable of damping the ringing.  I agree with your recommendations of devices with slower edges and series resistors at the driving source.  Either of these are a good way to reduce the ringing.  I don't know what specific AC part is being used to drive the HC logic, but I will find out and let you know.

    Regards,

    John