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SN74AHCT240: Why output of SN74AHCT240 oscillation greatly compared with SN74HCT240?

Emma Wang
Expert 5190 points
Part Number: SN74AHCT240
Other Parts Discussed in Thread: SN74HCT240, UCC23513

Tool/software:

Hi TEAM,

My customer want to upgrade the temperature range for their machine and the do some change from SN74HCT240 to SN74AHCT240.

But they found some output oscillation issue on output pin. I found there are some differences on output spec between these two devices. but I don't know which one is the biggest influencer this issue. Could you kindly tell me which differences on output structure will cause this ? and how to deal with the overstress on output?

Schematic

 

B-SN74AHCT240（125℃）ch2 input，ch4 output. positive voltage spike is 5.980V, over max VCC(5V)+0.5V.  The device is from SHE FAB.

C-SN74HCT240（85℃）ch1 is output, no obvious overstress. From RFAB

BR

Emma

  • 0
    Guru 316940 points

    Was this measured directly at the output pin?

    How long is the trace between U12 and R746? Was this measured with a ground clip or a ground spring (the latter would avoid oscillations inside the oscilloscope probe)?

  • 0 in reply to Clemens Ladisch
    TI__Expert 5190 points

    Clemens,

    They use ground clip.

    U12 and R746 is not on the same board, U9 is UCC23513, Switching frequency is 19.2KHz.

    the distance between U12 and R746 is about 10-15cm.

    and customer's question is the difference between the two devices' behaviors.

    BR

    Emma 

  • 0 in reply to Emma Wang
    Guru 316940 points

    Please measure with a ground spring; it would not make sense to protect against oscillations that are not actually there.

    Both devices show ringing, and if it is actually there, I suspect it happens in the cable between the boards. In general, I would recommend to use source termination, i.e., move R746 to pin 18, or add a resistor there. Also try increasing it to 22 Ω.

  • 0 in reply to Clemens Ladisch
    TI__Expert 5190 points

    Sorry I make a typo, they use Ground spring. And I also think move R746 to pin 18 is a good idea.

    BR

    Emma

  • 0 in reply to Clemens Ladisch
    TI__Expert 5190 points

    Clemens,

    Are there any die size or package difference between these two devices: 

    SN74AHCT240（125℃） 

    SN74HCT240（85℃）

    Since using same board, same test method,SN74AHCT240 has larger spike than SN74HCT240, customer guess the die to pin parasitic inductance is larger for SN74AHCT240 compared with SN74HCT240. 

    So if die size for SN74AHCT240 is smaller, then it makes sense, since they have same package.

    Emma

  • 0 in reply to Emma Wang
    Guru 316940 points

    They use different dies, and as far as I know, both got a new die design with the change to 300 mm wafers.