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PCA9517: PCA9517 issue

Nanfang Liao
Intellectual 2085 points
Part Number: PCA9517
Other Parts Discussed in Thread: TCA9517

Hello Team,

My customer use PCA9517, the appliation is as below.

When the SDA signal transmit from slave to master, they test point C(green) and point D(blue) waveform as below(refer to waveform1), there is a big overshooting of point C, and the voltage reach 1.159V. After removing our PCA9517A, they also test the waveforms of point c(refer to waveform2), there is no overshooting. Can you tell me how to result in this big overshooting? Is there any risk of this overshooting for our PCA9517? thanks

Waveform1:

Waveform2:

Regards,

Nanfang

  • 0
    TI__Mastermind 21055 points

    Hello Nanfang,

    The PCA9517 is a buffer which means it has a static voltage off-set.  This is part of the circuitry that determine which side of the translator is generating the low and then passing it through to the apposing side.  It is different from a pass FET translator.  buffers decoupling the node capacitance from the a side and b side, while pass FET translators do not. 

    Can you give me more information about your setup?  How long are the traces or cables?  How are you measuring the signal?  Do you have a long ground lead.  What are you pull-up resistor sizes?  Do you know your capacitive load? 

    Have you looked at other E2E posts that speak about the static voltage off-set?  I just want to make sure you know how the buffers work or if I need to explain more.  Examples E2E posts that talk about static voltage offset are:

    https://e2e.ti.com/support/interface/i2c/f/390/p/548966/2014638#2014638

    I see that there seems to be a large overshoot, I am wondering if there is either a measurement error or if there are some serious parasitic inductances that might be causing this problem.

    -Francis Houde

  • 0 in reply to fhoude
    TI__Intellectual 2085 points

    Hello

    You can check the schematic below, the data is from U5(eeprom) to PCA9517, and the pcb trace length between U5 and our part is about 2inch, the test point is R66 and close to PCA9517, and R66 is very close to PCA9517, you can find the overshoot of pin6.

    The cusotmer also use TCA9517 to replace PCA9517 and do the same test, there is no overshoot of pin6(blue), you can check the waveform below. Can you help to give some comments about this issue? thanks

    Regards,

    Nanfang