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TLK110: On boot-up - PFBOUT slowly decays to 0.5V, Rbias unstable

Anthony Oliveti
Prodigy 80 points
Part Number: TLK110

Hello,

I'm using a pair of TLK110 chips to run ECAT communications, and I am seeing intermittent issues with the TLK110 responsible for ECAT-OUT.

In normal operation, the chip auto-negotiates to 100Mbps rate - Pin 27 sinks current, LED turns on, and we can establish communications with downstream devices.

In the failure mode, here's what I've observed:

1. Pin 27 does not indicate that the data rate was auto-negotiated to be 100Mbps. LED does not turn on.

2. Rbias shows an oscillation at ~1.5MHz for ~4 seconds, then settles to ~0.7V

3. PFBOUT rises to ~1.7V, then linearly decays to ~0.5V over a period of ~4 seconds 

The 3.3V supply comes up normally and is stable in both cases, and, when the chip boots up normally, PFBOUT and Rbias are stable within their normal operating voltage range.

Can you share what is the function of the internal 1.55V LDO (PFBOUT), Rbias (1.215V across 4.87kohm resistor), and any idea why we might be seeing this behavior from the chip?

Thanks,

Anthony Oliveti

  • 0
    TI__Mastermind 26320 points

    Hi,

    TLK110 internally needs 1.55V supply to run the various components.  It has internal LDO to generate 1.55V from 3.3V. This 1.55V is available on PFBOUT and then supplied to modules thru PFBIN1 and PFBIN2.

    Few questions:

    1. Are you using single supply or two supply options ( providing 1.55V) externally ?

    2. If you are using single supply, can you try feeding 1.55V externally to PFBIN1 and PFBIN2 ?

    2. How many boards are you observing this issue ? All boards or some of them ?

    3. You mentioned issue observed is intermittent ? means same board some time links-up and some-time not ?

    4. Can you Share Phy schematics ?

    Regards,
    Geet

  • 0 in reply to Geet Modi
    Prodigy 80 points

    Hello Geet,

    Thanks for your response.

    1. Single supply.

    2. I will try this test.

    3. We've seen this issue on 2 boards out of a batch of 24. And yes that is correct, sometimes the board links up and sometimes it does not.

    4. Please see the attached schematic clip.

    Regards,

    Anthony Oliveti

  • 0 in reply to Geet Modi
    Prodigy 80 points
    Hello again Geet,

    Feeding an external 1.55V PSU to PFBIN1 and PFBIN2 seems to resolve the issue.
    Rbias is stable, and I am able to consistently establish ECAT comms on a PCBA that was previous intermittently failing.

    What is the significance of this Rbias pin being 1.215V+/- 5%?
    What do you suggest next? Is there any known causes for the internal 1.55V regulator to be unstable?

    Regards,
    Anthony Oliveti
  • 0 in reply to Anthony Oliveti
    TI__Mastermind 26320 points
    Hi Anthony,

    We have not observed instability issues with internal LDO. Further,

    a. this part did not work at all or got damaged with time ( say after testing few hours/days).
    b. Any handling issue you can think of during assembly ?

    Regards,
    Geet
  • 0 in reply to Geet Modi
    Prodigy 80 points
    Hello Geet,

    a. As far as I can tell, it never functioned normally. It was always intermittently failing to establish communication.
    b. Not that I can think of.

    One more piece of information for your consideration:
    With an external 1.55V source powering PFBIN1/2, I was able to mimic the response of the internal regulator in the "bad state".
    This did not cause the 1.5MHz oscillation at the Rbias pin. Have you ever observed an oscillation of this type on Rbias?

    Regards,
    Anthony Oliveti
  • 0 in reply to Anthony Oliveti
    TI__Mastermind 23675 points
    Hi Anthony,

    We have never seen an oscillation on the RBIAS pin.
    Can you send us the date codes on the top of a 'good' and 'bad' PHY?

    RBIAS is for our current mirror. If this is not maintained, our line driver gets thrown out of whack.
    RBIAS and PFBOUT are critical for proper operation.