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Will PCA9306 be damaged if Vref2 is connected directly to 3.3v

Scott Markinson
Prodigy 30 points
Other Parts Discussed in Thread: PCA9306

Will the PCA9306 chip be damaged if Vref2 is connected directly to 3.3v?

Note Vref1 is tied to 1.8v.

We have since reworked the cards to tie pin 7 & 8 together and using 200K resistor up to 3.3v and most are operating with no problems, but a few cards are having an issue with this bus. 

We continue to investigate, but were wondering if the chip could have been damaged due to the incorrect connections.

  • 0
    TI__Mastermind 30280 points

    Oh no!

    Yes, it could potentially be damaged.

    The 200-kOhm resistor is for current limiting purposes, because VREF1 and VREF2 are essentially connected as a diode configured FET when VREF2=EN.

    Without the resistor, you essentially have a diode turned on from VREF2 to VREF1 and there are 3 possible outcomes: (1) VREF1 can't sink anymore current, (2) VREF2 can't source anymore current, or (3) the device will blow if neither power supply limits the current. 

    With the resistor, the current is limited by the equation: ILEAK = (VREF2 - (VREF1 + VT))/R where VT = 0.6V (approx.)

  • 0 in reply to Brian Berner
    Prodigy 30 points

    Thank you for this information. I do have a followup question.

    We have a number of cards that were used with the Vref2 incorrectly wired that were use for a while. We have now reworked them to correctly connect Vref2.  (interesting that most cards did work OK with the mis-wire, which delayed us finding the problem)

    Now that these are reworked, ee are finding that a few of these are working for a while in a system, then start to fail. We remove them and retest on the bench, often they are good on the bench, or after reflowing the rework.

    We then return card to its normal system where it works for a while, (like days to weeks), then starts to fail again.

    Do you think this be due to earlier damage, or would the damage be more permanent, us intermittent as we are seeing?

    Thanks

  • 0 in reply to Scott Markinson
    TI__Mastermind 30280 points

    I'm sorry Scott, I can't tell from your post.

    Did you replace the PCA9306 on the boards?

    Or did you just re-wire them with a pull-up resistor in between VREF2 and VCC2? (for simplicity's sake, I will refer to the pin as VREF and the power supply as VCC)

    Are VCC1 and VCC2 static voltages? Or can they change in the application? If the voltages change on these pins, it could explain your intermittent failure.

  • 0 in reply to Brian Berner
    Prodigy 30 points

    Originally we had the pins tied directly to there power rails (Vref2 to 3.3v;  Vref1 to 1.8v) no resistor. which we later found was wrong. We then reworked the cards to move the Vref1 connection to tie with pin 7 (EN), so have the 200K resistor in the path, as is in the TI ref design.

    After rework we are finding most boards now working just fine, but a few are having an intermittent issue where this bus stops working. After a power cycle it usually comes back OK, at least for a while.

    Each of our cards has 4 ports each using one of these devices, we generally only see one of these 4 parts failing on a given card.

    On two of the 4 cards(1 port each) with the problem (out of about 30 cards that were reworked, meaning 120 chips), we replaced the part and they are now working reliably again. 

    We are trying to estimate it the earlier mis-wiring could have damaged the chips causing a later problem, or if the issues are unrelated. 

    Its basically about 4 of 120 chips that we have seen the intermittent state on.

    Thanks