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Original question:

TCA9803: external pull-up at B side

TCA9803: TCA9803 Abnormal waveform

Richard Yang1
Intellectual 1220 points
Part Number: TCA9803
Other Parts Discussed in Thread: TCA9548A, TCA9509

Hi Team:

    customer side meet an issue with TCA9803 when work with PCA9617+TCA9548, So Can you please help to comment for the questions as refer to the attached slides   thanks!

TCA9803 issue.pptx

  • 0
    TI__Mastermind 47328 points

    Hi Richard,

    1: TCA9548 is not with isolated function right? 

    Answer: I think you are getting at "is this a true buffer?" TCA9548 is a I2C switch, so this is not true buffering when the channel is ON - does not have isolated function except for disabling a channel. 

    2: in this topology, TCA9548 is a go-through solution? It do not have any impact on the signal from PCA9617? 

    Answer: TCA9548A is a I2C switch. Internally, it implements a passFET to connect the main I2C channel to a combination of 8 downstream I2C channels. There is no voltage offset or re-driving circuitry with this device. The only difference between the input and output signal of this device would be a slight voltage drop across Rds_on of the passFET. 

    Question2: when the master signal flow through PCA9617 + TCA9548 + TCA9803, why there is so much abnormal spike?

    Answer: TCA9803 cannot have external sources of current present on the B-side of the device. This includes any static voltage offset or the adding of pull-up resistors which could potentially hinder the device from satisfying IILC current requirements in order for a low to be passed properly through the device. 

    1: even if there is 0.5V low voltage plat from PCA9617 + TCA9548, but low voltage threshold of TCA9803 is 0.99V, it shall be no problem with this topology, right?

    TCA9803 cannot operate properly with SVO present on the B-side port of the device. The modified solution provided probably will not work correctly due to SVO present on PCA9617.

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    TI__Intellectual 1220 points

    Hi Tyler:

     Thanks very much for your quick reply and comment.

    so from your viewpoint, what is our suggested solution?  to see if TCA9509 is OK to replace TCA9803?  Thus in the board-B side, there is a solution of TCA9509+PCA9617. 

  • 0 in reply to Richard Yang1
    TI__Mastermind 47328 points

    Hi Richard,

    TCA9509 as a replacement device to the TCA9803 seems like it would work. TCA9803 would need pull-up resistors present on the B-side of the buffer. This would be fine since I believe there are pull-ups present on both sides of the TCA9548A. 

    I am also still trying to understand how this system is currently working with two PCA9617B B-sides connected together since they both implement static voltage offset. I would think there would be some type of oscillation occurring because of the B-sides being tied together. 

    We could offer the TCA9803 as replacement to 9509, but I think the error may be coming from the improper installation of the NXP devices. 

    Regards,

    Tyler