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TCA9517A: Application schematic check

Part Number: TCA9517A
Other Parts Discussed in Thread: TCA9802, , TCA9509, TCA9517, TCA9617B, P82B96, P82B715, TCA4311A

Hi there:

I'd like to check an application schematic design and seek some suggestion

The attached file is the currently design from customer 

Here are some concern would like to your help to confirm. 

1. consideration of VILC: 0.45V; Is that correct connection for this application? 

2. If the TCA9517A is not suitable, how about the TCA9802 could satisfy this design? 

3. If we change into TCA9802, do we still need to consider the VILC this parameter? (and what's the value of it?) 

I2C repeater.PPTX

  • Hi Kay,

    "1. consideration of VILC: 0.45V; Is that correct connection for this application?"
    Regarding the top 9517 and the bottom 9517, are you hooking up the B sides together? If so this device does not support connection between the B sides.

    "2. If the TCA9517A is not suitable, how about the TCA9802 could satisfy this design? "
    If you are connecting B side to B side then this is also not allowed.

    "3. If we change into TCA9802, do we still need to consider the VILC this parameter? (and what's the value of it?) "
    This device has a IoLc which means the master/slaves pull down FET must have an Rdson which is about 150 ohms or less.

    If you are trying to find a replacement for that TCA9517 which allows for B side to B side connection then you should look at TCA9509. It is similar to the TCA980x family but has its current source on A side. Note, to use this device you should not use a pull up resistor on A side of this device.

    Thanks,
    -Bobby
  • Hi Bobby:

    Thanks again for clarification.

    The further more question would like to check with this design.

    1. MB side's TCA9518A/PCA9517A can't have design change but Board A is OK, Is that OK with this connection?

    MB (B-side of TCA9517A) to Board A (B-side of TCA9509) 

    2. still suggest MB side change into TCA509 , the connection

    MB (B-side of TCA9509 ) to Board A (B-side of TCA9509) , both MB & Board A (A-side) no need the pull-up resistors

  • Hey Kay,

    "MB (B-side of TCA9517A) to Board A (B-side of TCA9509) "
    This should be fine. I need to mention that TCA9509: VccB>VccA+1V (B side needs to be atleast 1V larger).
    This means U4 needs to be changed to TCA9509 for this to work.

    "MB (B-side of TCA9509 ) to Board A (B-side of TCA9509) , both MB & Board A (A-side) no need the pull-up resistors"
    There should not be pull up resistors on A side of TCA9509. This is the left side of U4 in your picture connected to 1.8V.
    B side (the 3.3V logic side) needs pull up resistors.
    Some advice, your schematic should include pull up resistors on A side but leave them as DNP (do not populate) just incase.

    Thanks,
    -Bobby
  • Hi Bobby:

    Thanks again, one more item about VILC spec of TCA9509 would like to need your help; now we get 150mV (typical) saying in datasheet.

    How to we check it's OK if the VOL at A-side? (says VOL > 200mV (150mV + 50mV )  is OK?)

  • Hello Kay,

    ViLc typical is 150mV in your application. A side generates 1mA IoL. Therefore Rdson of any devices on A side of this device must be lower than 150mV/1mA. Therefore Rdson of the master/slave should be =< 150 ohms.

    You can check if this is true by looking at the master and slave's VoL/IoL. For example if VoL=0.4V at 3mA then the impedance is about 133.33 ohms. You can expect this to be even lower because our device will source 1mA instead of 3mA.

    Let me know if you are still confused.

    -Bobby

  • Hi Bobby:

    Thanks, from RD's concern about an description of datasheet, would that be an issue if low level on the A-side is about 0.45V (such as 1.8V of U4, vol= 0.45V (max) )? 

  • Hey Kay,

    Are you able to provide information on what device is being connected to A side of our device?

    To me, this shouldn't be an issue because VoL is generated by IoL which our device supplies on the A side.

    VoL of 0.45(max) is under a certain condition, what is IoL when VoL is 0.45? I suspect it is 3mA or greater. With a lower IoL we can expect the VoL max to be lower. The pull down FET of the master or slave on A side is likely in the linear region of operation which if that is the case then I would expect VoL to be closer to 0.15V max. If the FET is in saturation then the Rdson of the FET should not change much.

    To our device, if A side is sourcing current it will know a master or slave is pulling the line low and send a low to B side. If B side is pulling low it will turn on the pull down FET on A side and sink current. The contention comes in (what you highlighted) when B side pulls low, A side FET turns on and sinks the 1mA of current. If a slave/master pulls low, it needs to steal the current from the A side FET so the device sees it is sourcing current rather than sinking current to pull back low on B side. Therefore the Rdson of the slave/master needs to be low enough to steal this current.

    This means VoL max is not what we actually care about. We care about Rdson(max). This is what I meant in my previous post.

    Thanks,
    -Bobby
  • Hi Bobby:

    Thanks again!

    As we know, there is a 1mA current source equal inside the TCA9509, does this current source with the tolerance (such as min/typ/max?) and does it has the impact with temperature performance?
  • Hi Bobby:
    Except Kay's question, we want to know why use internal current source in TCA9509 without use external pull up in TCA9509?
    Can you provide the all of I2C repeater and Buffer matrix table for reference that include Vilc side information to us?
    Thank you.

    Best Regards
    Jason
  • Jason,

    TCA9509 supplies current to the I2C bus on A side, it looks at the current from A side to see if the current is leaving the device or if it is being sunk by the TCA9509. If the current is being sunk by TCA9509 then it knows B side is pulling low. If current is leaving the device, then it knows a master/slave on A side is pulling low. Adding an external pull up resistor could increase the current being sunk from a master/slave and and raise its Rdson. A larger Rdson will result in less current it is capable of sinking. TCA9509 requires a minimum amount of Rdson to pass a low during a contention situation such as clock stretching or an ACK.

    TCA9509||RiLc = 133 ohms<--A side

    TCA980x||RiLc = 150 ohms<--B side

    TCA9517||ViLc = 400mV<--B side

    TCA9617B||ViLc = 400mV<--B side

    P82B96||ViL = 600mV<-- S side

    -------------------

    TCA4311A and P82B715 use dynamic offsets

    -Bobby