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TLC5927 daisy chain question

mburger
Intellectual 360 points
Other Parts Discussed in Thread: TLC5927

Hello,

I my design I am planning to use up to 16 TLC5927 daisy chained, but I have some questions regarding such implementation:

1) Which is the capacitance each TLC5927 pin represents? I am worried that the clock, OE and LE signals of the MCU cannot drive the total capacitance represented by all 16 TLC's plus the PCB traces.

2) Does the TLC communication lines need a external pull up or the TLC has internal pull ups? In case a external one is needed, Which value is the most adequate? As you can see I will have 4 groups of 4 TLC's each, the distance between the SDO of one group to the SDI line of the next group can have a distance of 600mm (I think this distance can be the worst case scenario).

Attached is a small diagram showing the MCU-TLC interconnection, the problem here is that I do not know the exact trace length (the number shown is an approximation)

Best regards

Mark

  • 0
    TI__Genius 14890 points

    Mark,

    1.  I do not have the capacitance numbers handy, but am working on getting them.  I will post when they are available.

    2.  The SDI and CLK lines do not have internal pull-up resistors.  The SDO line is push/pull so the TLC5927 devices to be driven by another TLC5927 will not need any pull-up resistors.  The first device in the string may need a pull-up depending on your MCU.  Adding a pull-up or pull-down will not cause issues with the TLC5927.  What is the drive capability of your MCU?

    The LE pin does have a weak pull-down (~400kOhms at room temp) to prevent the pin from floating.  The OE pin has a weak puil-up (~400kOhms) for the same reason.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Intellectual 360 points

    Hello Dick,

     

    Sorry for the late reply, I just check you already reply me.

    1) You said that if a TLC is being driven a another TLC a pull up is not needed. Do you know if there is any distance constraint? My application will use 16 TLC but in 4 groups of 4 TLC, each group will interconnect between each other but I am estimating that the trace length between the SDO of one group to the SDI of the following one will be near 60 or 70 centimeters.

    2) I am using a Microchip MCU, the spec file says each I/O can sink 20mA. For a normal configuration it should be more than enough, but since I am using 16 TLC5927 plus all the PCB traces I am not sure if such current will be enough.

    Best regards

    Mark