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TXS0108E: PCB trace capacitive loading

Sanjeevi Thirumurugesan65
Prodigy 80 points
Part Number: TXS0108E
Other Parts Discussed in Thread: SN74LVC244A

Hello, 

I'm using 10 TXS ICs for level shifting between 3.3 V (VCCB) (microcontroller) and 1.2/1.5/1.8V (VCCA) and

4 TXS ICs for level shifting between 5V (VCCB)( a different microcontroller) and 1.2/1.5/1.8V  (VCCA). 

All lines are uni-directional from B to A: General Purpose Outputs from microncontrollers.

I'm starting to route the lines on 4-layer PCB and since the DUT and microcontrollers are in different boards and connectors have to be on the edge of the boards I'm designing, the traces are running a bit long: about 400mm for the longest line. Calculating for a regular PCB spec (Er: 4.2, Trace width: 6 mils, Trace height: 0.035mm, height of dielectric above return plane: 0.175mm.) The Capacitance for a 400mm line would be around 32pF. Is this too much? 

Thanks 

Sanjeevi

  • 0
    TI__Guru 59770 points
    Sanjeevi,

    32pF is good enough to work with the TXS for down translation.
    If the signals are unidirectional, i recommend using the SN74LVC244A for down translation instead of the TXS device.
  • 0 in reply to ShreyasRao
    Prodigy 80 points

    Thanks Shreyas. I was wondering, If I'm calculating the trace length and the lines are unidirectional, in my case (B to A), I need to worry about trace length only from the A port side to the DUT correct (Load side)?  In that case, the length wont be  more than 100mm. Am I correct in this?

    The SNxx won't work for me because the VCCA will go to 1.2V in some cases. 

    Thanks 

    Sanjeevi

  • 0 in reply to Sanjeevi Thirumurugesan65
    TI__Guru 59770 points
    Sanjeevi,

    Yes, having short trace length is ideal for TXB and being unidirectional, the load side will be critical.
    Sorry i overlooked the 1.2V. Yes, the LVC wont work if the supply goes down to 1.2V.