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logic problem with TSX0108

Steve Landas
Prodigy 240 points
Other Parts Discussed in Thread: OMAP3530

Hi, I designed a board that interfaces with another existing board. The other board drives some logic signals with a MAX3002 level shifter. The new board also uses a level shifter TSX0108. Most signals are uni directional with the 0108 driving the 3002. But the 3002 drives one signal into the 0108 and every thing works fine. But during a elevated temperature test we disconverd that when the input to the 3002 is driven low the output follows, as it should but the output slowly drifts from 0 up to a value greater than the logic low threshold even though the 3002 input is held low. It looks capacitive, I have no pull up or pull down signals on the signals between the 3002 and 0108.

Any thoughts on this?

 

Steve

  • 0
    TI__Genius 9085 points

    Hi Steve,

    Have you tried swapping out the part and seeing if the behavior persists? What temperature are you testing at?

  • 0 in reply to Hattie
    Prodigy 240 points

    Hi Hattie,

     

    We are testing with the ambient at 50C inside the unit it is probably 55 to 56C. This problem is repeatable on multiple units. Right now I am lifitng the 3002 input and output pins and patching in an open collector buffer to see if the behavior changes.

     

    Steve

     

  • 0 in reply to Steve Landas
    TI__Genius 9085 points

    Please let me know the results of your testing. 

    At higher temps, I would expect a decrease in the on resistance of the internal switch that propagates the LOW to the output side. This reduced resistance would cause a LOWER VOL level. I'll check with design and see if my assumption is wrong. 

    Hmm, which package are you using? What is the output connecting to?

  • 0 in reply to Hattie
    Prodigy 240 points

    Hi Hattie,

     

    I am using the RGY package and the output of the 0108 goes to an OMAP3530 GPIO (1.8V),

     

    Steve

  • 0 in reply to Hattie
    Prodigy 240 points

    Hi Hattie,

     

    Could I email you some plots?

  • 0 in reply to Steve Landas
    TI__Genius 9085 points

    h-spetla@ti.com

     

  • 0 in reply to Hattie
    Prodigy 240 points

    Hi Hattie,

     

    Actually I think I understand the problem.  The MAX3002 drives the txs0108 which has an internal pullup but the max data sheet says it can only sink 20uA with a 3.3V supply I think the 0108 is providing about 80uA. I am going to replace the MAX3002 with a TXS0108. I have another option also, I kept the MAX chip to maintain backwards compatibility but that basically means I have two level shifters. I could eliminate the MAX3002 and just change the A side of the 0108 from 3.3V to 5V. I'll let you know how this turns out.

     

    Thanks

     

    Steve

  • 0 in reply to Steve Landas
    TI__Genius 9085 points

    Please feel free to contact me with further questions either here or via e-mail. 

    The external current sink the 0108 requires for a LOW is a combination of the drop across both the internal pull-up resistors (and any external pull-ups.) So the current of VccA/10kOhm AND VCCB /10kOhm. 

    We also have an app note on working with the TXS type translators - SCEA044 .

  • 0 in reply to Hattie
    Prodigy 120 points

    I am using Avnet Wilink8 adaptor to add TI 1837MOD card to Xilinx FPGA
    here is the adaptor
    here is the Wifi module
    adaptor has TSX0108 level shifter to translate 3.3v (from FPGA) to 1.8vv (TI Wifi module). VCCA is connected to 1.8v and VCCB to 3.3v and pin B7 of TSX is directly connected to FPGA  (FPGA pull resistor is disabled)
    All the output from FPGA (3.3v) are translated correctlry to 1.8v. however the signal coming from Wifi module (1.8v) is not translated to 3.3v and it barely reaches 0.5v (the failing pin is B7)
    it seems the direction detection is not working fine. 
    any idea why this is happening (any flaw in the provided designs)

    --