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TXB0304: output reflection and oscillation when output trace is 30cm long.

Joyce Li
Intellectual 2170 points
Part Number: TXB0304

Hi Dear expert,

My customer uses TXB0304 on their board. Below is the schematic. 

I/OVLx is input signal, coming from FPGA, for each input, there is 20K resistor to GND which is not shown below. I/OVCCx is output, going to RF device working as a control signal. Near the RF device, customer also connect a 10pF cap to GND. The output trace is 30cm long. The input trace is also very long, 15cm.

We met two problems, one we have known the root cause. The issue is there is ringing at the output signal. It is due to long PCB trace which have large reflection, I suggest customer to remove the 10uf caps to GND near the RF device, and add the serial resistor at the output, we increase the resistor to 1.5K and the ringing is off. However, it make the low to high and high to low slew rate very low. 

See below pic: Before removing cap and add 1.5K resistor:

After removing the cap and add 1.5K resistor:

Can you please confirm the root cause we think is correct? Do we have ways to make the high to low and low to high slew rate higher?

Second issue we have found is that sometimes, the output will show sine wave like below:

If we use probe to touch the output, the output will become correct level. but the input wave is looking like sine wave.

Do you know why output looks like sine wave?

thanks for your reply!

BR, Joyce 

  • 0
    TI__Mastermind 45835 points
    Hi Joyce,

    Having a heavy capacitive load will cause the slew rate to be slow. I don't recommend them to have a 10uF cap on the output (10pF should be fine but I'm not sure which is the correct value). Having a long trace like that will increase the parasitic elements meaning there would be inductance and capacitance causing that ringing. Adding a resistor should help but you will create a bigger RC delay by adding such a large series resistor.

    I'm not sure what you mean by " If we use probe to touch the output, the output will become correct level. but the input wave is looking like sine wave.". The oscillations may be caused by the pull-downs. The datasheet recommends higher than 20 kohm and they used 20 kohm. I'd recommend making them larger or removing them all together.
  • 0 in reply to Dylan Hubbard
    TI__Intellectual 2170 points

    Hi Dylan,

    thanks a lot for your reply.

    For the first issue, we are clear about the root cause, like you mentioned.

    For the second issue, the output level is not constant 3.3 V, instead it looks like sine wave. It happens with some probability, after customer removed the parallel 10uF at the load, the probability is much lower. As I wrote in my original mail, " If we use probe to touch the output, the output will become correct level. but the input wave is looking like sine wave.". Customer told me when using probe to test output waveform, the waveform will change from sine wave to correct level. But if now probing the input pin, input pin waveform will become sinewave.

    Customer also tried removing input 20kohm pull down resistor, no change for the issue. From the tests, it is related to the output decaps. Do you know the root cause of sinewave?

    Thanks for your reply.

    Joyce

  • 0 in reply to Joyce Li
    TI__Mastermind 45835 points
    Hi Joyce,

    Could they probe both the input and output for me so I can see both in a scope shot? This will help me better understand what is going on.
  • 0 in reply to Dylan Hubbard
    TI__Intellectual 2170 points

    Hi Dylan,

    Please see below snapshot. C2 is input waveform, C1 is output, 

    The test condition is: we toggle input level from low to high to low to high, the TXB0304 input will show sine wave like below.

    Then we use probe to measure output waveform, then we use C1 as trigger signal, as soon as the probe touches the output pin, it triggers, and C1 will becomes a low level, and C2 will also recover to normal low level.

  • 0 in reply to Joyce Li
    TI__Mastermind 45835 points
    Hi Joyce,

    I think I follow what the issue is, correct me if I'm wrong:

    You send a pulse into the input of the device, which causes that input to oscillate at a really high frequency. When you probe the output the oscillations stop.

    It would help me to see both the output and input for the whole duration of the issue. I would like to see the original pulse and the start of the oscillation and see how the output reacts during that time. The schematic provided doesn't show much so I don't know what is connected to our device. I don't know any aspects of the PCB or devices connected to our device. The RF device, what frequency is it running at? Could it be coupling to the really long traces? Does the FPGA have enough drive strength? For the TXB device, the FPGA will need to have at least +- 3mA of drive strength.