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TXB0108

Saeb Abdallah
Intellectual 440 points
Other Parts Discussed in Thread: TXB0108

 I have a customer with the following question:

The TXB0108 port B (5V) is connected to AD7716 and AT24C16 and there are some 4.7K pullups on port B outputs. 

Port A (3.3V) is connected to a FPGA.  

According to the TXB0108 data sheet and application SCEA043, a strong pullup (< 50K ohms) on port B is not recommended.    

 

The legacy design engineer claimed he had tried that in the lab with 4.7Kohms pullup on port B outputs and they worked and did not see VOL problems (see attached bmp).

 

 

According to data sheet and app note, it should not work.  Do I miss anything……please help.

Thanks, Saeb

 

  • 0
    TI__Genius 9085 points

    The TXB drives with a 4kOhm buffer. A 4kOhm pull-up would create a voltage divider at the output of the device. This should cause the output to be ~1/2 VCC if the TXB device is driving low to the side with the pull-up.
    The VOL level problems would be seen on the B side (side with pull-ups) while the A side device is driving low. 
    Unfortunately the BMP did not attach.
    What signals are shown in the waveform? Which side is driving?
    Also, what is the purpose of the 4.7kOhm pull-ups?