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TXB0104-Q1: Why the pullup resistor needs to be higher than 50kohm for the TXB0104?

Part Number: TXB0104-Q1
Other Parts Discussed in Thread: TXB0104, ADS7951, DRA829, SN74AXC4T774

HI Team,

My customer has several question regarding the TXB0104B-Q1 and could you please kindly help:

1. In datasheet 8.3.5, it mentioned that the TXB0104 is designed to drive capacitive loads of up to 70 pF, how this 70pF come up? and what could happen if there are more than 70pF capacitance attach on the I/O pins?

Customer once had the issue use TXB0104 between DRA829(processor) and the ADS7951(ADC) for level shifting, however the SDO waveform not right and the data from the ADC is not correct. They add 68pF on the SDO wire and fix the issue. That's why above question1 come up. 

2. In datasheet 8.3.5, it also mentioned that "If pullup or pulldown resistors are connected externally to the data I/Os, their values must be kept higher than 50 kΩ to ensure that they do not contend with the output drivers of the TXB0104",   Why the pullup resistor needs to be higher than 50kohm?

In datasheet 8.3.1, it says that "The typical output impedance during output transition is 70 Ω at VCCO = 1.2 V to 1.8 V, 50 Ω at VCCO = 1.8 V to 3.3 V, and 40 Ω at VCCO = 3.3 V to 5 V." It seems the output impedance is not that high, why the pullup or pulldown needs to be higher than 50kohm??

Thanks

  • As shown in section 8.1 of the datasheet, the output impedance is 4 kΩ. The one-shot circuits lower the impedance for a short time during output transitions to allow for higher frequencies. Also see A Guide to Voltage Translation With TXB-Type Translators.

    1. If the load is higher than 70 pF, the one-shot circuit might time out before that output has reached the desired voltage level. With the 4 kΩ driver, the remaining transition will be much slower.

    2. The pullup needs to be weak enough so that it does not conflict with the 4 kΩ impedance.

    What voltages are being used? If at most 3.3 V, the SN74AXC4T774, which has proper buffers, would be a better idea.

  • Hi Ted,

    Please refer to the app note on the effects of pullup resistors on the TXB device below:

    The series 4kohm resistor would form a voltage divider when there is external pullup or pulldown.