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TXB0108 8-channel logic level translator IC Output not working

ISHAN PATEL
Intellectual 320 points
Other Parts Discussed in Thread: TXB0108

I've attached the datasheet of schematic of logic level shifter pcb. I need to connect 3.3V compatible pins of RPi to 5V compatible pins of some other sensor.
As shown in the circuit of the datsheet on page no. 5, I connect 3.3V to VCCA, 5V to VCCB, GND to OE, RPi pin to any of AX. I expect 5V pin on corresponding BX pin when 3.3V is applied to AX pin. Instead there is 0V on it.  6761.level.pdf

  • 0
    TI__Guru 62460 points
    hi Ishan ,

    I will move this into translation forum .
  • 0
    TI__Expert 6335 points

    Ishan,

    I have a few possible explainations:

    1. Either VCCA or VCCB is at GND (Isolation Mode)

    This would keep the TXB0108 in high impedance. Can you double check that VCCA and VCCB voltage at the TXB0108 pins are 3.3V and 5.0V?

    2. The 3.3V driver is not providing enough current

    The TXB0108 has a 4kΩ impedance buffer that is used to hold the I/O pins either high or low. In order to overdrive the voltage hold of this buffer, the driver connected to the TXB0108 must be capable of sourcing at least  +/-2 mA. This is described in Section 8.3.2 (Input Driver Requirements) of the datasheet. Can you measure the current going into the TXB0108 Ax pins?

    3. The TXB0108 outputs are connected to a low impedance load

    The TXB0108 outputs are intended to drive high impedance (i.e. low current) loads. If the outputs are connected to a low impendace load (e.g. 50Ω), then the output voltage will drop significantly. This is implied in the VOH equation that is provided in Section 9.2.2. (Detailed Design Procedure) of the datasheet.

    If you keep the Bx pins floating, and a connect a multimeter to measure the voltage, do you see 5.0V?

    Let me know your results, and we can proceed from there. Thanks.

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Intellectual 320 points
    Floating BX pins give 0V and I've reviewed all three considerations of yours and the components abide by them certainly. On not giving any voltage to OE pin i.e., keeping it disconnected and all other connections the same as mentioned in description of question I get a V of 2.86 V on output side and 1.96 V if I connect red led and 220 ohm resistor across it.

    Regards,

    Ishan
  • 0 in reply to ISHAN PATEL
    TI__Expert 6335 points
    Ishan,

    Sorry to hear that. Let's continue to debug:

    The OE pin should never be disconnected/floating. It should be driven either high (normal operation) or low (disables outputs). Can you confirm that "R1" (10k resistor) on the PCB is installed?

    The TXB0108 output does not have the drive strength to support a LED and 220 ohm resistor. LEDs require several mA of current to sufficiently conduct, which the TXB0108 is not capable of providing.

    Could you please try to execute the test shown below? This will verify that the TXB0108 device is at least operational:

    1. Connect the OE pin to VCCA (R1 must be installed, or you can directly connect OE to VCC instead).
    2. Power up VCCA = 3.3V
    3. Power up VCCB = 5.0V
    4. Measure voltage on OE pin. It should be at least (0.65)*(3.3V) = 2.145V. What do you measure?
    5. Apply 3.3V on Ax pin
    6. Measure voltage on Bx pin with a multimeter (nothing else connected to Bx pin). What do you measure?

    Best Regards,
    Nirav