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SN74LV1T34: Incorrect output voltage level at logic high input

SU ZHANG
Prodigy 210 points
Part Number: SN74LV1T34

We're using SN74LV1T34 in one of our designs but not seeing correct logic high voltage level. with VCC (pin 5) tied to 1.8V, pin 3 to GND, pin 2 tied to FPGA logic output with 100K pull down, and output (pin 4) is connected to test point through 49-ohm. when the FPGA logic output is measured close to 2.5V (confirmed by measuring voltage level at pin 2 of the SN74LV1T34), we're seeing pin 4 output is only appx 1V.

Was SN74LV1T34 tested to work with 1.65V voltage level? we're running at 1.8V nominal. 

  • 0
    TI__Expert 7730 points

    Hello Su Zhang,

    Here are some debugging procedures:

    1) If the 49 Ω resistor is unpopulated, what is the voltage at pin 2?

    2) Can we lower the pull-down resistor to 10k and observe the output.

    Lastly, can we replace the device with a new chip? Yes, the SN74LV1T34 can work at 1.65V

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Prodigy 210 points

    Hi Joshua, 

    1) the pin 2 voltage is measured with output open. there is a mechanical switch at the output connected to 49-ohm. the switch is open when the output voltage is measured; 

    2) we were measuring 2.5V at the input. The low logic state is fine. does the pull-down resistor value affect the logic high level at the output? 

    we're seeing the same behavior on two assemblies with the same circuit. 

  • 0 in reply to SU ZHANG
    Prodigy 210 points

    I also forgot to ask whether the part requires pull-up at the output. The datasheet appears to indicate no need for pull-up, but the IBIS file has tri-state output. 

    |*******************************************************************************

    |                                 Model Selectors

    |*******************************************************************************

    |

    [Model Selector] INPUT

    input_1p8   1.8V  digital input

    input_2p5   2.5V  digital input

    input_3p3   3.3V  digital input

    input_5p0   5.0V  digital input

    |

    [Model Selector] OUTPUT

    output_1p8   1.8V  digital 3-state

    output_2p5   2.5V  digital 3-state

    output_3p3   3.3V  digital 3-state

    output_5p0   5.0V  digital 3-state

  • 0 in reply to SU ZHANG
    TI__Expert 7730 points

    Hello Su Zhang,

    The output does not require a pull-up. The device is a buffer based translator so the output will follow VCC when input thresholds are met.

    What is the current of VCC? Have we verified that pin 4 is not shorting to gnd? With the output open, the voltage we should see is 1.8V. I'm wondering if the output pin is shorting to ground somewhere in the circuit.

    Regards,

    Josh