SN74HC125: VOH for 3.3VCC

Part Number: SN74HC125

Dear expert,

Customer use SN74HC125 in their application and face to VOH output voltage level issue.

They connect 3.3V for Vcc power supply, and measure output voltage is only 3.1V even there are no load connection at output side.

I have reference to E2E post below. If there are no load connection, IOH should be close to 0 and less voltage drop at output side.

https://e2e.ti.com/support/logic-group/logic/f/logic-forum/764984/faq-how-do-i-determine-the-output-voltage-voh-vol-or-output-current-ioh-iol-of-a-cmos-logic-device

My question are,

1.  Does 3.1V output voltage make sense if Vcc=3.3V?

2. if yes, what may cause output voltage drop if no load connection?

Ben

  • Hi Ben,

    The only thing I know of that will cause a voltage drop at the 'high' state output of a logic device is current being drawn through the device. Current doesn't have to come out of the pin to be drawn through the positive output driver though.

    0.2V of drop isn't much, but it indicates at least a few milliamps of current through the positive output driver transistor.

    It's possible that the device was damaged on the output, creating a weak short to ground through the ESD circuitry or low-side output driver.

    The positive output driver could also have been damaged - causing it to be more resistive then normal and dropping voltage with less current through it.

    If the device has been replaced and the issue still persists, my guess would be that there is some residual flux or other contaminant on the board that's causing a small current draw.

  • Hi Maler

    The output pin is been floated manually and we were measured on floated pin to check the voltage level. Otherwise he may think there is additional unexpected leakage current on the buffer output side and caused the issue.
    And The customer have checked on few DUTs and they have the same issue, so they think it is not the only case. 
    Besides the damage problem and ‘some residual flux or other contaminant’, is there any possibility to cause this issue from his perspective?

    Attached the measured waveform for signal input, signal output and its power supply voltage for you reference. You can see the supply voltage is ~3.25V and only ~3.01V on signal output.

    JTCK input

    3.3V : power supply

    JTCK output

    Many thanks

    Denny

  • Please ensure that none of the other inputs are floating.

    Does the output have a high impedance againt VCC and GND when you disable the output by forcing 3OE high?

  • Hi Clements

    The customer only reworked U3118.8 as floating manually, other pins of this device are still mounted on PCB and they are not floated.

    The customer are not sure if it has high impedance against VCC and GND since we didn’t disable output that OE#1-4 are connected to GND as enabled always.

    Many thanks

    Denny

  • Hi Clements

    Could you help to recommend?

    Many thanks

    Denny

  • As Emrys said, there is a current that flows somewhere.

    Did you measure VCC at the chip itself? The low voltage of the input signal will increase the power consumption.

  • Hi Clements

    I have provided waveform of VCC (3.3V : power supply).

    The customer measure the VCC_3V3(pin 14). Do you need more waveform to observe?

    If you need more information from the issue, please kindly suggest me.

    Many thanks

    Denny

  • Hi Denny,

    It would be best to see the supply and output operating together.

    Assuming the supply remains constant during operation, that's a drop of 0.24V at the output with a very light load (only the scope probe), which indicates to me that the output may be damaged. I'd recommend to start an FA with your supplier to get the device tested more thoroughly.