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CD74HC4050: buffer IC holds more current after input signal shut off

Part Number: CD74HC4050

Hello Guys,

Good day. Need your help in our customer concern on CD74HC4050E. Please see below in verbatim.

"I was using the CD74HC4050E buffer IC in between with a 100 Hz 5Vpk-to-pk 2.5V offset and 1% duty cycle input signal and a Darlington system input pin. Since the buffer IC Vcc = 5V, I set a power supply for it. The input signal and power supply were sharing the common ground. In addition, there is a 50 ohm pulldown resistor connected the input signal before the signal went to the buffer IC input. I left the buffer IC output open (Not Connect) to verify the output signal. There is a 0.1 uF capacitor connected between buffer IC Vcc pin and GND pin.

The power supply output current remained 0 Amp when input signal was off. And drain some current after input signal was on.
What I didn't understand was, after I turned off the input signal, the power supply current increased and stayed there until I turned off the power supply output."

Here is the schematic.


Thanks and regards,


  • Hey Art,

    I'm not fully following here. It sounds like the power supply was turned off to begin with and the input signal wasn't present. Following this a signal was applied to the inputs. This device doesn't support partial power down (ie signal at the inputs when Vcc = 0V) so the issue could be stemmed from there and some sort of back-powering of the device may be happening. Could you clarify this more?

    If I'm misunderstanding and the power supply is turned on to start, but no current is coming through to begin the test, could you confirm that the power supply isn't clamping the current?


  • Hello Rami,

    It seems the customer may have checked the waveform without the Vcc on, which can damage the part.

    I will let you know in case they provided other details.

    Thanks and regards,