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CD4025B, Vdd not powering up correctly

Other Parts Discussed in Thread: CD4025B

Hello,

I'm using the CD4025B NOR gate in a very simple circuit.  The problem I have is that it doesn't power up at the correct voltage on Vdd.  I use a voltage divider to reduce the supply voltage of around 24Vdc to around 5.3Vdc.  The majority of my boards work.  I will measure around 5.3Vdc on Vdd, but on the few that don't work the voltage at the Vdd pin seems to get stuck at 2.9V.  When I replace the CD4025B with a new on it fixes the power up problem.

Could the problem be with the IC ?  a few bad in a batch?

All the unused input pins are connected to 0V.  The unused output pins are open.

We tested 17 boards and two had this problem.  Then I tested 20 boards and 3 had this problem.

I can't figure out what's going on.

Also,  sometimes if I touch with my finger any one of the 3 input pins to the one NOR gate I'm using the circuit starts to work. The Vdd pin goes to 5.3Vdc.  But once power is removed and then turned back on the Vdd pin gets stuck at 2.9Vdc again.

Any comments or ideas on what's going on?

  • Hi Jim ,

    The voltage divider using resistor combination is well known method for voltage translation . however , it has its disadvantages of not being a stable supply as and when there is current consumption . The supply starts to droop with load .
    How good is the 24V power source . How much is current drawn when it is stuck at 2.9V . can you share schematics ?
    It is good that all the unused pins are held at known voltage rail and this is recommended,the outputs can be left open . Is this input directly to gnd or through a pulldown resistor ?
  • Thanks for your reply. I needed to have a pull down resistor on the input pins.  I will explain the circuit below.  I can't send a circuit diagram because it is going out for a patent.

    I have to use the voltage divider to drop the voltage to around 5V on Vdd.  I use a 5W 24V zener to supply the necessary amount of current to power the rest of the circuit.

    I had a 200 ohm resistor in series with a LED on the NOR gate input pins (only using one NOR gate out of the 3).  This cause an approximate 1.2V floating signal on the inputs.  I found 3 out of 24 boards had a problem with the output going high with this situation.  What made it even more weird was, when I replaced the NOR gate the circuit worked.  The output would be high even though the 3 inputs were floating around 1.2-1.3V.

    So I put a 15k resistor in parallel with the 200-ohm resistor and LED.  (The LED is only designed to run off of about 10mA).  This 15k resistor pulled down the input voltage to near 0V.  This 15k resistor did not have any effect on the light output intensity of the LED when it was switched in to the circuit (alarm condition).  That was one of my concerns.

    The boards that had the output going high without the 15K resistor installed now works correctly with it in the circuit.

    I do not have any more questions on this subject.  My circuit is working and it needed a pull down resistor since the other circuit connected to the inputs didn't keep it low enough.

  • Glad to hear it worked . yes , its never suggested to have floating inputs 'cos there is no guarantee on what voltage it takes on.
    There is good app note on inputs which shouldn't be left floating and the issues which could be caused due to it which is attached here .
    www.ti.com/.../scba004c.pdf