CD4072B 4 Input OR Gate - Output always HIGH

Roger Dallas

Other Parts Discussed in Thread: CD4072B

Hello,

I'm using the Cd4072B device, TSSOP-14 package. Operating with a 3.3V filtered and no noise supply. The device is bypassed

at pin 14 with a 100n ceramic cap.

Let's look at just device A. Pins 2,3,4,5 are connected to the circuit that senses a high condition (across a 3V LED) when a fault occurs. There is no fault, the LED's are off, the voltage is for all intents and purposes, LOW. Ans so it is at the all the input pins. Some typical noise at 0 to 40mV. Scoping theose inputs shows no noise or oscillation.

Pin 1 is lifted off the pad to ensure nothing could be affecting it. Yet, the output is HIGH. If I fault the detect circuits, that is, actually feed the LED voltage to one of the pins (actually 2.8V) it does not change the output. It's behaving as if there is a high on one of the inputs.

Connecting pin 1 to the pad, and trying to pull it to ground with a 1K resistor brings Vout to 2.9V, but essentially still HIGH. It really is....HIGH.

Can anyone explain this behavior? I've got a whole bunch of boards I need to use this device on, and no one else in the world makes a 4 input OR gate. Is this a single device problem, a batch issue, or...?

Any help, soon, would be very appreciated.

Regards,

Gary

over 8 years ago

0 Emrys Maier over 8 years ago

TI__Guru* 96591 points

Hi Gary,

Can you provide a schematic and scope shots of the input signals? The engineer that will respond to you is out of town until Monday, but having these will help him when he gets back. Also, is this happening with all of the CD4072B devices you have, or just one? I have not worked with this specific device, so I don't know how it behaves so close to it's minimum voltage limit.

0 Roger Dallas over 8 years ago in reply to Emrys Maier

Prodigy 160 points

Emyrs,

I didn't look before I leaped...posted...jumped...
As much care as I took when installing (using ESD protective mat and wrist strap), the device had 1 blown gate. Discovered by lifting/verifying 1 input at a time. It may have been this way when I received it, unknown. In any case, I installed a new device and all is well.

Sorry to bother and thanks for your reply (some things just aren't obvious in the face of reality !)

Gary

0 Roger Dallas over 8 years ago in reply to Roger Dallas

Prodigy 160 points

Was I thinking this was a device problem vs a bad chip? (nahhhhhh...not me.... 8--)

Thanks again,
Gary

0 ShreyasRao over 8 years ago in reply to Roger Dallas

TI__Guru 55970 points

Thanks for letting know ,glad it works well now .

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CD4072B 4 Input OR Gate - Output always HIGH