CD14538B: Should pins 1 and 15 be grounded or left ungrounded?

Kyle Hansen

Part Number: CD14538B

Tool/software:

Hi all,

I am currently working on a design that previously used a Fairchild part that has since been discontinued. I switched to the CD14538BE and I started having some issues with it. There are now timing issues with the activation of the braking resistor. I am operating this chip at 12Vdc with an input of 10V as logic high and 0V as logic low. When I place my braking switch into the brake position, I am no longer receiving a signal out of the monostable that leads to a d-flip-flop that activates a MOSFET. This was working 100% of the time with the Fairchilid part and now with the TI part it works about 20% of the time. I am just not sure why I am getting such unreliable behavior. Any help would be greatly appreciated. Thank you for your time in advance.

30 days ago

0 Clemens Ladisch 30 days ago

Guru 262920 points

As shown in figure 1, pins 1 and 15 are internally grounded.

Please show the schematic of your circuit, and an oscilloscope trace of the relevant I/O pins.

0 Kyle Hansen 30 days ago in reply to Clemens Ladisch

Prodigy 20 points

The monostable does not always catch the falling edge of TR-. (Not every Negative going pulse gets translated into a Q pulse).

0 Clemens Ladisch 29 days ago in reply to Kyle Hansen

Guru 262920 points

Please show an oscilloscope trace of TR− and Q with such a missed edge.

0 Kyle Hansen 28 days ago in reply to Clemens Ladisch

Prodigy 20 points

Hi Clemens,

After some more testing being done on my circuit, I realized that it is the other half of the mono that is not working properly. Attached are oscilloscope waveforms and a picture of that part of the circuit.

Below is a waveform with it working properly with the onsemi part. (same pinout of the postit note above).

0 Clemens Ladisch 28 days ago in reply to Kyle Hansen

Guru 262920 points

There is nothing wrong with the input signal. Apparently, the chip is damaged. Try replacing it.

0 Kyle Hansen 28 days ago in reply to Clemens Ladisch

Prodigy 20 points

Unfortunately, this is an ongoing issue through multiple chips. The issue that I am having, with the TI chips, is that they work unreliably. About 20-50% of the time that I enable the brake circuit, they work, but the other 50--80% of the time they do not. There was never a reliability issue with the onsemi chips.

Logic

Logic forum

CD14538B: Should pins 1 and 15 be grounded or left ungrounded?