SN74LVC1G123: rising/falling edge detector reliability problem

Hi Wesley,
I have notified the appropriate applications engineer to respond to you. Can you also provide the circuit for the relays/what is beyond "Pulse n out"?

Hi Emrys
Thanks for the reply.

The "Pulse n out" is connected directly to Radiall R570.432.100 coaxial switch terminal E1 and E2 respectively. The current draw on E1 and E2 are given as 800 uA at 5V in the datasheet.

Hi Emrys

Sorry for the long delay in reply. We decided to go with a completely different design based on the Atmel Tiny26 to generate the pulses to switch the relay.

I am still interested in finding the root cause of the failure.

With reference to your first guess; normally that would be the most likely cause.

In my case, the input to the relay is a TTL control, which is connected to a separate 12 VDC supply. 

The actual current draw from the Q output into the TTL control of the relay is about 5 uA.

Unfortunately it is difficult to see if the clamping diodes will make a difference because the problem is very difficult to reproduce. We have only been able to recreate the fault once in the lab with about 3 weeks of testing.

The fault occurred after the main power to the test bench failed.

Is it possible that the latch-up problem could be due to the large capacitor on the Rext/Cext pin to ground?

I read on another product datasheet that a latch up condition could be created if the Vcc drops off very quickly and the Rext/Cext capacitor stores enough charge to be above the Vcc voltage level.

I suppose the solution in this case would be to put a larger capacitor across Vcc and GND.

I would appreciate your further thoughts on the above

  

We do typically recommend adding a series resistance to the capacitor for anything larger than 0.1uF of ~51Ω. This is just to limit the discharge current through the MMV's internal circuitry.

Since the failure is happening with a system level event (ie power failure), it's likely that there's more going on than just this. It would be interesting to monitor the power rail, inputs and outputs of the SN74LVC1G123 during such a failure.

Hi Emrys
Thanks for the information. I actually did see the recommendation for the 51Ω resistor for other devices but I could not find anything in the datasheet that it applies specifically to our used device. Our capacitor used was 1uF, so I can definitely see a place to improve the design.

Can you double check from you side? Maybe I just missed it.

The other datasheet i am referring to above is SCHS123E (for CD54HC4538, CD74HC4538, CD54HCT4538, CD74HCT4538 page 10 specifically)

If it is the case, then maybe you can add this to the datasheet to prevent confusion in future.

With reference to the power rails, it was running directly off a bench power supply. The circuit was drawing 6mA. I did not see a rise in current during the fault and the failure seemed to be directly related to pin 7 of U4 (shown above). I did not put a scope on the voltage rails during the failure.

While the failure mode was present, I did measure the input and output pins with no pulses applied. I compared the levels to the working device. The only difference i found was that the voltage measured at pin 7 of U4 to ground was not as expected (2.3VDC) and pin 5 of U4 was constant 5VDC.

Kind regards,

Wesley

It's not explicitly stated in the datasheet, however it is good practice. I haven't ever seen large capacitors cause a problem - and it sounds like this issue was very rare and is probably specific to the system.

Hi Emrys

Thanks for the information. It looks like we won't be able to get the root cause without investing more resources.

I feel that I have learnt about the device during this process so I feel that the design can be improved when we use it again.

Can we close this discussion?

Thanks for visiting the forums - and good luck with the rest of your design!