POR with using NAND Logic

Hiroshi Sato

Other Parts Discussed in Thread: SN74HC132

Please give us your advice.

Our customer had designed Power On Reset circuit (when startup and once OUT is high, it will outputs continuously high.) with two NAND logic.

But, the circuit did not worked well , the output had oscillated, as figure I attached.

Then, the customer found that R34(1Kohm) had removed and the oscillation has stopped.

If possible, please tell us why the first customer's cirucuit had not worked good

and is there any issue with this usage(LVC00 with R34 removed) ?

Or please show us your recommended circuit example.

Thanks,

Hiro

over 10 years ago

0 Emrys Maier over 10 years ago

TI__Guru* 97016 points

Hiroshi-san,

When slow inputs are used, a part with Schmitt-trigger input circuitry is the best solution to avoid this type of problem. The SN74HC132 would be a better solution for this situation.

If you are interested to know exactly why these oscillations occur, please refer to our application report, "Implications of Slow or Floating CMOS Inputs."

0 Hiroshi Sato over 10 years ago in reply to Emrys Maier

Expert 2590 points

Hello Emrys-san,

Thank you for your quick replying.

And these are very useful comment and information for us.

Thanks and best regards,

Hiro

0 Hiroshi Sato over 10 years ago in reply to Emrys Maier

Expert 2590 points

Hello Emrys-san,

Please tell us just one more thing.
Could you tell us the reason why the customer's circuit worked good with R34 changed from 1kohm to 0ohm(dummy)
instead of they still use normal NAND logic(not schmitt-triggerd)?

Thanks and best regards,
Hiro

0 Emrys Maier over 10 years ago in reply to Hiroshi Sato

TI__Guru* 97016 points

Can you tell me what Texas Instruments parts are being used in this device? From the schematic, it appears that Toshiba NAND gates are being used, and I can't really explain how another company's parts work.

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POR with using NAND Logic