CD74AC05: Output of CD74AC05 is limited to 3V only

nghnjeet

Part Number: CD74AC05
Other Parts Discussed in Thread: SN74LVC06A

I am using CD74AC05 for switching ON the opto isolator. Since CD74AC05 is open drain, I am using 390 Ohm as pullup with 5 V. Supply to CD74AC05 is given as 3.3 V. Normally for any open drain or open collector, output is equal to pullup voltage which 5 V in this case irrespective of supply of IC. But here for low input voltage observed at output pin as 3 V which is causing sinking current from pullup and through 5 V. This result is switching on of opto LED and making it ON.

Is this behaviuor correct i.e. why CD74AC05 is giving 3 V at output pin for pull voltage of 5 V?

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 316690 points

The output itself is off, but there is an ESD protection diode from the pin to V_CC. The output is not overvoltage tolerant.

LVC devices (e.g., SN74LVC1/2/3G06, SN74LVC06A) have overvoltage-tolerant outputs.

0 nghnjeet over 4 years ago in reply to Clemens Ladisch

Prodigy 80 points

Does this mean that we can't given pull up voltage more than Vcc? If so, it must be mentioned in the datasheet to avoid any confusion.

0 Emrys Maier over 4 years ago in reply to nghnjeet

TI__Guru* 97046 points

Thanks for bringing this to our attention.

The datasheet is currently contradictory - showing that there's a clamp diode in the Output clamp current (abs max ratings table), but not showing it in the Output voltage (rec op conditions table).

I've submitted a ticket to have this fixed by updating V_O(max) to "Vcc" instead of "5.5V"

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CD74AC05: Output of CD74AC05 is limited to 3V only