SN74LVC08A: Y output issue

user4979899

Expert 3080 points

Part Number: SN74LVC08A

Hi Sir,

May I ask is there any input voltage requirement for SN74LVC08A to have Y pin output?

We using this SN74LVC08APWR IC for AND two PWM signal.

We found out that if the A/B input voltage about 2.4V, there will no Y output signal.

If we pull the A/B side voltage level to about 3V, there will have Y output signal.

From the SPEC, we are wondering why there are no output from Y pin, our voltage level at A/B side have meet the SPEC requirement which > 2V.

Below is the 3V waveform and 2.4V waveform and the schematic, FYI.

Could you please help to advice it?

Thank you

FAN8_PWM/FAN9_PWM voltage level is about 3V.
FAN10_PWM/FAN11_PWM voltage level is about 2.46V.

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 243920 points

The typical switching threshold is near V_CC / 2, so a failure to switch at 2.4 V could imply a higher supply voltage. Is that supply really 3.3 V, or 5 V?

0 user4979899 over 3 years ago in reply to Clemens Ladisch

TI__Expert 3080 points

Hi Clemens,

Yes, the VCC is 3.3V.

May I know is there any other idea from you to clarify it?

Thanks

0 Clemens Ladisch over 3 years ago in reply to user4979899

Guru 243920 points

Pins 9 and 10 (and any other currently unused inputs) should not be left open. I doubt that floating inputs can cause these symptoms, but cross currents or oscillations might affect the power supply negatively. Please ask for an oscilloscope trace including pin 14.

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SN74LVC08A: Y output issue