Schematic review

WU YISYUAN

Expert 2165 points

Part Number: SN74LVC1G74

hi,

my design as below, but no output, VCC and VIN is normal.

could you give me some suggest in my schematic?

thanks.

over 5 years ago

0 Emrys Maier over 5 years ago

TI__Guru* 96591 points

Hi Wu,
I can't really tell what's going on from that tiny scope shot -- can you provide the full scope shot including timing and voltage settings?

From the tiny snippet you provided, it appears that the input has a pause at about Vcc/2, which would be very bad for a CMOS device like the SN74LVC1G74.

0 WU YISYUAN over 5 years ago in reply to Emrys Maier

Expert 2165 points

Hi,
I was update waveform already.
thanks.

0 Emrys Maier over 5 years ago in reply to WU YISYUAN

TI__Guru* 96591 points

Thanks Wu,
On the valid signal, I can clearly see that the clock input is stopping at about 1/2*Vcc for about 50-100us on most rising edges. This is _very bad_ for a CMOS device. It will cause excessive current, heat, oscillation, and possibly damage to the device.

On the 'invalid signal' scope shot, I can't really tell what's happening. your time scale is 500us, and this device responds within 10ns. Can you zoom in on the time scale and see what's happening when the output triggers?

0 WU YISYUAN over 5 years ago in reply to Emrys Maier

Expert 2165 points

Hi,
About your one answer, what's your recommended maximum VCC voltage?

thanks.

0 Emrys Maier over 5 years ago in reply to WU YISYUAN

TI__Guru* 96591 points

Hi Wu,

I'm not sure what that has to do with my answer, but I will be happy to get that for you.

The datasheet shows the recommended maximum supply voltage in the Recommended Operating Conditions table:

I think the information you may want is located here:

[FAQ] How does a slow or floating input affect a CMOS device?

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Schematic review

[FAQ] How does a slow or floating input affect a CMOS device?