Electronic – HC vs LVC for 3.3V Logic


I am designing a circuit in which Vdd=3.3V. I have checked datasheets of hex inverters 74HC04 and 74LVC04 and both of these can work good at 3.3V logic circuits. If I have to select one of them then which one is recommended?

Best Answer

74LVCxx is designed to work at 3.3 volts, with a working range of 1.8 to 3.6 volts. 74HCxx can work at 3.3 volts, with a range of 2 to 5.5 volts, and is considered as being "5 volt logic". The 74HCxx series is limited to ~20MHZ but has strong output drivers with a +/- 25 mA limit. We used them to drive bi-color LED's on the high-side. With proper decoupling capacitors and light loads ground bounce is minimal.

The HCT series was designed to be compatible with older 74LSxx logic which without pull-up resistor could not source a voltage over Vcc - 1.2 volts. A logic 'high' for the 74LSxx series was ~ 3.75 volts. HCT and ACT could work with that, but not 74HC or 74AC. A 4.7K or 10K pull-up resistor allowed for 'mixed' 5 volt logic families with various loading and drive capabilities.

The 74LVCxx series is 10 times faster. Rise and fall times are a couple of nanoseconds. Ground bounce can be significant if data outputs have most any capacitive loading. A more elaborate decoupling scheme is needed. Termination resistors may be needed.

I would avoid the speed Demon 74LVCxx in favor of the much more tame 74HCxx / 74HCTxx series, but the 74HCTxx is not meant to work with 3.3volt logic. The exception would be clocking rates above 20MHZ up to 200MHZ, but then the board layout is much more complex as well.