The 74HC14N Chip

[NC Cams]

Just because a fet can be driven by 5v, that does not mean that it can be properly or quickly driven to FULL enhancement via a 5v HC/TTL device. The HC's are specifically built for FAST swithing and/or logic interfacting, not fet driving.

From personal experience, we did a LOT of experimentation with FET driving and learned a lot in the process. Yes, TTL chips can/will drive a logic level fet - so can 324 op amps. HOWEVER, when you start to pull current or ask for fast switching or need to prevent "shoot thru", the simple TTL IC or an op amp won't drive them fast or hard enough. This is where fet drivers become CRITICAL.

Moreover, 5V logical level fets will work at 5V but they are NOT as fully enhanced as they can be if you operate them at 10V - check the enhancement curves offered by the manufacturer. The amount of heat that say an IRLZ40 generates switched at 10 amps with 5Vgs is noticeably HIGHER than the same device with a 10Vgs triggering voltage.

Can you drive fets directly via HC TTL? Yes. Will they work better if "properly" driven? Yes. That's why they make and encourage the use of fet drivers.

BTW: if you want a low cost alternative, a simpe NPN/PNP totem pole (2N4401 + 2N4403) will suffice in place of a fet driver but take up as much board space as a fet driver and be more of a PITA to create.

Solution: have the 7414 trigger a true and dedicated fet driver.

[Mariss Freimanis]

I never use a 74HC14 as a driver; its output current is only a few milliamps. Rather, a 74HC14 is very useful as an input buffer and signal conditioner.

Let's say you have a limit switch input that has multiple '0' and '1' transitions every time it closes or opens. Or you have a switch input that should show closed, but because of interfering noise, shows 'open' for a few microseconds maybe thousands of times a second.

The 74HC14 comes to the rescue when it's used with an RC low-pass filter. Let's say you want to reject any signal pulse narrower than 10uS: A 10K resistor and 1nF capacitor has a 10uS RC time constant. This RC network is placed between the signal and the input of the 74HC14. The 74HC14 then 'squares up' the filtered signal to a single clean logic edge every time the input has a valid change. It rejects (doesn't 'see') any input change that lasts less than 10uS; noise is stripped from the signal.

Mariss