Schmitt Trigger and Driver, inverting vs non-inverting

Hi All



Regarding the attached diagram, to drive a N-channel Mosfet switch, what is the difference between using "combination of an inverting schmitt trigger & an inverting gate driver" and "combination of a non-inverting schmitt trigger & a non-inverting gate driver"?
I don't understand why many designers would prefer the inverting combination!

Thanks a lot for any hint, in advance
Politecnico

Politecnico said:

I don't understand why many designers would prefer the inverting combination!

Click to expand...

Inverting gates usually have less propagation delay than non-inverting gates: a simple non-inverting gate always consists of 2 inverting gates in series. This isn't necessarily valid for Schmitt-Trigger and driver circuits, but most probably the twice inverting combination would have lower propagation delay than the twice non-inverting combination.

Hi,

it´s hard to find non inverting schmittt trigger devices. like 74LVC1G17

Easy to find inverting schmitt triggers: 74HC14


Klaus

Hi Klaus

You mean in the market?
but it's available on Digi-key
What about the Propagation Delay issue?

Thanks

KlausST said:

Hi,

it´s hard to find non inverting schmittt trigger devices. like 74LVC1G17

Easy to find inverting schmitt triggers: 74HC14


Klaus

Click to expand...

Politecnico said:

Hi All

View attachment 110663

Regarding the attached diagram, to drive a N-channel Mosfet switch, what is the difference between using "combination of an inverting schmitt trigger & an inverting gate driver" and "combination of a non-inverting schmitt trigger & a non-inverting gate driver"?
I don't understand why many designers would prefer the inverting combination!

Thanks a lot for any hint, in advance
Politecnico

Click to expand...

ALL single stage transistor switches are Inverting when saturated.

Non-inverting is either 2 stages of inverting ...

... or Drain or Emitter followers, which are not saturating.

SunnySkyguy said:

ALL single stage transistor switches are Inverting when saturated.

Non-inverting is either 2 stages of inverting ...

... or Drain or Emitter followers, which are not saturating.

Click to expand...

Dear SunnySkyguy

Thanks a lot for the explanations
Just could you please explain more on "ALL single stage transistor switches are Inverting when saturated."?

Like Common Emitter , Collector out, MOSFETs switch with Common Source , Drain Out, so both are inverting

Look at any CMOS Logic schematic inside chip, all CMOS are complementary Common Source, so they are naturally inverting for drivers.

Attached is the Schmitt Trigger. notice ALL such devices are 3 stages of gain with positive feedback, (OEM secret) to implement the hysteresis between two stages. This shows 4 stages, but implementation using a non-inverting buffer is old school.

Also shown is the 74LVC14A inverter. Buffered types change the A to a B and have 3 stages of inversion. each stage has a linear gain of 10 and I have used this in some occasions for 1000 gain linear amp with AC coupled input and a feedback R.

SunnySkyguy said:

Like Common Emitter , Collector out, MOSFETs switch with Common Source , Drain Out, so both are inverting

Look at any CMOS Logic schematic inside chip, all CMOS are complementary Common Source, so they are naturally inverting for drivers.

Below is the 74LVC14A inverter.

Click to expand...

Thanks a lot for the explanation.
So, assuming we have a dc-dc buck converter, with a N-ch Mosfet as Switch, should the pulse train which drives the Switch be inverting or non-inverting?

Politecnico said:

Thanks a lot for the explanation.
So, assuming we have a dc-dc buck converter, with a N-ch Mosfet as Switch, should the pulse train which drives the Switch be inverting or non-inverting?

Click to expand...

read again until you understand where I pointed out the difference between saturating and non-saturating driver.

Both are used , and have tradeoffs for inherent deadband and dropout voltage.