Sometimes I see they use schottky barrier in their circuits but I don't know when should I use them. really What is a schottky barrier when should we use them? e.g. look at this circuit:

How this circuit works?
Another question: Why schottky barriers are ultra-fast?

Here in the datasheet they have written :

How can we use them to protect our circuits? What does mean "Blocking diodes"?

 

Hi,

Schottky diodes have several advantages over regular Si diodes.
First, they drop less voltage. Typically 0.5v instead of 0.7v, which makes a big difference in some circuits.
Seconds, they recover much faster than regular Si diodes, so they are used in switching circuits.
The disadvantage is more reverse leakage, but it's not often a concern because it is still very acceptable.

A blocking diode would be a diode used to 'block' current in one direction but not the other. For example, on a solar panel where you dont want current to flow back into the array from the storage capacitors. A diode in series (any diode really) with the array would prevent this, and it could be one of several models such as 1N4004 which is a common Si rectifier diode. The only requirements being the current and voltage specs of the diode have to be selected properly for the application.

Another widely used application for the Schottky diode is in powered USB hubs, where the computer hub 5v line has Schottky diode in series with it. This prevents the computer from getting the 5v from the wall wart that came with the hub when it is used with the hub. The wall wart is then able to take over all the power demands for the hub rather than have the computer itself supply the power. If the wall wart is unplugged then the diode lets the 5v line connect to the externally plugged in USB device.
This is used in every USB hub i have seen that also comes with a wall wart for higher power, or has a jack on the side for such a wall wart.
Any diode there drops the voltage somewhat, but a Schottky drops it less than say a 1N4004 diode so it helps a little. In any case, that's a called a blocking diode too.

Schottky diodes are fast because of the way they are made, which makes them have less recovery time, much less. Recovery time is the time it takes for the diode to start acting like a diode again after it has been conducting (so it can block current again). The difference between a regular diode and the Schottky can be a lot too. Using a regular Si diode in a switching circuit at say 20kHz would be very bad, but a Schottky does this very well.

 

The circuit clamps the voltage of the VGA_CRT_DET line to between about +3.4V (where the diode 3-2 starts to conduct) and -0.4V (where diode 3-1 starts to conduct).

Schottky diodes have a lower forward voltage drop than a normal silicon diode and are have a much faster reverse recovery time.
See Wikipedia