choosing power sully switcher

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I have a 3.3 V in that i will convert,so basically what is better switcher or Ldo?

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Convert to what?

If you want to convert to 12V, then you need a switcher (boost) converter.

If you want to convert to 2.5V, then you can use a linear regulator, or a switcher. You determine which one to select based on current.

For example, conversion of 3.3V to 2.5V using a linear regulator involves a voltage drop of 3.3V - 2.5V = 0.8V over the linear regulator. The linear regulator will dissipate P = VI = 0.8V * I, where I is your load current. If your circuit needs 1A, then you will dissipate P = 0.8W. If you look in the linear regulator data sheet, it will tell you the thermal resistance of the package for a given PCB layout. Lets say its 60-degrees C per W. This means that at 25C room temperature, your linear regulator die temperature will be T = 25C + 0.8 W * 60C/W = 73C. You then need to check what the maximum die temperature allowed by the linear regulator is. If its 75C, then you do not have much margin in your system.

As a typical rule of thumb, if your load current is under 500mA, just use a linear regulator, if its above 1A, use a switcher, and if its between 500mA and 1A, use either, depending on what the circuit is, and your preferences.

Cheers,

Dave