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-   -   Difference between boot strap and level shift (http://www.diyaudio.com/forums/class-d/185918-difference-between-boot-strap-level-shift.html)

levelshift 27th March 2011 09:03 PM

Difference between boot strap and level shift
 
Hi. Is boot strap a kind of level shift or what's the difference? I see some circuits using both these methods, why? Thanks.

nigelwright7557 27th March 2011 09:12 PM

Boot strap is usually where positive feedback is used to increase the input impedance.

Pafi 27th March 2011 09:57 PM

In ClassD the bootstrap usually refers to a power supplying method of high side gate driver. Level shifting is a different thing, but often they go together.

nigelwright7557 27th March 2011 10:02 PM

Level shifting in class d is altering the levels of the signals to match the mosfet driver.
The output of the comparator is usually based around zero volts and the input to the mosfet driver is based around B-.

levelshift 27th March 2011 10:16 PM

Thanks but not enough to make me understand though, could you try to explain a bit more, treat me like a noob, cause that's what I am.

AP2 27th March 2011 10:29 PM

Hi,
Pafi says right on the bootstrap method.
Level shift is used when there is a potential difference between the stage of the mosfet and the stadium before (this is often referred to GND).
usually resolves with two bjet the low side MOSFET (VN)

nigelwright7557 27th March 2011 10:33 PM

Quote:

Originally Posted by AP2 (Post 2518671)
Hi,
Pafi says right on the bootstrap method.
Level shift is used when there is a potential difference between the stage of the mosfet and the stadium before (this is often referred to GND).
usually resolves with two bjet the low side MOSFET (VN)

This can usually be done with a single transistor stage.

AP2 27th March 2011 10:39 PM

It depends on what is the potential difference.
consider that this stage is critical for the transient response, and then must work with low load resistances. this (depending on the voltage difference that must absorb) the energy will dissipate.

nigelwright7557 27th March 2011 10:45 PM

With my discrete class d amp I had to shift grom +12v/0v to -35v/-20v and a simple transistor stage worked fine. I did put a diode to the -20v rail and a resistor in series with it to clip the voltage. Worked very well.
I have also seen another design with a very similar circuit.

levelshift 28th March 2011 08:15 AM

Ok, so a level shift is not necessary to drive the upper transistor, the boot strap is? And a level shift might also be needed in some cases? I'm sorry it's a bit hard to understand, all these English terms.. If anyone has time and wants, a picture might help. Sorry for being slow.


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