Low Drop Off voltage regulators

[walshlg]

How do these things work? From what I'm reading it appears tha you can get say 11.7V output from a 12V source at 1A -> is this correct? How does it work if you need 1.7V across the transistor minimum? I'm considering using this as a voltage follower in place of a power transistor as I could get a higher output voltage.

[I'm trying to make a fan speed controller buffer to handle multiple fans on a single controller line (eg there are 3 fans on my PC watercooling radiator) but I want to be able to get as close to 12V as possible for max speed when things get hot. Pulse width modulation (with 3 lead fans) makes the tach signal worthless so can't go that way btw]

 

[Speakerguy]

They use a different topology than the standard NPN pass transistor regulators (like LM317s and 7805s). It probably uses a single P type FET as the pass transistor.

 

[Nigel Goodwin]

They use a different topology than the standard NPN pass transistor regulators (like LM317s and 7805s). It probably uses a single P type FET as the pass transistor.

Or (more likely) a PNP transistor, as used in 12V portable TV's for decades.

 

[ericgibbs]

hi,
This was recently posted by a member. 'thespeakerguy'

Explains standard Vregs and LDO.

 

[Speakerguy]

actually that was me

 

[ericgibbs]

actually that was me

This was recently posted by a member. 'the speakerguy'

hi,
I did post the credits naming you..

Useful pdf.

 

[walshlg]

Thanks! I really liked that tutorial. I was just wondering about the physics of the gating etc as to how the PNP can turn on with such a small voltage drop. Sometimes I can understand such explanations like the basics of how transistors work, so I was just wondering what is different about this configuration. Just curious.

 

[Hero999]

If you use an n-channel MOSFET and a small boost converter so you can drive the gate at a higher voltage than the supply, you can get a very low drop out voltage.

It's a wonder that some ICs don't use this technique. There could be an on-chip voltage doubler so all you need to do is add a couple of ceramic capacitors to give an ultra low dropout regulator.