Low drop regulators

[vic]

Hi,

I'm wondering what are the principles used in low drop regulators.

I 'm trying to make one using standard components, I started by using a
zener and a resistance, the closer I could think to a low drop
regulator. However there is a constant current draw independant of the load.

I tried then to use a transistor with a fixed voltage at the base
(provided by a LED), and then using the fact that VBE is constant =
0.6V. However there is still a voltage drop across the transistor.

So, can one achieve almost 0V drop in some regulators ?

Thanks,
vic

 

[John Fields]

Hi,

I'm wondering what are the principles used in low drop regulators.

I 'm trying to make one using standard components, I started by using a
zener and a resistance, the closer I could think to a low drop
regulator. However there is a constant current draw independant of the load.

I tried then to use a transistor with a fixed voltage at the base
(provided by a LED), and then using the fact that VBE is constant =
0.6V. However there is still a voltage drop across the transistor.

So, can one achieve almost 0V drop in some regulators ?

---
Use a PNP for your series pass element (with the emitter going to
your raw +DC and the collector going to your load) and you'll be
able to get the voltage drop across the transistor to Vce(sat),
which is a lot better than than you can get with the transistor
wired as an emitter follower.

 

[John Popelish]

Hi,

I'm wondering what are the principles used in low drop regulators.

I 'm trying to make one using standard components, I started by using a
zener and a resistance, the closer I could think to a low drop
regulator. However there is a constant current draw independant of the
load.

I tried then to use a transistor with a fixed voltage at the base
(provided by a LED), and then using the fact that VBE is constant =
0.6V. However there is still a voltage drop across the transistor.

So, can one achieve almost 0V drop in some regulators ?

The low drop regulators all use a pass element (either bipolar
junction transistor or mosfet) operating in inverting mode. For a
positive regulator, this implies either a PNP transistor or a P
channel mosfet. This also means that the pass element is acting, not
as a follower (with its inherent fast negative feedback) but as a
voltage amplifier that needs external voltage feedback to control the
output voltage. It is also a lot harder to stabilize over a wide
range of output current, and in the case of the PNP pass element, adds
the base current requirement to the load current requirement to come
up with the total input current.

You might look at the equivalent schematics for some of the integrated
low drop regulators to see what kind of circuit performs this function.