LDO pass device question

[devop]

hi,
If design a ldo with pmos pass device.I use Vsat<Vdropout to calculate the W/L of the pass device.is that right?
why can we use the pass device in the linear region?when Vg of the pmos equal to zero.the Vdropout will smaller
thanks

 

[safwatonline]

PMOS in linear gives lower rejection

 

[devop]

You are always the good man to answer my question.thanks very much

and happy new year!!!!!!!

 

[devop]

then I want to know how to define the Vdropout?
If Vsat=200mv,and Vout=3v.Imax=100mA,I use Vsat<Vdropout to calculate the W/L.
then simulate the circuit,when Vin=3.05V,the circuit can also provide 100mA current,Vout=3V.but the Pass device has gone to linear region.Can I say the Vdropout<50mV???????

 

[safwatonline]

i don't get ur info correctly as from what u said u want Vout=3V so if u work till Vin=3.05 then the dropOut=50mV, so if u have this case and ur specs are fulfilled (PSR, Load regulation, Line regulation, PM) then of course u can say the DropOut=50mV (even if the Mpass is in linear), if u want to have the Mpass in saturation then yes u need to increase the Mpass size

 

[devop]

I got your mean.Vdropout is the minum voltage between Vin and Vout when the other spec meet,no matter the pass device works in saturation region or not.
so if we just talk about the Vdropout and leave other spec alone,it's meanless

 

[devop]

So,I have a question when measure the Vdropout
the picture shows how to measure the Vdropout,It's the minim voltage between Vin and Vout,do not care about other spec.And I think the pass device here must in linear region.Can anyone explain?

 

[safwatonline]

as i said the difference is called DropOut voltage BUT u sure dont care about the difference itself u only care about the difference in the range that u r regulating.
like in ur figure at the small Vin the out is following the input i.e. it is not regulating at all ! , so if this is acceptable (like in case of small supply u dont have to regulate it but u have to provide a small Drop-Out voltage) then this is ur system and specs.
hope i didnt confuse u.
regards

 

[mallikarjun]

when output is follwing the input then its is not a regulator, its just resistor in triode region. Is that what you mean?

 

[zyyang]

does your LDO's psr pass the spec, i don't think so.

 

[safwatonline]

when output is follwing the input then its is not a regulator, its just resistor in triode region. Is that what you mean?

yep, regulating means that ur output follows a "Clean Reference" and is not affected by the input.
when the output follows the input its a "Buffer"

 

[devop]

"The picture illustrates how to measure the Vdropout of the LDO.The second(from left to right) arrow is the Vdropout,but it's not easy to get the point,so we measure the first arrow,the length is equal to the second one."
I get it from a paper,but I am not sure about that ,so I pop out the question .

 

[safwatonline]

"The picture illustrates how to measure the Vdropout of the LDO.The second(from left to right) arrow is the Vdropout,but it's not easy to get the point,so we measure the first arrow,the length is equal to the second one."
I get it from a paper,but I am not sure about that ,so I pop out the question .

hello,
from what i understand from this sentence is that the Drop-Out voltage is the difference between the Vin and Vout at the point which the output "stops" regulating and starts to follow the input (referred in the paper by the second arrow from the left) but since he cannot get this point accuratly he says that the drop out will be the same as the one in the first.
i am not sure if the two voltages are the same (DropOut1 == DropOut2) , but from the sentence u can understand that the dropout voltage is the one at the second arrow from the left ( just before the regulator stops regulating)

seems like i confused u more "Sorry"

 

[devop]

But I think the pass device has gone to linear region at the second arrow,with low psrr
So I doubt the measure method