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what passive deivce mean in LDO?

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chang830

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ldo linear region saturation

Hi,
In LDO design, the output stage is a big PMOS device acting as a scurrent source. It is always called "passive deivce". Why it is called passive deivice? It is coompared to the active deivce?

thanks for the explainations.
 

ldo linear region

Hi chang830, often the PMOS device is called "pass device", not "passing device", it's just mean the pass element, how do you think about it?
 

This PMOS in LDO can be regarded as a pass transistor of a voltage-controlled current source.
As such, it is like having a load-insensitive (ideally) source resistance (which is a passive element but non-linear). Hence you can also regard this a passive.
 

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    chang830

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In LDO, the power MOSFET acts as a resistor(always operating in the linear region), so you can name it as a passive component.
 

to liuyonggen_1:you are totally wrong, in the active region.
 

pass device or pass fet
 

niezimei said:
to liuyonggen_1:you are totally wrong, in the active region.
Click to expand...

i am sorry, i know little about LDO.
could you explain why the PowerMosfet operate in active region?
does the PowerMosfet sometimes operate in saturation region, sometimes operate in linear region??
 

liuyonggen_1 said:
niezimei said:
to liuyonggen_1:you are totally wrong, in the active region.
Click to expand...

i am sorry, i know little about LDO.
could you explain why the PowerMosfet operate in active region?
does the PowerMosfet sometimes operate in saturation region, sometimes operate in linear region??
Click to expand...

It always works in active regio.
 

Your must make a mistake . It should be "pass device"
 

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    chang830

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fanrong said:
Your must make a mistake . It should be "pass device"
Click to expand...

Yes, it should be called pass device instead of passive device.

I found few articel call it passive device, but I believe it is not canonical.

Thanks fanrong!

Also apologize for the mistake and thanks all of you..
 

Hi liuyonggen_1

Almost all LDO pass device operate in saturation region instead of linear region. Some recent authurs
published papers operating LDO pass device in linear region to minimize device area. However, for pass device in linear region, it hurts PSRR as linear region will directly couples any noise from gate or input source to the output, almost no PSRR at all.
 

the minimum voltage to keep the power MOSFET work in the saturation region, because if the power mosfet leave saturation region ,it will not supply constant current to the load
 

pass device bc all the power pass through it
 

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