Help: feedback in LDO - explanatios needed

[lokay]

Help: feedback in LDO

In a LDO system, the reference and feedback voltage is stable at a given output voltage. But the gate voltage of the pass element mos is different when the load current is different? how did this happen?

I think:when the input of the error op is stable,the ouput of the op is stable, But it's wrong, why? pls help me.

 

[Blackuni]

Re: Help: feedback in LDO

Hi,

do u mean vgs r voltage and gate w.r.t gnd?

Can u share u r schmatic/ blk dia?

Whn load is varied thn it will have impact over VDS. but howmuch it is makes the difference.

Thanks,

 

[LvW]

Re: Help: feedback in LDO

In a LDO system, the reference and feedback voltage is stable at a given output voltage. But the gate voltage of the pass element mos is different when the load current is different? how did this happen?
I think:when the input of the error op is stable,the ouput of the op is stable, But it's wrong, why? pls help me.

To avoid confusion or misunderstandings, please avoid the term "stable" when you mean "constant". A stable/unstable system is something different.
To your question: When the gate voltages changes, the feedback voltage (which is the cause for the change) is NOT constant. However, because the amplifier has a very large gain the change of the feedback voltage is rather small if compared with the gate voltage change. Did this answers your question ?

 

[lokay]

Re: Help: feedback in LDO

I got it, Thank you very much, LvW.

 

[surianova]

Re: Help: feedback in LDO

In a LDO system, the reference and feedback voltage is stable at a given output voltage. But the gate voltage of the pass element mos is different when the load current is different? how did this happen?

I think:when the input of the error op is stable,the ouput of the op is stable, But it's wrong, why? pls help me.

actuall very simple, when load change, the current change, then Vgs must change to sink that amount of current.