the state of nmos or pmos in LDO

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berryfan

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hi,all
In LDO ,there is one mos which provide current to load.I'm adoubting that need I confirm that the state of mos is always in saturation?
when load current is very low ,it's must not in "saturation region"
thanks all ~

Added after 16 minutes:

I have the nmos pass transistor,i get someone points:it must in saturation.
i agree this point .but I really mean is that:my state appears when light load: vgs<vth vds >vdsat,is it ok ? it's in saturation,but not good vdsat
 

For a NMOS pass transistor, we would ideally like it to be in linear mode, but most of the time we have no choice as for it to be in the linear mode, the gate drive must be at least one threshold above the line voltage.

Therefore, it is in saturation most of the time, where you would need at least one VGS of dropout. The smaller the load current would only result in a smaller gate overdrive.

Lastly, for LDO, unless we have another voltage rail above the line voltage, we usually use a PMOS pass transistor for LDO, as we can easily operate it in linear mode.
 

checkmate,i mean that:should I keep the vgs>vth?in every load current
 

berryfan said:
checkmate,i mean that:should I keep the vgs>vth?in every load current
Click to expand...
You mean that your pass transistor went into cutoff? That implies that you have failed to satisfy the minimum dropout!
However, this should occur with heavy loads and not light loads. If it occurs in your case with light loads, then you should re-check your feedback loop. Under the lightest load of zero current, the NMOS should still be in saturation with zero vdsat.
 

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