Should I calculate the lowest value of supply for LDO opamp?

[jutek]

LDO's opamp

hello

when i design an opamp for LDO, i should calculate it for the lowest value of supply?

and size of PMOS pass device i should also choose for these conditions to ensure max I load?

regards

 

[hoonsk]

Re: LDO's opamp

Yes, dropout condition, min VDD and max I to determine your power PMOS.

 

[sunking]

LDO's opamp

of course you should take care of it.

 

[jutek]

Re: LDO's opamp

and how to assure 500mV of drop-out voltage?

 

[hoonsk]

Re: LDO's opamp

and how to assure 500mV of drop-out voltage?

A couple of ways :
1.) Make the power transistor W/L big !
2.) Make sure the gate (CMOS) of power device can have wide swing i.e. the previous stage driving the gate is not limiting the swing.

 

[allan_guo]

Re: LDO's opamp

If you gain is high, the power mos can work in liner region.

 

[pillar_chen]

LDO's opamp

yes, there is no doubt that we want to get big regulated voltage, so the PMOS is made fat.