LDO's pass device size selection

[jutek]

hello

i should specify the pmos size as one of the first, to know what
capacity i'll have to drive, right?

so i have to do it for the lowest values of Vin. for example:

Vdrop_out=500mV
Vin=1.3V
Vout=0.8V

what is the lowest value of vg(opamp's output) which i can set to turn
on the PMOS. How close can i approach to zero? And when i turn off the
pass device how close can i approach to Vin?

These values will specify opamp's output range.

So if i know the overdrive voltage for the lowest vin case, i have to
specify W/L ratio to give me the max load current i want to. right?

It'll determine input capacity. I know reverse bulk polarization can
give more current with lower W/l so lower input capacity, but i consider
it later

please confirm if i'm right or correct if wrong.

regards

luke

 

[Chris Jones]

hello

i should specify the pmos size as one of the first, to know what
capacity i'll have to drive, right?

so i have to do it for the lowest values of Vin. for example:

Vdrop_out=500mV
Vin=1.3V
Vout=0.8V

what is the lowest value of vg(opamp's output) which i can set to turn
on the PMOS. How close can i approach to zero? And when i turn off the
pass device how close can i approach to Vin?

These values will specify opamp's output range.

So if i know the overdrive voltage for the lowest vin case, i have to
specify W/L ratio to give me the max load current i want to. right?

It'll determine input capacity. I know reverse bulk polarization can
give more current with lower W/l so lower input capacity, but i consider
it later

please confirm if i'm right or correct if wrong.

regards

luke

If you want good line rejection, then you might not want to let the pass
device go into the triode region, which would mean that you need a bigger
device so that it can handle the current with |Vgs| < |Vds+Vth|. I think
many designs allow the pass device to go into the triode region and
therefore have poor line rejection at intermediate and high frequencies.

Chris