LDO's pass device size selection

Jan 06, 2006 1 Replies
J

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


Vote
Twitter Facebook LinkedIn
C

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

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required