LDO's pass device size selection
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Posted by jutek on January 6, 2006, 4:18 pm
Please log in for more thread options 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 | ||||||||||||||||
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Posted by Chris Jones on January 6, 2006, 11:30 pm
Please log in for more thread options jutek wrote: 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 | ||||||||||||||||
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>
> 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