ysc wrote on Aug 15th, 2006, 6:04am:The power supply voltage can vary from 2.8v to 5.25v. The output of this circuit should be less than or equal to 4v. In other words this circuit does not do anything for input voltages less than 4.0v. when the input voltage crosses 4.0v this limits the output voltage to 4.0v .
I'm confused, as your last post contradicts your previous post...
anyway I think what you need is a regulated output supply of 4V. This means for supply voltages 4V to 5.5V you get a limited output at 4V max. For supply voltages 2.8V to 4V, the output is limited at the supply voltage.
You can design a low drop-out regulator using a banggap refence voltage and multplying it up with an oaamp and an output PMOs and resistive divider. Google voltage regulator or LDO for info.
I hope this answers what you want.