Question on ideal and non-ideal Voltage Source

[freescale_bharat]

ideal voltage source

If there is ideal voltage source and load is connected to it, being the ideal voltage source it can source upto infinite current to the load and the voltage level will not drop.

Now coming to non-ideal voltage source or Voltage regulator; if the regulator is designed to source upto 100mA, it will regulate the output voltage till the load demands 100mA.
Let's say if load needs 110mA, regulator will loose its regulation and it will start droping to its earlier voltage.

My question is why does it drop? I am not looking for decoupling cap in the answer.

Thanks
-Bharat

 

[JoannesPaulus]

non ideal voltage source

Typically an LDO uses a PMOS pass transistor biased with a suitably low gate voltage. If the output current increases, the PMOS transistor needs an even lower gate voltage. At some point - usually more than 10% - the gate voltage cannot decrease any longer and the regulator stops regulating...