ICMR for differential amplifier

[niya7820]

hii,
how can ICMR can be decided for designing error amplifier in LDO...????
Vdd=3.3V
on what does icmr depends and how can we decide its value.please reply.
Thank you

 

[SIDDHARTHA HAZRA]

Hi niya,

The ICMR (i.e. the reference in LDO) is generally lower than or equals to the output voltage of the LDO, because the feedback voltage (same as the reference) can be easily generated by using a simple resistor divider.
Say the output is 2.5V, so Vref can be anything below it. But lower the Vref lower is the beta value and so lower is the loop-gain. To increase loop gain you need a multistage op-amp which will need stabilization. So sometimes using higher beta may help. Also the input diff-pair of the error amplifier is decided sometimes based on PSRR requirement. So that might also effect the choice of ICMR.
Generally, since a bang-gap reference is available at 1.2V or below upto 0.5V so anything in that range can be selected.

Hope this helps