LDO dropout point dependence

Thread Starter


Joined Apr 23, 2015
I designed an LDO whose output is 1V and a 200mV dropout in mind. Thus I use a 1.2V supply upon sizing the transistors.
I've managed to arrive at the specifications I have set. The only thing that is confusing me is the DC Vout VS Vin graph. (I cannot upload the file, says Error uploading. I dunno why.) Here is my understanding:
So we can divide Vout VS Vin graph into dropout region, regulating region, and off region. Wherein at dropout region, the pass element acts as a resistor and thus we have a linear response of Vout VS Vin. In the regulating region, the error amplifier already controls the pass element that acts as a variable resistor. And in off region, the pass element is in cutoff which makes Vout drop to around 0V.
The thing is the dropout point (where Vout starts to become 1V) is not actually at Vin=1.2V but in a lower Vin value (around 1.1V). I'm wondering where does this dropout point depend? What parameter will I change if I want to move the graph from left or right?
Attempt of solution: Is the output swing of the error amplifier the cause of this regions of Vout VS Vin? increasing the swing by means of increasing the gain would expand the range of the regulating region?
Please enlighten me.
Thank you.