Having trouble understanding a PSU design.

[frasdoge]

Hi everyone, I've come across this excellent series from Dave on building your own Lab Power Supply.

I think I understand how both the constant current and constant voltage modes work, although when placed in series I don't understand how the constant current generator turns into a current limiter. The CC circuitry suddenly has no effect until the load starts drawing more than the set amount.

I've had a go at simulating them individually in LTSpice and they all work as intended, but I'm failing to see the logic behind why this happens.

I've noticed the SPICE model only works as intended as well if I simulate the power rails for the second opamp as well which I've attached. 

[stafil]

Blind leading the blind here.

But the way I understand it, is by connect the ADJ pin after the resistor, you "measure" the voltage after the voltage drop of the resistor and adjust the LDO output accordingly. The higher the current the more the voltage drop and the less the voltage feed to the ADJ pin.

[Manul]

Not sure if I understand your question correctly. Maybe you have a confusion about ideal and practical constant current source. Ideal current source will rise voltage as much as needed to achieve the set current. For eg. if we have ideal constant current source of 1A, and connect 1K resistor, the voltage will rise to 1000V to achieve that. In practice, constant current circuit is powered by some limited voltage source. So it may reach its limit of voltage output before it reaches set current and effectively becomes a voltage source itself. That is a practical current source. So if output does not draw enough current, CC section goes into voltage "starvation" mode, reaching its maximum possible output voltage and thats it, it stays there. CV is happy to regulate voltage, because it is getting enough input voltage from CC section output. If output is drawing current above CC set current, CC circuit goes out of voltage starvation and starts to work as an actual constant current source. And that causes CV section to go into input voltage starvation so it can not maintain output voltage regulation, so output voltage starts droping. Something like that.