LM2576 Constant Current Circuit

[aiq25]

Hi. I was looking online and found an interesting design with a LM2576 constant voltage/constant current power supply design but I'm a bit confused on how the constant current part works and I was wondering if someone can help explain it.

Attached is the schematic. Here is the link to the project: https://www.pcbway.com/blog/technology/How_to_build_an_adjustable_switching_power_supply_using_LM2576_[Buck_Converter__CC_CV].html

I wanted to do a variant with this design but use an MCU for the control. I understand how the voltage regulator works but confused about the current regulation.

[Benta]

So...
Where's the load?
And what do four 0.5 ohm resistors in parallel connected to ground at both ends do?
And...
No, I won't continue.

[sleemanj]

In short, the 4 parallel resistors are the current shunt, the opamp applies a higher voltage to the feedback of  the lm2576 when the current increases above the setpoint, this "fools" the lm2576 into thinking the output voltage is too high so it reduces it, thereby reducing the current, and at some point equilibriam is found.

Take careful note of GND and LGND in the schematic, same symbol but different things. 

[aiq25]

So...
Where's the load?
And what do four 0.5 ohm resistors in parallel connected to ground at both ends do?

Those are the current sense resistor. The load is connected between "Vout" and "LGND".

And...
No, I won't continue.

No problem. You don't have to continue. Thanks for responding.

[John B]

The current sense resistors are connected between LGND and GND, so the load isn't regular GND referenced. The op amp compares the voltage at LGND to the inverting input. The output will be ~0V which causes the LM2576 to increase it's duty cycle, hence increase the output voltage and current and therefore increase the voltage at LGND until it reaches equilibrium. The op amp will have around 1.8V on it's output, which is approximately a diode drop plus the ~1.2V reference on the LM2576. I guess R7 sets the constant current, and R6 is some trimmer, which also gives the diode D2 a discharge path.

[sleemanj]

The output will be ~0V which causes the LM2576 to increase it's duty cycle

The diode prevents this.  This is a cv/cc circuit, cc only kicks in when the trigger is reached.

I guess R7 sets the constant current, and R6 is some trimmer, which also gives the diode D2 a discharge path.

R6 is the CV trimmer, it sets the voltage in absense of the CC limiting it (by pushing feedback higher).

[John B]

I meant the output of the op amp would be 0V as an initial condition until the loop takes control of the LM2576.