As I mentioned before, this circuit is a battery charger with programable output current (control voltage fed to FB - feedback - through D2) and voltage limit set around 5.5V (control voltage from this branch is fed to FB through D4) ..
As you can see, both control voltages, one from current control, one from voltage control, "meet" at the FB pin and whichoine is higher takes control over the LM2576 ..
Without anything connected to output terminals, there is no current flowing through Rsense(0.05Ω) so the voltage on the anode of D2 will be low and the LM2576 will "try" to generate as high output voltage as possible .. but, there is another control voltage, generated by voltage divider (R5-9.1k/R8-1k) and this control voltage (fed through D4) will actually set the output voltage level to:
Vout * [1kΩ/(9.1kΩ+1kΩ)] +0.7V(D4) = Vref(1.25V)
Then: Vout ≈ 5.5V
As far as the current control is concerned, current flow is sensed on R=0.05Ω and amplified by 1/2 LM358. The output is fed to 2/2 LM358 and compared with the control voltage from D/A converter: the higher the voltage from the D/A (the lower the FB voltage) and the higher the output voltage (- read: current) supplied to the output ..
Regards,
IanP