thanks, there is an output capacitor,its 10uF, - if you look at pages 16 and 17 of this datasheet, you can see the equations for the RHPZ and power stage pole of a buckboost LED driver..........(equations 17 and 18)
**broken link removed**
If you calculate it , you see that the RHPZ for this buckboost is 1340Hz, and the power stage pole frequency is 4440Hz.
..As you can see, that is a disaster for doing any kind of loop compensation.
The output capacitor cannot be any bigger due to cost, space and the internal ambient being too hot for bigger electrolytics.
You can see that the RHPZ is a disaster, its too low, due to the low dynamic impedance of the leds.....but...
...I actually think that putting the LEDs dynamic resistance into those feedback loop equations is the wrong thing to do, I think that Resistors R13 and R18 act as an "upper divider resistor"...and for the circuit of the top post, I believe that the LEDs dynamic resistance should not be a factor in the feedback loop equations....do you agree?