@ EasyB
I never said that the lower mode is the cause for the saturation of the core ! I said that IF , in lower modes, you have a significant increase of current ,not only in led , but in the whole circuit , this is a sign that the inductor is not able to store , and give back the amount of energy required . Maybe I was not so clear in explanations, could be , English is not my native language...
First of all, " a sense resistor" is NOT the same thing as " a limit resistor" , and the difference between this two is about HOW they act in a driver.
A sense R , is "reading" the current , and the voltage across it , is sent to the Switching Regulator Controller , as a feed back , in order to hold the current constant. If the led is heating,it is gone "ask" for more current , but that means that the voltage in the feed back loop is going up , so the controller is gone reduce the duty-cycle to the gate of the FET , regardless of what duty-cycle is getting from generator ,to the point where the system is in good ballance. So , it is able to change , within some limits the duty-cycle. This is one of the limits I was talking about.
If , due to some reason , the feed back is exceeding the upper limit of voltage ( for ex. more than 0.25 V , depending on the type of the Controller ) , the internal architecture of it , is gone "latch", sending to the gate of FET a continuous pulse , as a 100% duty-cycle . In that moment the FET's gate is driven with a continuous voltage , and not pulses of variable duration.This is translated in loosing the modes.If the cause that generated the latch is not removed , you will have only the highest mode. That's why you can use a FET like a ON-OFF switch by connecting the gate to +V or -V , depending if it is a N or P channel FET.
Not all the drivers has these Switching Regulator Controller ( SRC) that could be designed for N channel or P channel FET transistors. Also , there are cheap SCRs and not so cheap ones , the last ones are designed with internal circuits , in order to prevent the latch . A more complex SCR requires more external components , so the cost of production will increase. So , the manufacturer are designing a driver as a final product , and not a one that could be improved by the customer...
About "limit Resistor" :
Some of drivers are just simple , Pulse Generator + FET with no feed back , and a Limit Resistor . The idea is that , at 100% duty-cycle , the sum of FET Rds on , plus the limit R, to limit the current to a safe value for the load :
V led= V supply - V r limit.
This type of drivers will not loose the UI, no matter how hard you drive them. Also you will see that the constant current is not so "constant".You can include here the , so much controversial DDdriver...
I'll continue tomorrow ..., damn , is already tomorrow , I need rest... See you...