I am trying to regulate the current across an LED so that I can tightly control its intensity since I will be using this light as a source of illumination for a sensor.
I am using a CAT4002A-D LED driver, in the configuration shown in the schematic below.
I am powering it using 5V from a lab bench power supply, so I used 3*100 ohm resistors to drop the excess voltage.
Now when I set the Rset to 3.74kohm, as per the datasheet I should be getting 20mA whereas I get only around 4mA which is strange since the current drop in series and the resistors shouldn't affect that!!
But when I remove that resistance and use 3.3V as VDD (to prevent the LED from blowing up) then I get current as expected.
I don't understand why should a series resistor cause the current to drop?