mrboni:
I'm trying to understand the circuit in figure 12, on page 6 of this data sheet - http://www.onsemi.com/pub_link/Collateral/NSI45090JD-D.PDF'Dimming using PWM'
I want to use this constant current led driver with a ws2801 pwm driver (also cc) to allow me to control higher power leds with the ws2801, while retaining constant current functionality. I'm loaded with questions -
I think I understand this - you use the 2801 in constant voltage drive mode and use the NSI45090JD to set current and dissipate the extra power (rather than fry the WS2801).
Note that this device is a particularly poor constant current diode, reducing current to 65% of nominal at 2V, 20% of nominal at 0.5V, and only
properly entering constant current zone at 7V or above - pretty much unsuitable for this task. There are chips specifically designed for
providing constant current drive to LEDs which this isn't (despite what the datasheet claims).
I suggest telling us which LEDs you are thinking of and what power supply you will be using.
[edit: for an example of a decent LED driver http://www.onsemi.com/pub_link/Collateral/CAT4101-D.PDF ]