Hi,
I got this design from "TI Cook Book Q2 2010"
There are two issues I encountered:
1) When I used 220K for R1, R2 and R3 as suggested in the schematic, it could not startup the IC, the Vcc voltage of the IC was measured to be less the 10.5V. My input is 230V (AC). Once I decrease the total resistence of (R1 ,R2 and R3) to 120K, it started to work well.
Question: If 220k were used for R1, R2 and R3, according to the calculation((230*(2^0.5))/600K) , there is only about 0.5mA supply current for IC. How can the IC start up with this small supply current?
2) While it is working, the efficiency of the circuit varies a lot with different loads, for instance, efficiency was about 65% with 4 High Brightness LEDs, total output power is 4.5W
79.2% for 9 HB LEDs, with total power is 9.4 W.
Question: Can this circuit achieve high efficient (over 80%) with light load (3~6 LEDs)?
