Question on Resistor Voltage dropper values for VCC supply of LED driver IC

[aaj]

Hi,

So I was designing the PCB for a 10W LED driver with the AL1676 controller(datasheet attached). From the datasheet, the application circuit shows that the driver takes its VCC supply through R1 and R2. The max value for VCC is 18V out of the approx 381V mains after rectification. Can someone help me figure out the formulas/calculations for these resistor droppers and for the value of capacitor C3.

The Evaluation module(attached) shows the values of the droppers as R2=R3=180k. The IC uses Startup Current: 100µA, Operation Current: 170µA (5KHs Switching) which means with 180K*2 and 170µA, they should be dropping only 61.2V out of the 381V supplied to it. So 319V is available at the VCC pin when it can handle only 18V. How does that work!!

[Ian.M]

Did you see the Zener symbol in the block diagram to the right of 'Vcc management', labelled 'Vcc Clamp'? Later on it says the typ. clamping voltage is 15.5V.

However the datasheet appears rather deficient as it doesn't specify a max. clamping current so all you can do is blindly use the dropper values from the eval board.