The half bridge and power MOSFETs are inside the IC generating a square wave between 40 kHz and 100 kHz with an amplitude equal to the voltage on CBUF. In the tank circuit Llamp is the impedance that determines the lamp current. The inductor Llamp has a value of 2-3 mH depending on lamp power and mains voltage. Any square wave consists of sine waves of the 1st , 3rd ,5th etc harmonics (Fourier transform). Only the 1st harmonic will pass the inductor, the higher ones are filtered out by the resonant tank inductor Llamp, giving close to a sine wave at the resonant tank node Llamp plus Clamp. The DC blocking capacitors take the DC component of the square wave, which is half the Vin bus voltage. The difference between the UBA2211 and UBA2213 is that the UBA2213 has a boost feature for a shorter run-up time; this will be discussed later.