Looking inside the UBA2211, the most important components are the two MOSFET switches, HSPT and LSPT. Together they form the half bridge. The IC starts by preheating the burner. The preheat frequency is controlled such that the current through the resonant tank during preheat is kept constant. This preheat current is sensed by an RSENSE  resistor. The sense voltage is fed into the IC. The preheat time can be set by the value of capacitor CSW (typical 0.7 s). After preheat and ignition the same RSENSE resistor is used in the control loop and limits the lamp current in case the mains voltage rises above the nominal mains. The Rosc and Cosc components determine the starting and minimum frequency. During operation the frequency will be running at this minimum unless hard switching or the RSENSE control cause it to increase.