AN833 STMICROELECTRONICS | Alldatasheet
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PCB layout, active switch performance and also some particular circuital solutions. performs the frequency modulation function by using Pin16, that in this version is named FREQ-MOD. This pin (see fig. 1) is internally connected (with a current mirror) to an input of the multiplier. not affected by the line variations (see also Pin 16 description in AN628). Figure 1. Frequency Modulation with L4981B.
2 POLES
R fm is the programming current resistor. rectified voltage increases, so the minimum fsw occurs at the input peak voltage and current (see fig. b) The switching losses increase with the frequency (and obviously with the current). the higher current ripple produces an higher EMI. Figure 2. Modulation Frequency Normalized in a Half Cycle of the Mains Voltage. (eg. RFM = 1100kΩ , PFC application in terms of EMI, it is recommended to modify the starting frequency (oscillator).
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