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FC9920 Datasheet(PDF) 4 Page  First Silicon Co., Ltd 

FC9920 Datasheet(HTML) 4 Page  First Silicon Co., Ltd 
4 / 7 page FC9920 2012. 10. 31 4/7 Revision No : 0 0.7 0.6 0.5 0.4 0.3 0.2 0.1 When the switching MOSFET turns on, the capacitance CP is discharged into the DRAIN pin of the IC. The discharge current is limited to about 210mA typically. However, it may become lower at increased junction temperature. The duration of the leading edge current spike can be estimated as: TSPIKE = [(VIN · CP) / (ISAT)] + trr (4) In order to avoid false triggering of the current sense comparator, CP must be minimized in accordance with the following expression: CP < ISAT · (TBLANK(MIN)  trr) / VIN(MAX) (5) where TBLANK(MIN) is the minimum blanking time of 200ns, and VIN(MAX) is the maximum instantaneous input voltage. . Estimating Power Loss Discharging the parasitic capacitance CP into the DRAIN pin of the FC9920 is responsible for the bulk of the switching power loss. It can be estimated using the following equation: PSWITCH = [(VIN2 · CP / 2) + VIN · ISAT · trr] · FS (6) where FS is the switching frequency, ISAT is the saturated DRAIN current of the FC9920. The switching loss is the greatest at the maximum input voltage. The switching frequency is given by the following: FS = (VIN – η1 · VO) / VIN · TOFF (7) where η is the efficiency of the power converter. When the FC9920 LED driver is powered from the fullwave rectified AC input, the switching power loss can be estimated as: PSWIYCH ≈ [1 / (2·T OFF)] · (VAC· P + 2·ISAT·trr) · (VAC  η1 · VO) (8) VAC is the input AC line voltage. The switching power loss associated with turnoff transitions of the DRAIN pin can be disregarded. Due to the large amount of parasitic capacitance connected to this switching node, the turnoff transition occurs essentially at zero voltage. Conduction power loss in the FC9920 can be calculated as: PCOND = (D · IO2 · RON) + [IDD · VIN · (1–D)] (9) where D = VO / (η · VIN) is the duty ratio, RON is the onresistance, IDD is the internal linear regulator current. When the LED driver is powered from the full wave rectified AC line input, the exact equation for calculating the conduction loss is more cumbersome. However, it can be estimated using the following equation: PCOND = (KC · IO2 ·RON) + (KD · IDD · VAC) (10) where VAC is the input AC line voltage. The coefficients KC and KD can be determined from the minimum duty ratio of the FC9920. Fig.1. Conduction Loss Coefficients KC and KD EMI Filter As with all offline converters, selecting an input filter is critical to obtaining good EMI. A switching side capacitor, albeit of small value, is necessary in order to ensure low impedance to the high frequency switching current of the converter. As a rule of thumb, this capacitor should be approximately 0.1  0.2µF/W of LED output power. A recommended input filter is shown in Figure 2 for the following design example. Design Example Let us design an FC9920 LED lamp driver meeting the following specification: Input: Universal AC, 85135VAC Output Current: 100mA Load: String of 24 LED (Power TOPLED OSRAM ® VF = 2.5V max. each) Step 1. Calculation L1. The output voltage VO =24 x VF =60V (max.). Use equation (1) assuming a 30% peaktopeak ripple. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 DM KD (DM) KC (DM) 
