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## KA7541 Datasheet(PDF) 5 Page - Fairchild Semiconductor

 Part # KA7541 Description Simple Ballast Controller Download 14 Pages Scroll/Zoom 100% Manufacturer FAIRCHILD [Fairchild Semiconductor] Direct Link http://www.fairchildsemi.com Logo

## KA7541 Datasheet(HTML) 5 Page - Fairchild Semiconductor

 5 / 14 pageKA75415Start-up CircuitStart up current is supplied to the IC through the start up resistor (Rst). In order to reduce the power dissipation in Rst, the Rstis connected to the full wave rectified output voltage.The following equation can be used to calculate the size of RstThe size of start up capacitor (Cst) is normally decided in terms of the start up time and operating current build up time withauxiliary operating current source.The turn off snubber capacitor (Cq2) and two diodes (D1, D2) constitute the auxiliary operating current source for the IC. Thecharging current through the Cq2 flows into the IC and also charges the start-up capacitor. If the size of Cq2 is increased, theVCC voltage of the Cst is also increased.Figure 1. Start up circuitOscillatorThe gate drive output frequency is as half as that of the triangular waveform in timing capacitor (Ct) at pin #2. In normaloperating mode, the timing capacitor charging current is 50µA. The discharging current is seven times of the charging current(7× 50µA). The charging period of the timing capacitor is the on duty of the gate drive. The discharging period is the off dutyof the gate drive.The rising slope and falling slope of the triangular waveform are as following.Rising slope: dv / dt = i / C = 50µA / CtFalling slope: dv / dt = i / C = 7× 50µA / CtFor example, when the timing capacitor is 180pF,∆Tch = 6.69µ∆Tdis = 0.956µRstVin ac()2Vth st() max,–×Ist max,---------------------------------------------------------------------------<85210.5–×0.25103–×--------------------------------------440kΩ==PRStVin ac_max()2Vcc–⋅()2RSt------------------------------------------------------------------------0.5W≤=RSt2Vin ac_max()2Vcc–⋅()2×≥RSt260K≥260KRSt440K≤≤∴Q1Q2Cq2D1D2RectifierOutputRstCst+−To VCC(Pin 8)

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