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CPC9909 Datasheet(PDF) 8 Page - IXYS Corporation |
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CPC9909 Datasheet(HTML) 8 Page - IXYS Corporation |
8 / 12 page ![]() INTEGRATED CIRCUITS DIVISION CPC9909 8 www.ixysic.com R03 The switching frequency is determined by: Where: • D = duty cycle • t off = Off-time In general, switching frequency selection is based on the inductor size, controller power dissipation, and the input filter capacitor. The typical off-line LED driver switching frequency, fS, is between 30 kHz and 120 kHz. This operating range gives the designer a reasonable compromise between switching losses and inductor size. The internal off-time one-shot has an accuracy of ±20%. The figure below shows the RT resistor selection for the desired off-time. 2.8 Inductor Design The inductor value is defined by the LED/inductor ripple current, minimum off time, and the output voltage. The minimum off time is determined by the duty cycle and switching frequency. The duty cycle is given by: Where: • V LEDstring is the LED string voltage at the desired average LED current. • V in(min) is the minimum DC input voltage. The minimum inductor value for a given ripple current is: Where: • I L = Ripple Current The inductor peak current is given by: 2.9 Gate Output Drive The CPC9909 uses an internal gate drive circuit to turn on and off an external power MOSFET. The gate driver can drive a variety of MOSFETs. For a typical off-line application, the total MOSFET gate charge will be less than 25nC. FS 1 D – toff ------------- = R T vs Off-Time 0 5 10 15 20 25 30 35 40 45 0 500 1000 1500 2000 2500 3000 R T (KΩ) t off ( μS) D VLEDstring Vin min -------------------------- = LMIN VLEDstring I L -------------------------- toff = ILPeak ILED 0.5IL + = |
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