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FAN6863WTY Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN6863WTY Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 16 page  © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6863W • Rev. 1.0.1 11 Operation Description Startup Operation Figure 24 shows a typical startup circuit and transformer auxiliary winding for a typical application. Before switching operation begins, FAN6863W consumes only startup current (typically 8µA) and the current supplied through the startup resistor charges the VDD capacitor (CDD). When VDD reaches turn-on voltage of 16V (VDD- ON), switching begins and the current consumed increases to 2mA. Power is then supplied from the transformer auxiliary winding. The large hysteresis of VDD (7V) provides more holdup time, which allows using a small capacitor for VDD. The startup resistor is typically connected to AC line for a fast reset of latch protection. Figure 24. Startup Circuit Green-Mode Operation The FAN6863W uses feedback voltage (VFB) as an indicator of the output load and modulates the PWM frequency, as shown in Figure 25, such that the switching frequency decreases as load decreases. In heavy-load conditions, the switching frequency is 65kHz. Once VFB decreases below VFB-N (2.5V), the PWM frequency starts to linearly decrease from 65kHz to 22.5kHz to reduce the switching losses. As VFB decreases below VFB-G (2.2V), the switching frequency is fixed at 22.5kHz and FAN6863W enters “deep” Green Mode, where the operating current decreases to 600µA (maximum), further reducing the standby power consumption. As VFB decreases below VFB-ZDC (1.6V), FAN6863W enters Burst-Mode operation. When VFB drops below VFB-ZDC, switching stops and the output voltage starts to drop, which causes the feedback voltage to rise. Once VFB rises above VFB-ZDC, switching resumes. Burst Mode alternately enables and disables switching, thereby reducing switching loss in Standby Mode, as shown in Figure 26. Figure 25. PWM Frequency Figure 26. Burst-Mode Operation Frequency Hopping EMI reduction is accomplished by frequency hopping, which spreads the energy over a wider frequency range than the bandwidth measured by the EMI test equipment. An internal frequency hopping circuit changes the switching frequency between 61.0kHz and 69.0kHz with a period of 4.4ms, as shown in Figure 27. It covers the whole frequency range in hopping function and shrinks the period with operation frequency proportionally. Figure 27. Frequency Hopping |
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