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FAN6862 Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FAN6862 Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 15 page  © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6862 • Rev. 1.0.1 10 Operation Description Startup Operation Figure 21 shows a typical startup circuit and transformer auxiliary winding for a FAN6862 application. Before FAN6862 begins switching operation, it 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), FAN6862 begins switching and the current consumed increases to 3mA. Then, the power required is supplied from the transformer auxiliary winding. The large hysteresis of VDD (8.5V) 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 21. Startup Circuit Green-Mode Operation The FAN6862 uses feedback voltage (VFB) as an indicator of the output load and modulates the PWM frequency, as shown in Figure 22, such that the switching frequency decreases as load decreases. In heavy load conditions, the switching frequency is 65KHz. Once VFB decreases below VFB-N (2.2V), the PWM frequency starts to linearly decrease from 65KHz to 22.5kHz to reduce the switching losses. As VFB decreases below VFB-G (2.1V), the switching frequency is fixed at 22.5kHz and FAN6862 enters “deep” green mode, where the operating current decreases to 2.5mA (maximum), further reducing the standby power consumption. As VFB decreases below VFB-ZDC (1.7V), FAN6862 enters burst-mode operation. When VFB drops below VFB-ZDC, FAN6862 stops switching 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 23. V V FB -N FB -G FB V Frequency FB-ZDC V PWM Frequency 65kHz +4.2kHz -4.2kHz 22.5kHz +2.9kHz -2.9kHz Figure 22. PWM Frequency VFB (1.7V) Ids Vo Switching Disabled Switching Disabled VFB.ZDC Figure 23. 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 60.8kHz and 69.2kHz with a period of 4.4ms, as shown in Figure 24. Figure 24. Frequency Hopping |
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