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FAN100 Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN100 Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 16 page  © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN100 Rev. 1.0.2 11 Temperature Compensation Built-in temperature compensation provides constant voltage regulation over a wide range of temperature variation. This internal compensation current compensates the forward-voltage drop variation of the secondary side rectifier diode. Green-Mode Operation The FAN100 uses voltage regulation error amplifier output (VCOMV) 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 fixed at 42KHz. Once VCOMV decreases below 2.8V, the PWM frequency starts to linearly decrease from 42KHz to 550Hz to reduce the switching losses. As VCOMV decreases below 0.8V, the switching frequency is fixed at 550Hz and FAN100 enters into “deep green” mode, where the operating current reduces to 1mA, reducing the standby power consumption. Figure 25. Switching Frequency in Green Mode Leading-Edge Blanking (LEB) At the instant the MOSFET is turned on, a high-current spike occurs through the MOSFET, caused by primary- side capacitance and secondary-side rectifier reverse recovery. Excessive voltage across the RCS resistor can lead to premature turn-off of the MOSFET. FAN100 employs an internal leading edge blanking (LEB) circuit to inhibit the PWM comparator for a short time after the MOSFET turns on. External RC filtering is not required. 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. FAN100 has an internal frequency-hopping circuit that changes the switching frequency between 39.4kHz and 44.6kHz with a period of 3ms, as shown in Figure 26. Figure 26. Frequency Hopping Startup Figure 27 shows the typical startup circuit and transformer auxiliary winding for FAN100 application. Before FAN100 begins switching, it consumes only startup current (maximum 10μA) and the current supplied through the startup resistor charges the VDD capacitor (CDD). When VDD reaches turn-on voltage of 16V (VDD-ON), FAN100 begins switching, and the current consumed increases to 3.5mA. Then, the power required for FAN100 is supplied from the transformer auxiliary winding. The large hysteresis of VDD provides more hold-up time, which allows using small capacitor for VDD. Figure 27. Startup Circuit |
Número de pieza similar - FAN100 |
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Descripción similar - FAN100 |
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