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FAN6755 Fiches technique(PDF) 6 Page - Fairchild Semiconductor |
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FAN6755 Fiches technique(HTML) 6 Page - Fairchild Semiconductor |
6 / 9 page AN-6755 APPLICATION NOTE © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.2 • 1/14/11 6 Output Driver / Soft Driving The output stage is a fast totem-pole gate driver capable of directly driving external MOSFETs. An internal Zener diode clamps the driver voltage under 18V to protect the MOSFET gate from over voltage. Due to integrated circuits that control the switching speed, the external resistor RG (Figure 13) may not be necessary to reduce switching noise. Figure 13. Gate Driver High / Low Line Compensation in VIN Pin The conventional pulse-by-pulse current-limiting scheme has a constant threshold for current limit comparator, which results in higher power limit for high line voltage. FAN6755 has a current-limit threshold that decreases as line voltage increases to make the actual power limit level almost constant over different line voltages of universal input range, as shown in Figure 14. In the FAN6755, the peak-current-limiting threshold is adjusted by the peak voltage of the VIN pin. When the circuit senses the bulk capacitor input voltage (VBulk) at the VIN pin and for R1, R2, and C1 set to 20MΩ (10MΩ +10MΩ), 160KΩ, and 2.2µF; then when VBulk is around 126V and the threshold voltage for current limit is around 0.83V as measured from the reference board design. Figure 14. VLimit Level vs. VIN VIN GND FAN6755 V Bulk R1 R2 C1 Figure 15. Input Voltage Compensation for Constant Output Power Limit Brownout Protection in VIN Pin Since the VIN pin is connected through a resistive divider to the bulk capacitor voltage, it can also be used for brownout protection. If the VIN voltage is less than 0.7V, the PWM output is shut off. As the VIN voltage reaches 0.9V, the PWM output is enabled again. The hysteresis window for ON/OFF is around 0.2V. The recommended values for R1, R2, and C1 are 20MΩ (10MΩ +10MΩ), 160KΩ, and 2.2µF. Using these values in the test board, the power supply is turned off at 62V (maximum load) and recovered at 80V. The VIN-ON and VIN-OFF are calculated by: 2 / ) R R R 9 . 0 ( ) RMS ( V 2 2 1 ON - IN (8) 2 / ) R R R 7 . 0 ( ) RMS ( V 2 2 1 OFF - IN (9) Recovery Function in VIN Pin The recovery function using the VIN pin is available in the FAN6755. When VIN is higher than 5.3V, FAN6755 stops operation, then restarts. Figure 16 shows the external circuit for secondary-side output OVP. If output voltage (VO) is higher than the Zener diode voltage (VZ), the VIN pin is pulled HIGH and the FAN6755 is in recovery. Figure 16. External Circuit for Second OVP |
Numéro de pièce similaire - FAN6755 |
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Description similaire - FAN6755 |
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