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FAN6751MRMY Folha de dados(PDF) 11 Page - Fairchild Semiconductor |
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FAN6751MRMY Folha de dados(HTML) 11 Page - Fairchild Semiconductor |
11 / 13 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6751MR • Rev. 1.0.0 11 VDD Over-Voltage Protection (OVP) VDD over-voltage protection is built-in to prevent damage due to abnormal conditions. Once the VDD voltage is over the over-voltage protection voltage (VDD- OVP), and lasts for tD-VDDOVP, the PWM pulses are disabled until the VDD voltage drops below the UVLO, then starts again. Over-voltage conditions are usually caused by open feedback loops. Brownout Protection Since the VIN pin is connected through a resistive divider to the rectified AC input line voltage, it can also be used for brownout protection. If the VIN voltage is less than 0.7V, the PWM output is shut off. If the VIN voltage over 0.92V, the PWM output is turned on again. The hysteresis window for on/off is around 0.22V. The brownout voltage setting is determined by the potential divider formed with RUpper and RLower. To calculate the resistors: ) ( , V unit V R R R V AC Upper Lower Lower IN = × + = (1) Thermal-Overload Protection Thermal-overload protection limits total power dissipation. When the junction temperature exceeds TJ = +135 °C, the thermal sensor signals the shutdown logic and turns off most of the internal circuitry. The thermal sensor turns internal circuitry on again after the IC’s junction temperature drops by 25 °C. Thermal- overload protection is designed to protect the FAN6751 in the event of a fault condition. For continual operation, do not exceed the absolute maximum junction temperature rating of TJ = +150°C. Limited Power Control The FB voltage increases every time the output of the power supply is shorted or overloaded. If the FB voltage remains higher than a built-in threshold for longer than tD-OLP, PWM output is turned off. As PWM output is turned off, VDD begins decreasing. When VDD goes below the turn-off threshold (~10.5V), the controller is totally shut down. VDD is charged up to the turn-on threshold voltage of 16.5V through the startup resistor until PWM output is restarted. This protection feature continues as long as the overloading condition persists. This prevents the power supply from overheating due to overloading conditions. Noise Immunity Noise on the current sense or control signal may cause significant pulse-width jitter, particularly in continuous- conduction mode. Slope compensation helps alleviate this problem. Good placement and layout practices should be followed. Avoiding long PCB traces and component leads, locating compensation and filter components near the FAN6751, and increasing the power MOS gate resistance improve performance. |
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