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SG6846B Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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SG6846B Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 8 page AN-6846 APPLICATION NOTE © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.3.2 • 9/26/08 6 Figure 13.Universal Line Voltage Compensation for Constant Output Power Limit Two-level Over-Current Protection (OCP) The cycle-by-cycle current limiting shuts down the PWM immediately when the switching current is over the peak- current threshold. In addition, an over-current protection circuit is built-in. When the switching current is higher than the OCP threshold, the internal counter starts counting up. When the switching current is lower than the OCP threshold, the internal counter counts down. When the total accumulated counting time is more than OCP delay time (SG6846A: 1600ms; SG6846C: 110ms), the controller is latched off. By adjusting the RS resistance to let the peak-current at maximum load under OCP threshold and over this threshold at peak load condition, the two-level OCP protection is enabled. These functions are especially designed for an SMPS with surge current output, such as for printers, scanners, and motor drivers. Figure 14.Timing Chart for Two-level Over-Current Protection (OCP) VDD Over-Voltage Protection (OVP) VDD over-voltage protection prevents the controller from over-voltage destruction. The VDD voltage rises when an open-loop failure occurs. Once the VDD voltage exceeds 23.6V for around 100µs, the power supply latches off. Thermal Protection A constant current, IRT, is provided from the pin RT. The resistor connected to pin RI determines its magnitude: IRT = 1.8V / RI (8) For example, IRT = 70µA if RI = 26KΩ. For over-temperature protection (OTP), an NTC thermistor RT in series with a resistor Ra can be connected between the RT pin and ground. When the voltage of the RT pin drops below 1.065V, PWM output is latched off. A debounce time around 100µs is added to prevent false triggering. After the latch is reset and cleared (SG6846CX: VRT>1.165V; SG6846LX: AC unplugged), PWM turns on again. If the RT pin is not used, connect a 100k Ω resistor between the RT pin and ground to disable this thermal protection function. Beside NTC and Ra, a capacitor should be connected to RT pin to eliminate switching noise. This capacitor’s value is less than 1nF (shown as Figure 14). Figure 15.T C1 (<1nF) Connect to RT Pin Lab Note Before rework or solder/desolder on the power supply, discharge the primary capacitors by an external bleeding resistor. Otherwise, the PWM IC may be destroyed by external high voltage during solder/desolder. This device is sensitive to ESD discharge. To improve production yield, the production line should be ESD protected according to ANSI ESD S1.1, ESD S1.4, ESD S7.1, ESD STM 12.1, and EOS/ESD S6.1. |
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