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FAN6754A 数据表(PDF) 6 Page - Fairchild Semiconductor |
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FAN6754A 数据表(HTML) 6 Page - Fairchild Semiconductor |
6 / 9 page AN-6754A APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 10/8/10 6 Figure 15. Brownout Circuit Figure 16. Brownout Protection Behavior Overload / Open-Loop Protection (OLP) When output is overloaded, the drain current reaches its pulse-by-pulse current limit level, limiting the input power. Then, the output voltage drops and no current flows through the opto-diode, which causes the feedback voltage to increase above the OLP protection threshold (4.6V). A similar response occurs when the feedback loop is open and no current flows through the opto-diode. When the feedback voltage is higher than 4.6V for longer than the OLP delay time, the OLP protection is triggered, as shown in Figure 17. Figure 17. OLP Behavior VDD Over-Voltage Protection (VDD_OVP) VDD over-voltage protection protects the VDD pin from damage from over-voltage. The VDD voltage rises when an open-feedback loop failure occurs. Once the VDD voltage exceeds 25V (VDD-OVP) for longer than 165µs, the power supply is latched off until VDD is discharged below VDD-LH. Sense Pin Short-Circuit Protection (SSCP) This protection is used for Limited Power Source (LPS) safety testing. This test is performed with RSENSE shorted to ground, thus increasing the output power. If the output voltage rating is set to 19V, this protection should be triggered before the output wattage increases to 100W. Figure 18 shows this detection method. When RSENSE is shorted to ground, the FAN6754A detects the voltage on the SENSE pin during the gate’s on time. After debounce time, the gate turns off and VDD goes into hiccup mode. The sense resistor short-circuit protection is triggered if all of the following occur: gate is in a HIGH state; and the on-time of gate; tON-SSCP is over 4.4µs and VSENCE< 50mV; and during the debounce time of 120µs, if all pulse conditions are as above or the SSCP counter is reset. Because of the fixed gate’s on-time checking, the peak current of the inductor is different between high line and low line. The FAN6754A sets VSENSE detection levels for all input voltage ranges. Figure 18. SSCP Detection Method To prevent miss-triggered protection in normal mode (RSENSE not shorted to ground); especially at lower AC line input voltages and light-load conditions, the transformer’s inductance should not be too high in inductance. The following is a design example; (1) If the RHV is 200KΩ, the brownout voltage should be 80VAC; assume the ripple is 15V at light-load condition. (2) RSENSE is 0.15Ω, and has ±5% tolerance. (3) The minimum specification of tON-SSCP is 4µs from the datasheet. (4) The maximum specification of VSSCP-L is 70mV from the datasheet. |
类似零件编号 - FAN6754A |
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类似说明 - FAN6754A |
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