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FAN6862TY Datenblatt(PDF) 11 Page - Fairchild Semiconductor |
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FAN6862TY Datenblatt(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6862 • Rev. 1.0.1 11 Protections Self-protective functions include VDD Over-Voltage Protection (OVP), Open-Loop / Overload Protection (OLP), Over-Current Protection (OCP), Short-Circuit Protection, and Over-Temperature Protection (OTP). OLP, OCP, and SCP are auto-restart mode protections; while OVP and OTP are latch-mode protections. Auto-Restart Mode Protection: Once a fault condition is detected, switching is terminated and the MOSFET remains off. This causes VDD to fall because no more power is delivered from auxiliary winding. When VDD falls to VDD-OFF (8.5V), the protection is reset and the operating current reduces to startup current, which causes VDD to rise. FAN6862 resumes normal operation when VDD reaches VDD-ON (16V). In this manner, the auto-restart can alternately enable and disable the switching of the MOSFET until the fault condition is eliminated (see Figure 25). Latch-Mode Protection: Once this protection is triggered, switching is terminated and the MOSFET remains off. The latch is reset only when VDD is discharged below 4V by unplugging AC power line. Fault Situation 8.5V 16V VDD VDS Protection Triggers Fault Removed Normal Operation Normal Operation Power On Operating Current 3mA 8uA Figure 25. Auto Restart Operation Over-Current Protection (OCP) FAN6862 has over-current protection thresholds. It is for pulse-by-pulse current limit, which turns off MOSFET for the remainder of the switching cycle when the sensing voltage of MOSFET drain current reaches the threshold. The other threshold is for the over-current protection, which shuts down the MOSFET gate when the sensing voltage of MOSFET drain current is above the threshold longer than the shutdown delay (56ms). Open-Loop / Over-Load Protection (OLP) When the upper branch of the voltage divider for the shunt regulator (KA431 shown) is broken, as shown in Figure 26, no current flows through the opto-coupler transistor, which pulls up the feedback voltage to 5.2V. When the feedback voltage is above 4.6V longer than 56ms, OLP is triggered. This protection is also triggered when the SMPS output drops below the nominal value longer than 56ms due to the overload condition. 5.2V VFB-OLP OLP Shutdown Delay Time OLP Triggers VFB (4.6V) Figure 26. OLP Operation VDD Over-Voltage Protection (OVP) VDD over-voltage protection prevents IC damage caused by over voltage on the VDD pin. The OVP is triggered when VDD reaches 25V. A debounce time (typically 30µs) prevents false triggering by switching noise. Over-Temperature Protection (OTP) The OTP circuit is composed of current source and voltage comparators. Typically, an NTC thermistor is connected between the RT and GND pins. Once the voltage of this pin drops below a threshold of 1.0V, PWM output is disabled after tDOTP debounce time. If this pin drops below 0.7V, it triggers the latch-off protection immediately after tDOTP2 debounce time. |
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