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FSCM0565RJ Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FSCM0565RJ Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 20 page  FSCM0765R 11 2.2 Leading Edge Blanking (LEB): At the instant the internal SenseFET is turned on, there usually exists a high current spike through the SenseFET, caused by primary-side capacitance and secondary-side rectifier reverse recovery. Excessive voltage across the Rsense resistor can lead to incorrect feedback operation in the current mode PWM control. To counter this effect, the FSCM0765R employs a leading edge blanking (LEB) circuit. This circuit inhibits the PWM comparator for a short time (TLEB) after the SenseFET is turned on. 3. Protection Circuit: The FSCM0765R has several self protective functions such as over load protection (OLP), over voltage protection (OVP) and thermal shutdown (TSD). Because these protection circuits are fully integrated into the IC without external components, the reliability can be improved without increasing cost. Once the fault condition occurs, switching is terminated and the SenseFET remains off. This causes Vcc to fall. When Vcc reaches the UVLO stop voltage of 8V, the current consumed by the FSCM0765R decreases to the startup current (typically 25uA) and the current supplied from the DC link charges the external capacitor (Ca) that is connected to the Vcc pin. When Vcc reaches the start voltage of 12V, the FSCM0765R resumes its normal operation. In this manner, the auto-restart can alternately enable and disable the switching of the power SenseFET until the fault condition is eliminated (see Figure 19). Figure 19. Auto Restart Operation 3.1 Over Load Protection (OLP): Overload is defined as the load current exceeding a pre-set level due to an unexpected event. In this situation, the protection circuit should be activated to protect the SMPS. However, even when the SMPS is in the normal operation, the over load protection circuit can be activated during the load transition. To avoid this undesired operation, the over load protection circuit is designed to be activated after a specified time to determine whether it is a transient situation or an overload situation. Because of the pulse-by-pulse current limit capability, the maximum peak current through the SenseFET is limited, and therefore the maximum input power is restricted with a given input voltage. If the output consumes beyond this maximum power, the output voltage (Vo) decreases below the set voltage. This reduces the current through the opto-coupler LED, which also reduces the opto- coupler transistor current, thus increasing the feedback voltage (Vfb). If Vfb exceeds 2.5V, D1 is blocked and the 5.3uA current source (Idelay) starts to charge CB slowly up to Vcc. In this condition, Vfb continues increasing until it reaches 6V, when the switching operation is terminated as shown in Figure 20. The delay time for shutdown is the time required to charge CB from 2.5V to 6.0V with 5.3uA (Idelay). In general, a 10 ~ 50 ms delay time is typical for most applications. Figure 20. Over Load Protection 3.2 Over Voltage Protection (OVP): If the secondary side feedback circuit were to malfunction or a solder defect caused an open in the feedback path, the current through the opto-coupler transistor becomes almost zero. Then, Vfb climbs up in a similar manner to the over load situation, forcing the preset maximum current to be supplied to the SMPS until the over load protection is activated. Because more energy than required is provided to the output, the output voltage may exceed the rated voltage before the over load protection is activated, resulting in the breakdown of the devices in the secondary side. To prevent this situation, an over voltage protection (OVP) circuit is employed. In general, Vcc is proportional to the output voltage and the FSCM0765R uses Vcc instead of directly monitoring the output voltage. If VCC exceeds 19V, an OVP circuit is activated resulting in the termination of the switching operation. To avoid undesired activation of OVP during normal operation, Vcc should be designed to be below 19V. Fault Situation 8V 12V Vcc Vds t Fault occurs Fault removed Normal Operation Normal Operation Power on V FB t 2.5V 6.0V Over Load Protection T 12= Cfb*(6.0-2.5)/Idelay T 1 T 2 |
Similar Part No. - FSCM0565RJ |
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Similar Description - FSCM0565RJ |
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