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FAN7530 데이터시트(PDF) 14 Page - Fairchild Semiconductor |
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FAN7530 데이터시트(HTML) 14 Page - Fairchild Semiconductor |
14 / 37 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.1.3 14 Figure 20. Error Amplifier Compensation Circuitry 7. Output Voltage Feedback (FB) The feedback pin receives the divided-down output voltage of the converter. In regulation, the FB pin should be 3V, which is the reference used at the non-inverting input of the error amplifier. Due to the gM type error amplifier, the FB pin is always proportional to the output voltage and can be used for over-voltage protection as well. A non-latching over-voltage detection circuit monitors the FB pin and prevents the boost MOSFETs from turning on when the FB voltage exceeds 3.25V. Operation resumes automatically when the FB voltage returns to its nominal 3V level. The open feedback detection circuit is also connected to the FB pin. Since the output of the boost converter is charged to the peak of the input AC voltage when power is applied to the power supply, the detection circuit monitors the presence of this voltage. If the FB pin is below 0.5V, which would indicate a missing feedback divider (or wrong value causing dangerously-high regulation voltage), the FAN9611/12 does not send out gate drive signals to the boost transistors. External Components Internal Circuits 8 FB To SS gM Error Amplifier To Phase / Gain Control VOUT +200 mV, –500 mV, Clamp Figure 21. Output-Voltage Feedback Circuit 8. Secondary Output Voltage Sense (OVP) A second-level latching over-voltage protection can be implemented using the OVP pin of the controller. The threshold of this circuit is set to 3.5V. There are two ways to program the secondary OVP. Option 1, as shown in Figure 22, is to connect the OVP pin to the FB pin. In addition to the standard non- latching OVP (set at ~8%), this configuration provides a second OVP protection (set at ~15%), which is latched. In the case where redundant over-voltage protection is preferred (also called double-OVP protection), a second separate divider from the output voltage can be used, as shown by Option 2 in Figure 22. In this case, the latching OVP protection level can be independently established below or above the non-latching OVP threshold, which is based on the feedback voltage (at the FB pin). If latching OVP protection is not desired at all, the OVP pin should be grounded (Option 3). Figure 22. Secondary Over-Voltage Protection Circuit |
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