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FAN7530 데이터시트(PDF) 25 Page - Fairchild Semiconductor |
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FAN7530 데이터시트(HTML) 25 Page - Fairchild Semiconductor |
25 / 37 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.1.3 25 Step 12: Soft-Start Capacitor FB2 MAX OUT, FB2 FB1 OUT SS R I 0.3 R R C μA 5 C (19) where 5μA is the charge current of the soft-start capacitor and 0.3·IOUT,MAX is the maximum output current charging the output capacitor of the converter during the soft-start process. It is imperative to limit the charge current of the output capacitor to be able to maintain closed-loop soft-start of the converter. The 0.3 factor used in the CSS equation can prevent output over voltage at the end of the soft-start period and provides sufficient margin to supply current to the load while the output capacitor is charging. Step 13: Compensation Components FB2 FB1 FB2 2 0 OUT MAX OUT, M LF COMP, R R R f π 2 C 4.1V I g C (20) where 4.1V is the control range of the error amplifier and f0 is the desired voltage loop crossover frequency. It is important to consider that the lowest output ripple frequency limits the voltage loop crossover frequency. In PFC applications, that frequency is two times the AC line frequency. Therefore, the voltage loop bandwidth (f0), is typically in the 5Hz to 15Hz range. To guarantee closed-loop soft-start operation under all conditions, it is recommended that: SS HF COMP, C 4 C (21) This relationship is determined by the ratio between the maximum output current of the gM error amplifier to the maximum charge current of the soft-start capacitor. Observing this correlation between the two capacitor values ensures that the compensation capacitor voltage can be adjusted faster than any voltage change taking place across the soft-start capacitor. Therefore, during startup the voltage regulation loop’s response to the increasing soft-start voltage is not limited by the finite current capability of the error amplifier. LF COMP, 0 COMP C f π 2 R 1 (22) COMP HFP HF COMP, R f π 2 C 1 (23) where fHFP is the frequency of a pole implemented in the error amplifier compensation network against high- frequency noise in the feedback loop. The pole should be placed at least a decade higher than f0 to ensure that it does not interfere with the phase margin of the voltage regulation loop at its crossover frequency. It should also be sufficiently lower then the switching frequency of the converter so noise can be effectively attenuated. The recommended fHFP frequency is around 120Hz in PFC applications. Step 14: Over-Voltage Protection Setting (OVP) OVP LATCH OUT, OV2 P V 3.5V R (24) where 3.5V is the threshold voltage of the OVP comparator and POVP is the total dissipation of the resistive divider network. Typical POVP power loss is in the 50mW to 100mW range. OV2 LATCH OUT, OV1 R 1 3.5V V R (25) ROV1 can be implemented as a series combination of two or three resistors; depending on safety regulations, maximum voltage, and or power rating of the selected resistor type. Step 15: Input Line Voltage Sense Resistors INSNS MIN , LINE MAX , LINE 2 IN P V 2 V V 925 . 0 R 2 (26) where 0.925V is the brown-out protection threshold at the VIN pin. VLINE,MIN is the minimum input RMS operating voltage. Its divided down level at the VIN pin corresponds to the 0.925V brown out protection threshold. VLINE,MAX is the maximum input RMS voltage anticipated in the design and PINSNS is the total power dissipation of the RIN1 - RIN2 divider when the input voltage equals VLINE,MAX. Typical PINSNS power loss is in the 50mW to 100mW range. IN2 MIN LINE, IN1 R 1 0.925V V R 2 (27) RIN1 can be implemented as a series combination of two or three resistors; depending on safety regulations, maximum voltage, and or power rating of the selected resistor type. μA 2 V . 0 R R R V 2 R IN2 IN1 IN2 ON LINE, INHYST 925 (28) where 0.925V is the threshold voltage of the line under- voltage lockout comparator and 2μA is the sink current provided at the VIN pin during line under-voltage (brownout) condition. The sink current, together with the terminating impedance of the VIN pin determines the hysteresis between the turn-on and turn-off thresholds. |
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