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ML4821CS Arkusz danych(PDF) 7 Page - Fairchild Semiconductor |
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ML4821CS Arkusz danych(HTML) 7 Page - Fairchild Semiconductor |
7 / 12 page PRODUCT SPECIFICATION ML4821 REV. 1.0.2 6/19/01 7 Figure 3. Error and Current Amplifier Configuration Figure 4. Error Amplifier Open-loop Gain and Phase vs. Frequency. 6 7 9 – + S.S INV +8V VREF +8V 6.2k Ω EA OUT VCC = 15V VO = 1.0V TO 5.0V RL = 100k Ω TA= 25 °C GAIN PHASE 100 80 60 40 20 0 –20 10 100 1.0k 10k 100k 1.0M 10M 0 –30 60 90 120 150 180 f, FREQUENCY (Hz) Figure 5. Output Saturation Voltage vs. Output Current. Figure 6. K-factor. Gain Modulator gain with respect to the voltage at VRMS. SOURCE SATURATION VCC TA = 25 °C TA = –55 °C TA = –55 °C TA = 25 °C VCC = 15V 80 µs PULSED LOAD SINK SATURATION GND 0 .0 2.0 3.0 2.0 .0 0 0 200 400 600 800 IO, OUTPUT LOAD CURRENT (mA) 0.5 0.4 0.3 0.2 0.1 0 135 7 46 0 85VAC 0.23 DESIGN FOR NORMAL OPERATIONS THIS IS THE MINIMUM OPERATING VOLTAGE POINT THIS GAIN CURVE TAKES OUT THE 1/(VIN)2 DEPENDENCY OF THE VOLTAGE CONTROL LOOP 120VAC OPERATING BOUNDRY 220VAC K 2 VRMS DESIGN FOR BROWNOUT The output of the gain modulator is a current which appears on IA+ to form the reference for the current error amplifier and is given as: IGM = K × ISINE × (VEA – 0.8) where: ISINE is the current in the dropping resistor, VEA is the output of the error amplifier and K is a constant deter- mined by the VRMS input. The output current of the gain modulator is limited to: This sets the system current limit.The multiplier output current is converted into the reference voltage for the current (IA) amplifier through a resistor to ground on IA+. Figure 6 shows the gain adjustor (K) with respect to the voltage at VRMS. The curve has been separated in two parts. The right hand part is for operation under normal conditions in the voltage range from minimum line voltage to maximum line voltage (90VAC to 260VAC). 85VAC on the curve has been chosen to account for tolerances. Under normal operat- ing conditions as input voltage decreases the gain increases compensating for the drop in the loop gain. Under brownout conditions (below 85VAC) the gain decreases to limit the amount of current that is drawn from the line thus preventing an overload condition. This is a very useful feature since in many cases the load for a PFC is a constant power load. The input current has to go high to compensate for a drop in the input voltage. I GM MAX () 2.5 R T -------- = |
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