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AN827 Datasheet(PDF) 9 Page - STMicroelectronics |
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AN827 Datasheet(HTML) 9 Page - STMicroelectronics |
9 / 20 page 9/20 AN827 APPLICATION NOTE Pin 3 OVP (Overvoltage protection input) The overvoltage protection trip level is determined by the voltage divider across the output bulk capacitor, C6. The resistor values R11, R21 and R22 are calculated as follows: Where ∆Vout = 47V is the maximum overvoltage limit. The overvoltage limit selection is dependent upon the voltage rating of the output bulk capacitor (450VDC) and the power mosfet (500BVdss). Care must be taken that the level is not set too low, thus causing false tripping of the OVP. Pin 4 IAC (AC current input) This pin must be connected through resistors R1 and R2 to the rectified line to drive the multiplier with a current IIAC proportional to the instantaneous line voltage as shown below: Thus IIAC ranges from 77µA to 231µA. The relationship between IIAC and multiplier output current, Imult, is de- scribed in section Pin 8 (MULTOUT). Pin 5 CA-OUT (Current amplifier output) The current amplifier output delivers its signal to the PWM comparator. An external network defines the suitable loop gain to process the multiplier output and the inductor current signals. To avoid oscillation problems, the maximum inductor downslope (Vout/L) must be lower than the oscillator ramp-slope (Vsrp*fsw). The current am- plifier high frequency gain can be described as follows: Where: Vsrp = 5.0V is the oscillator ramp peak-peak voltage Gca is the current amplifier gain fsw = 80kHz is the switching frequency Rsense = 0.033Ω is the parallel combination of R30-32 Thus, use R14=R16=2.7k, and R15=36K. To define the value of the compensation capacitor, C9, it is useful to consider the open loop current gain, defined by the ratio of the voltage across the sense resistor and the current amplifier output voltage. The crossover fre- quency is given by the following equation: To ensure a good phase margin, the zero frequency, fz, should equal approximately fc/2. R21 R22 + R11 ----------------------------- V out ∆V out + V ref --------------------------------- 1 400 47 + 5.1 ----------------------- 1 909k 909k + 21k --------------------------------- = – = – = I IAC 88 V () V i npk R1 R2 + ---------------------- 288 ⋅ 806k 806k + --------------------------------- 77 µA == = I IA C 264 V () 2264 ⋅ 806k 806k + --------------------------------- 231 µA == G ca R15 R14 ----------- 1 V sr p f SW L ⋅⋅ V out R sens e ⋅ ---------------------------------- 5.0 80k 0.5m ⋅⋅ 400 0.033 ⋅ ----------------------------------------- = ≤ + = f c f sw 2 π ⋅ ----------- 80k 2 π ⋅ ----------- 12.7kHz === |
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