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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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AN3112 Datenblatt(PDF) 20 Page - STMicroelectronics |
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AN3112 Datenblatt(HTML) 20 Page - STMicroelectronics |
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20 / 36 page  Designing a fixed-off-time PFC AN3112 20/36 Doc ID 16820 Rev 3 Pin 1 (INV) is connected both to the inverting input of the E/A and to the OVP circuitry. A resistive divider is connected between the boost regulated output voltage and this pin. The internal reference on the non-inverting input of the E/A is 2.5 V (typ.), the output voltage (Vout) of the PFC pre-regulator is set at its nominal value, by the resistors ratio of the feedback output divider. RoutH and RoutL are then selected considering the desired nominal output voltage and the desired output power dissipated on the output divider. For example for a 50 mW output divider dissipation: Equation 39 With the commercial value selected RoutH = 3 M Ω: Equation 40 Equation 41 RoutL = 62 k Ω in parallel to a 27 kΩ can be selected. Please note that for RoutH a resistor with a suitable voltage rating (>400 V) is needed, or more resistors in series must be used. Pin 6 (PFC_OK - feedback failure protection): The PFC_OK pin is dedicated to monitoring the output voltage with a separate resistor divider. This divider is selected so that the voltage at the pin reaches 2.5 V if the output voltage exceeds a preset value (Vovp), usually larger than the maximum Vout that can be expected, also including worst-case load/line transients. For a maximum output voltage Vout max of 430 V and selecting a 50 µA current flowing into the divider: Equation 42 By selecting a commercial value of 51k Ω: Equation 43 Connecting in series, two 3.3 M Ω resistors and one 2.2 MΩ resistor, a total value of 8.8 MΩ can be obtained. Note that both feedback dividers connected to the L6564 pin #1 (INV) and pin #6 (PFC_OK) can be selected without any constraints. The unique criterion is that both dividers must sink mW 50 ) V 5 . 2 V ( R 2 OUT outH − = Ω = − = M 160 . 3 mW 50 ) V 5 . 2 V 400 ( R 2 outH 1 V 5 . 2 V R R out outL outH − = 159 1 V 5 . 2 V 400 R R outL outH = − = 159 R R outH outL = Ω = Ω = k 8 . 18 159 M 3 R outL divider OK _ PFC _ REF L I V R = Ω = µ = k 50 A 50 V 5 . 2 R L ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − ⋅ = 1 V V R R OK _ PFC _ REF MAX _ OUT L H Ω = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⋅ Ω = M 721 . 8 1 V 5 . 2 V 430 k 51 R H |
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