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AN3027 Datasheet(PDF) 19 Page - STMicroelectronics |
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AN3027 Datasheet(HTML) 19 Page - STMicroelectronics |
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19 / 41 page  AN3027 Designing a TM PFC Doc ID 16134 Rev 4 19/41 the desired output power dissipated on the output divider. For example, considering a power dissipation of 50 mW: Equation 37 With the commercial resistor selected, RoutH = 3 MΩ. Equation 38 Equation 39 The RoutL = 62 kΩ resistor in parallel to 27 kΩ has been selected for giving a total resistance close to the calculated value. Please note that for RoutH a resistor with a suitable voltage rating (>400 V) is needed, or more resistors in series have to be used. Please note also that the maximum value of the resistor divider is limited by the L6563S INV pin input bias current given in the datasheet. To guarantee correct output voltage regulation, the current flowing in the resistor divider must be significantly higher than the current flowing into the pin. Pin 7 (PFC_OK - feedback failure protection): the PFC_OK pin has been dedicated to monitor 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 (7), 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 imaging a 50 µA current flowing into the divider: Equation 40 By selecting a commercial resistor of 51 k Ω: Equation 41 Using two 3.3 M Ω resistors and one 2.2 MΩ resistor, a total output PFC_OK high resistor of 8.8 M Ω has been obtained. Notice that both feedback dividers connected to L6563S pin #1 (INV) and pin #7 (PFC_OK) can be selected without any constraints. The unique criterion is that both dividers have to sink a current from the output bus which needs to be significantly higher than the current biasing the error amplifier and PFC_OK comparator. The OVP function described above is able to handle “normal” overvoltage conditions, i.e. those resulting from an abrupt load/line change or occurring at startup. In case the 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 RL ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − ⋅ = 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 RH |
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