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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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AN3009 датащи(PDF) 21 Page - STMicroelectronics |
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AN3009 датащи(HTML) 21 Page - STMicroelectronics |
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21 / 36 page  AN3009 Designing a transition mode PFC Doc ID 16032 Rev 4 21/36 By selecting a commercial value of RoutH equal to 3 MΩ, we get: Equation 38 Equation 39 We have selected RoutL = 62 kΩ in parallel to 27 kΩ. Note that for RoutH a resistor with a suitable voltage rating (>400 V) is needed, or else additional in-series resistors must be used. Also note that the maximum value of the resistor divider is limited by the L6564’s 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 6 (PFC_OK - feedback failure protection): the PFC_OK pin is dedicated to monitoring the output voltage by a separate resistor divider. This divider is selected so that the voltage at the pin reaches 2.5 V (typ.) if the output voltage exceeds a preset value VOVP (7), usually larger than the maximum Vout that can be expected, and also including worst-case load/line transients. For a maximum output voltage VOVP of 430 V and imposing a 50 µA current flowing into the divider, we obtain: Equation 40 By selecting a commercial value of 51 k Ω, we then get: Equation 41 By connecting in series two 3.3 M Ω and one 2.2 MΩ resistors, a total value of 8.8 MΩ is obtained. Note that both feedback dividers connected to the L6564’s pin #1 (INV) and pin #6 (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 can handle “normal” overvoltage conditions, that is, those resulting from an abrupt load/line change or occurring at start-up. If the overvoltage is generated by a feedback disconnection for instance, when one of the upper resistors of the output divider fails to open, an additional circuitry detects the voltage drop of pin INV. If the voltage on pin INV is lower than 1.66 V (typ.) and at same time the OVP is active, a feedback failure is assumed. 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 OVP L H Ω = ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ − ⋅ Ω = M 721 . 8 1 V 5 . 2 V 430 k 51 RH |
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