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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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LM3445MX Arkusz danych(PDF) 19 Page - National Semiconductor (TI) |
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LM3445MX Arkusz danych(HTML) 19 Page - National Semiconductor (TI) |
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19 / 26 page  SETTING THE LED CURRENT The LM3445 constant off-time control loop regulates the peak inductor current (I L2). The average inductor current equals the average LED current (I AVE). Therefore the average LED cur- rent is regulated by regulating the peak inductor current. 30060325 FIGURE 18. Inductor Current Waveform in CCM Knowing the desired average LED current, I AVE and the nom- inal inductor current ripple, Δi L, the peak current for an appli- cation running in continuous conduction mode (CCM) is defined as follows: Or, the maximum, or "undimmed", LED current would then be, This is important to calculate because this peak current mul- tiplied by the sense resistor R3 will determine when the internal comparator is tripped. The internal comparator turns the control MOSFET off once the peak sensed voltage reach- es 750 mV. Current Limit: Under normal circumstances, the trip voltage on the PWM comparator would be less than or equal to 750 mV, depending on the amount of dimming. However, if there is a short circuit or an excessive load on the output, higher than normal switch currents will cause a voltage above 1.27V on the ISNS pin which will trip the I-LIM comparator. The I- LIM comparator will reset the RS latch, turning off Q2. It will also inhibit the Start Pulse Generator and the COFF com- parator by holding the COFF pin low. A delay circuit will prevent the start of another cycle for 180 µs. VALLEY FILL CAPACITORS Determining voltage rating and capacitance value of the val- ley-fill capacitors: The maximum voltage seen by the valley-fill capacitors is: This is, of course, if the capacitors chosen have identical ca- pacitance values and split the line voltage equally. Often a 20% difference in capacitance could be observed between like capacitors. Therefore a voltage rating margin of 25% to 50% should be considered. 19 www.national.com |
Podobny numer części - LM3445MX |
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Podobny opis - LM3445MX |
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