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Zakładka z wyszukiwarką danych komponentów |
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LM3445MX Arkusz danych(PDF) 18 Page - National Semiconductor (TI) |
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LM3445MX Arkusz danych(HTML) 18 Page - National Semiconductor (TI) |
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18 / 26 page  The off-time of the LM3445 can be programmed for switching frequencies ranging from 30 kHz to over 1 MHz. A trade-off between efficiency and solution size must be considered when designing the LM3445 application. The maximum switching frequency attainable is limited only by the minimum on-time requirement (200 ns). Worst case scenario for minimum on time is when V BUCK is at its maximum voltage (AC high line) and the LED string voltage (V LED) is at its minimum value. The maximum voltage seen by the Buck Converter is: INDUCTOR SELECTION The controlled off-time architecture of the LM3445 regulates the average current through the inductor (L2), and therefore the LED string current. The input voltage to the buck converter (V BUCK) changes with line variations and over the course of each half-cycle of the input line voltage. The voltage across the LED string is relatively constant, and therefore the current through R4 is constant. This current sets the off-time of the converter and therefore the output volt-second product (V LED x off-time) remains constant. A constant volt-second product makes it possible to keep the ripple through the in- ductor constant as the voltage at V BUCK varies. 30060340 FIGURE 17. LM3445 External Components of the Buck Converter The equation for an ideal inductor is: Given a fixed inductor value, L, this equation states that the change in the inductor current over time is proportional to the voltage applied across the inductor. During the on-time, the voltage applied across the inductor is, V L(ON-TIME) = VBUCK - (VLED + VDS(Q2) + IL2 x R3) Since the voltage across the MOSFET switch (Q2) is rela- tively small, as is the voltage across sense resistor R3, we can simplify this to approximately, V L(ON-TIME) = VBUCK - VLED During the off-time, the voltage seen by the inductor is ap- proximately: V L(OFF-TIME) = VLED The value of V L(OFF-TIME) will be relatively constant, because the LED stack voltage will remain constant. If we rewrite the equation for an inductor inserting what we know about the circuit during the off-time, we get: Re-arranging this gives: From this we can see that the ripple current ( Δi) is proportional to off-time (t OFF) multiplied by a voltage which is dominated by V LED divided by a constant (L2). These equations can be rearranged to calculate the desired value for inductor L2. Where: Finally: Refer to “Design Example” section of the datasheet to better understand the design process. www.national.com 18 |
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Podobny opis - LM3445MX |
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