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LM3464AMHX 数据表(PDF) 11 Page - National Semiconductor (TI) |
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LM3464AMHX 数据表(HTML) 11 Page - National Semiconductor (TI) |
11 / 22 page System Operation In order to provide failure protection to the LEDs, the rail volt- age is pulled up by the LM3464/64A from a relatively low voltage level at system startup until the rail voltage reaches certain preset level. Figure 3 shows the change of the rail voltage of the LM3464/64A LED lighting system upon the pri- mary power source is powered. The Lm3464/64A can be interfaced to an off-the-shelf con- verter to form a LED lighting system with simple connections. Figure 1 shows the typical application circuit of a lighting sys- tem using the LM3464/64A with a fly-back AC/DC converter. In this application, the output voltage of the AC/DC converter is mainly governed by a voltage reference IC, LM431 and a voltage divider consists of R 1 and R2. The LM3464/64A influ- ences the output voltage of the AC/DC converter by sinking current from the junction of the voltage divider (R 1 and R2) to realize dynamic headroom control. The operation of the LM3464/64A upon startup can be divided into several phases according to the changes of the rail volt- age as shown in Figure 3. When the AC/DC converter is powered, the rail voltage increases and stays steady when its native nominal output voltage, V RAIL(nom) is reached. This volt- age is defined by the output voltage feedback resistor divider of the AC/DC converter. At this voltage level, the LM3464/64A is powered already. After certain delay defined by C DHC, the LM3464/64A starts to push the rail voltage up by sinking cur- rent into the OutP pin from the voltage feedback node of the AC/DC converter until the rail voltage reaches V DHC_READY. V DHC_READY is the highest rail voltage in normal operation and should be enough to turn on all the LED strings with current regulation (defined by R SNSx). As VRAIL reaches VDHC_READY, the LM3464/64A turns on all the output channels. This dis- charges the output capacitor of the primary power supply and causes the rail voltage to decrease to certain level that system efficiency is maximized (V LED). 30115039 FIGURE 3. Changes of Rail Voltage Upon Power Up Application Information SETTING (V RAIL(nom)) The nominal rail voltage V RAIL(nom) is the nominal output volt- age of the primary power supply (AC/DC) prior to DHC begins. The selection of V RAIL(nom) is primarily depend on the forward voltages of the LED arrays and should follow the equation shows below: V RAIL(nom) ≤ Vf(all_temp) + VVDHC In the equation, V f(all_temp) is the lowest forward voltage among all the LED strings under all possible temperature. And V VD- HC is the voltage headroom which equals to the voltage at the VDHC pin. Normally, the forward voltage of an LED drops as the ambient temperature increases. This could create large variation of total forward voltage of a LED sting under different temperature. In order to ensure proper system startup, the variation of LED forward voltage against temperature must be considered in calculations. SETTING V DHC_READY AND VRAIL(peak) DHC begins when the voltage at VLedFB pin reaches 2.5V, which is defined by the values of R FB1 and RFB2: Where V DHC_READY < VRAIL(peak) At this stage, the current of the LED strings are regulated and the rail voltage decreases in order to maintain minimum volt- age drop and power dissipation on the MOSFETs. In case the OutP pin is accidentally shorten to ground, the rail voltage will increase and end up exceeds V DHC_READY. To avoid damaging the AC/DC converter, the possible peak out- put voltage, V RAIL(peak) can be roughly defined by the forward voltage of the LED strings and must set below the rated volt- age of the components at the output of the AC/DC converter. In order to limit the power dissipation on the external MOS- FETs, V RAIL(peak) is set to to no more than 10VDC higher than the forward voltage of the LED string. The following equations define the maximum output voltage of the AC/DC converter that can be pushed up by the LM3464/64A: V RAIL(peak) = VR1 + VREF(AC/DC) = (R1 x IR1) + VREF(AC/DC) for V REF(AC/DC) = 2.5V also since 11 www.national.com |
类似零件编号 - LM3464AMHX |
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类似说明 - LM3464AMHX |
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