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LM3405 Fiches technique(PDF) 4 Page - National Semiconductor (TI) |
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LM3405 Fiches technique(HTML) 4 Page - National Semiconductor (TI) |
4 / 6 page where V F1 and VF2 are the respective forward voltages of LED1 and LED2 at the primary current level, and V F3 is the forward voltage of LED3 at the secondary current level. I F1, I F2, and IF3 are the currents in LED1, LED2, and LED3, re- spectively. In Figure 6, VBST can be connected to an external DC source of 3V to 5V. VBST can also be derived from the cathode of LED3. If VBST is derived from the cathode of LED3, this VBST can be written as (assuming V F1=VF2=VF3=VF=3.8V): VBST = 205 mV + V F1 + VF2 - VF3 ≅ 205 mV + VF ≅ 4V Note that the average current through LED3 will now be high- er since the average current to charge C3 is supplied through LED3 as discussed in the previous section. Driving Parallel LEDs with the LM3405 LED Driver Multiple LEDs can be paralleled and connected between V OUT and FB as shown in Figure 7. 30022911 FIGURE 7. Driving Multiple LEDs in Parallel The voltage at the FB pin is regulated to 205 mV by the control loop and therefore the current in R1 is fixed. V OUT is deter- mined by the LED with the highest forward voltage. This solution has the advantage of having a single resistor set the total currents in the LEDs but has no control of equal current sharing between the LEDs. Conclusion This article describes a few application solutions using the LM3405 buck regulator. These solutions provide the user with highly-compact, driving and dimming solutions for high-bright- ness LEDs. The LM3405 data sheet may be referred to for a detailed description of operation and component design guidelines. www.national.com 4 |
Numéro de pièce similaire - LM3405_08 |
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Description similaire - LM3405_08 |
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