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LM3405 Fiches technique(PDF) 4 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No de pièce LM3405
Description  Powering and Dimming High-Brightness LEDs with the LM3405 Constant-Current Buck Regulator
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3405 Fiches technique(HTML) 4 Page - National Semiconductor (TI)

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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


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