LM3405 datasheet(3/6 Pages) NSC

3 / 6 page

30022907

FIGURE 5. Deriving a Self-Biased Boost Voltage from the LED String When Driving Two or More LEDs in Series

The anode of the boost diode D2 is connected to the anode

of LED3. The voltage at the anode of LED3 is approximately

equal to 205 mV plus the forward voltage VF of LED3. This

voltage is approximately in the range of 3V to 4V (depending

on the I

F3 current setting) and meets the BOOST to SW volt-

age requirement. It must be noted that the current through

LED1 and LED2 will be slightly larger than the current in LED3

due to the fact that the average charging current for the boost

capacitor C3 will now be provided through LED1 and LED2.

Therefore, the LED currents are:

where I

F1, IF2, and IF3 are the currents in LED1, LED2, and

LED3, respectively and I

C3 is the average current charging C3

over one switching cycle. This approach can be used if exact

current matching (and hence brightness) is not required be-

tween I

F1 and IF3 or between IF2 and IF3.

Driving Multiple LED Strings with

the LM3405 LED Driver

The LM3405 LED driver can be used to drive multiple LED

strings in parallel as shown in Figure 6. The current in the

primary branch (with LED1 and LED2) will be tightly regulated

by the feedback loop.

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FIGURE 6. Driving Multiple LED Strings

The current in the secondary branch (with LED3) will be reg-

ulated based on V

OUT, forward voltage of LED3, and R2. The

value of R2 can be adjusted to get the desired brightness for

LED3.

LM3405 Datasheet(PDF) 3 Page - National Semiconductor (TI)