AN2823 datasheet(4/33 Pages) STMICROELECTRONICS

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Basic principles of LED driving

AN2823

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Doc ID 15015 Rev 1

Basic principles of LED driving

This paragraph deals with different ways to arrange a set of LEDs to generate the required

lumen output. Advantages and disadvantages for each topology are also provided.

The easiest and cheapest way to drive LEDs is to connect them in series, as shown in

Figure 2. However, this implies that the LED driver must provide an output voltage that is the

sum of the forward voltage of the LEDs.

Figure 2.

Basic schematic of series-driven LEDs

Therefore, in other cases the preferred solution is to drive LEDs in parallel, matching the

different current sources (Figure 3).

Figure 3.

Basic schematic of parallel-driven LEDs

An easy and cost-effective way to design a constant current source to drive LEDs is to use a

DC-DC converter, as shown in Figure 4. Since the voltage control loop of the device

regulates the voltage at the FB pin, a constant current source can be obtained by simply

connecting a resistor between this pin and GND. Application note AN1891 from

STMicroelectronics provides helpful information for designing a similar application using

monolithic DC-DC converters.

AN2823 Datasheet(PDF) 4 Page - STMicroelectronics