AN2115 datasheet(29/33 Pages) STMICROELECTRONICS

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AN2115

Application ideas

Doc ID 11165 Rev 5

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voltage). The forward voltage across a white LED is approximately 3.6 V and so, depending

on the input source, appropriate topologies must be used.

10.2.1

Driving white LEDs: buck topology

The simple buck topology can be used when the input voltage source is higher than

approximately 4.5 V, which is the case, for example, with a USB bus.

Figure 24.

Buck LED

In this case, the maximum device current (800 mA, continuous) can be delivered to the LED.

Moreover, in this topology the efficiency is maximized.

10.2.2

Driving white LEDs: boost topology

When the input voltage source is always lower than 3 V (which is the case, for example, of 2

NiMH battery cells) a boost topology must be implemented, as shown in Figure 25.

Figure 25.

Boost LED

In this case, according to the boost topology, the maximum current that can be delivered

depends on the duty cycle. The relation between the output current and the internal switch

current (assuming a negligible current ripple and 100% efficiency) is given in Equation 24:

AN2115 Datasheet(PDF) 29 Page - STMicroelectronics