AN1882

Application ideas

Doc ID 10209 Rev 4

27/30

Equation 24

This topology is possible because the input source is a battery, and thus must not be

referred to ground.

A drawback of this approach, intrinsic to the boost topology, is that a path between the input

and output is always present. This does not allow effective short-circuit protection and can

generate a battery discharge also when the device is turned off.

9.2.3

Driving white LEDs: buck/boost topology

In case a single Li-Ion cell is used at the input, a buck/boost topology can be used, as shown

in

Figure 25.

Figure 26.

Buck boost topology schematic

The relationship between the output current and the switch current is the same as the boost

topology. An advantage of this topology compared with the boost, is that when the device is

turned off, there is no current path between the input and the output. This allows an effective

short-circuit protection and minimizes the current drawn from the battery when the device is

turned off.

A dimming control can be developed by turning on and off the device with a frequency of

around 100-200 Hz in order to avoid LED flickering. Another way to implement the led

dimming is to reduce the voltage drop across the resistor in series to the LED to a partition

of the FB voltage. The

Figure 27 and Figure 28 shows the relative circuits.

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OUT

IS

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