AN2115 Datasheet, PDF (30/33 Pages) STMicroelectronics – This application note details the main features

Application ideas
AN2115
10.2.3
Equation 24
I_OUT = I_SWITCH(1 – D)
This topology is possible only because the input source is a battery, and so it must not be
referred to ground. A drawback of this approach, which is intrinsic in 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
even when the device is turned off.
Driving white LEDs: buck boost topology
When a single Li-Ion cell is used at the input, a buck boost topology can be employed, as
shown in Figure 26.
Figure 26. Buck boost LED
The relationship between the output current and the switch current is the same as in the
boost topology. An advantage of this topology compared with the boost topology, however, is
that when the device is turned off there is no current path between the input and the output.
This allows 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 the
device on and off with a frequency of around 100 - 200 Hz in order to avoid LED flickering
Figure 27. Another way to implement LED dimming is by changing the voltage of the
dimming resistor Figure 28.
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Doc ID 11165 Rev 5