AN1882

Application ideas

Doc ID 10209 Rev 4

25/30

9

Application ideas

9.1

Buck boost topology

In portable applications, the input voltage changes significantly due to the battery discharge

profile, which often depends on parameters like temperature, discharge rate, battery ageing,

etc. Moreover, in certain applications, the output voltage requirements can also change.

This could imply that is not possible to provide the desired regulated output voltage by using

the simple buck topology. This problem is often present, for example, in systems using a

single Li-Ion cell, whose voltage profile changes from 4.2 V down to 2.7 V or less. In fact, in

these systems, a 3.3 V output is normally required to power processor I/O, memory and

logic. Adopting the buck topology, the 3.3 V output can be regulated until the battery voltage

is approximately 3.4 V, also depending on the minimum dropout of the regulator. Depending

on the battery type and conditions, this would leave unused some 20%-30% of its capacity.

Another application, even more critical, is the power management of 3G phones, where a

3.7 V or more can be required to power the RF power amplifier (PA). In order to use the full

battery capacity also in these applications, a positive buck/boost topology can be used.

Figure 22 shows how to implement it. This topology can be more suitable, compared to a

standard buck, depending on the battery discharge profile and the load conditions. In fact,

the efficiency loss of the buck/boost topology can be translated into an equivalent loss in

battery capacity. This can then be compared with the gain in battery capacity due to the fact

that it is used over the full voltage range.

Figure 23.

Positive buck boost application. 1 Li-Ion cell to 3.3 V@0.25 A

9.2

White LED

White LEDs are now widely used both for LCD backlighting and for illumination. Since their

brightness is proportional to the current flowing through them, a current control loop must be

implemented instead of a voltage one. The device can be used in current control

architecture by simply inserting a sense resistor between the FB and GND pins and

connecting the LED in series with it. The loop will set 0.6 V across the sense resistor, and

so, a constant current flowing through the LED. The current, and by consequence, the

brightness, can be adjusted by changing the resistor value or the voltage across it (by

partitioning the FB pin 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.