Application ideas

AN2115

28/33

Doc ID 11165 Rev 5

10

Application ideas

10.1

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 using a

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 the 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, depending also on the minimum dropout of the regulator. Depending

on the battery type and conditions, this would leave unused some 20 - 40% of its capacity.

Another even more critical application is the power management of 3G phones, where 3.7 V

or more can be required to power the RF power amplifier (PA).

In order to use the full battery capacity in these applications, a positive buck boost topology

can be used. Figure 23 shows how to implement this topology using the L6928. This

topology may be more suitable than 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

10.2

White LEDs

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 rather than a voltage control loop. The L6928 can be used in a 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

thus a constant current flow through the LED. The current, and therefore the brightness, can

be adjusted by changing the resistor value or the voltage across it (by partitioning the FB pin