L6985F datasheet(24/65 Pages) STMICROELECTRONICS

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Functional description

L6985F

24/65

DocID027836 Rev 2

4.5.2

Low consumption mode (LCM)

The low consumption mode maximizes the efficiency at the light-load. The regulator

prevents the switching activity whenever the switch peak current request is lower than the

ISKIP threshold (350 mA typical). As a consequence the L6985F device works in bursts and

it minimizes the quiescent current request in the meantime between the switching operation.

In the LCM operation, the pin SYNCH/ISKIP level dynamically defines the ISKIP current

threshold (see Table 9).

The ISKIP programmability helps to optimize the performance in terms of the output voltage

ripple or efficiency at the light-load, that are parameters which disagree each other by

definition.

A lower skip current level minimizes the voltage ripple but increases the switching activity

(time between bursts gets closer) since less energy per burst is transfered to the output

voltage at the given load. On the other side, a higher skip level reduces the switching activity

and improves the efficiency at the light-load but worsen the voltage ripple.

No difference in terms of the voltage ripple and conversion efficiency for the medium and

high load current level, that is when the device operates in the discontinuous or continuous

mode (DCM vs. CCM).

Figure 14 and Figure 15 report the efficiency measurements with VOUT = 3.3 V to highlight

the ISKIPH and ISKIPL efficiency gap at the light-load also in comparison with the LNM

operation. The same efficiency at the medium / high load is confirmed at different ISKIP

levels.

Figure 14. Light-load efficiency comparison at different ISKIP - linear scale

Table 9. ISKIP current level

SYNCH / ISKIP (pin 4)

ISKIP current threshold

LOW

ISKIPH = 0.35 A typical

HIGH

ISKIPL = 0.1 A typical

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