PM6686 datasheet(29/50 Pages) STMICROELECTRONICS

29 / 50 page

PM6686

Device description

Doc ID 15281 Rev 4

29/50

8.1.4

SKIP

To improve the efficiency at light load the PM6686 implements pulse skip operation mode.

When SKIP pin is tied to GND the inductor current is sensed and if it is equal to zero the

synchronous MOSFET is turned off. As a consequence the output capacitor is left floating

and the discharge depends only on the current sourced by the load. The new TON starts

when the output reaches the voltage regulation. As a consequence at light load conditions

the switching frequency decreases improving the total efficiency of the converter. Working in

discontinuous current mode, the switching and the conduction losses are decreased

skipping some cycles.

If the output load is high enough to make the system work in CCM, skip mode is

automatically changed in PWM mode.

Figure 48.

Inductor current and output voltage in SKIP mode

8.1.5

Non audible SKIP (NA SKIP)

To avoid audio noise the NA SKIP operation mode can be selected, connecting SKIP pin to

VREF2 or leaving it floating. In this condition if a new cycle doesn't start within 30

μs typ.

from the previous one the PM6686 turns on the low-side MOSFET to discharge the output

capacitor. The inductor current goes negative until the output reaches the voltage regulation

voltage allowing a new cycle to begin. If the switching frequency is above 33 kHz the device

works in SKIP mode.

This operation mode is useful to avoid audio noise but it lowers the efficiency at light loads if

it is compared to the SKIP mode.

Output

Inductor

current

Vreg

TON TOFF

PM6686 Datasheet(PDF) 29 Page - STMicroelectronics