PM6675 datasheet(23/47 Pages) STMICROELECTRONICS

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PM6675

Device description

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No-Audible Pulse-Skip Mode

Some audio-noise sensitive applications cannot accept the switching frequency to enter the

audible range as it is possible in Pulse-Skip mode with very light loads. For this reason, the

PM6675 implements an additional feature to maintain a minimum switching frequency of

33kHz despite a slight efficiency loss. At very light load conditions, if any switching cycle has

taken place within 30

µs (typ.) since the last one (because of the output voltage is still higher

than the reference), a No-Audible Pulse-Skip cycle begins. The low-side MOSFET is turned

on and the output is driven to fall until the reference point has been crossed. Then, the high-

side switch is turned on for a TON period and, once it has expired, the synchronous rectifier

is enabled until the inductor current reaches the zero-crossing threshold (see Figure 10).

Figure 10.

Inductor current and output voltage at light load with non-audible pulse-skip

For frequencies higher than 33kHz (due to heavier loads) the device works in the same way

as in Pulse-Skip mode. It is important to notice that in both Pulse-Skip and No-Audible

Pulse-Skip modes, the switching frequency changes not only with the load but also with the

input voltage.

Output

voltage

Inductor

current

Vreg

TMAX

TON TOFF

TIDLE

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