PM6675AS datasheet(25/48 Pages) STMICROELECTRONICS

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PM6675AS

Device description

25/48

No-audible pulse-skip mode

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

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

PM6675AS implements an additional feature to maintain a minimum switching frequency of

33 kHz despite of 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 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 28).

Figure 28.

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

For frequencies higher than 33 kHz (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

PM6675AS 数据表(PDF) 25 Page - STMicroelectronics