PM6670AS datasheet(29/54 Pages) STMICROELECTRONICS

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PM6670AS

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

PM6670AS implements an additional feature to maintain a minimum switching frequency of

33 kHz 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 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 32

Figure 32.

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.

VDDQ

Output

Inductor

current

Vreg

TMAX

TON TOFF

TIDLE

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