400DAFBA2 datasheet(32/47 Pages) ONSEMI

32 / 47 page

NCP12400

www.onsemi.com

Quiet−Skip

To further avoid acoustic noise, the circuit prevents the

burst frequency during skip mode from entering the audible

range by limiting it to a maximum of 800 Hz. This is

achieved via a timer tquiet that is activated during

Quiet−Skip. The start of the next burst cycle is prevented

until this timer has expired. As the output power decreases,

the switching frequency decreases. Once it hits minimum

switching frequency fOSC(min), the skip−in threshold is

reached and burst mode is entered − switching stops as soon

as the current drive pulses ends – it does not stop

immediately.

Once switching stops, FB will rise. As soon as FB crosses

the skip−exit threshold, drive pulses will resume, but the

controller remains in burst mode. At this point, a 1250

(typ) timer tquiet is started together with a count to nP,skip

pulses counter. This nP,skip pulses counter ensures the

minimum number of DRV signal pulses in burst. The next

time the FB voltage drops below the skip−in threshold, DRV

pulses stop at the end of the current pulse as long as nP,skip

drive pulses have been counted (if not, they do not stop until

the end of the nP,skip −th pulse). They are not allowed to start

again until the timer expires, even if the skip−exit threshold

is reached first. It is important to note that the timer will not

force the next cycle to begin – i.e. if the natural skip

frequency is such that skip−exit is reached after the timer

expires, the drive pulses will wait for the skip−exit

threshold.

This means that during no−load, there will be a minimum

of nP,skip drive pulses, and the burst−cycle period will likely

be much longer than 1250

ms. This operation helps to

improve efficiency at no−load conditions.

In order to exit burst mode, the FB voltage must rise higher

than Vskip(tran) level. If this occurs before tquiet expires, the

drive pulses will resume immediately – i.e. the controller

won’t wait for the timer to expire. Figure 63 provides an

example of how Quiet−Skip works, while Figure 62 shows

the immediate leaving the quiet skip mode by crossing the

transient enhancement level Vskip(tran).

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