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NCP12401EBAAA1 Datasheet(PDF) 32 Page - ON Semiconductor |
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NCP12401EBAAA1 Datasheet(HTML) 32 Page - ON Semiconductor |
32 / 44 page NCP12401 www.onsemi.com 32 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 ms (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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