SSL21081AT datasheet(7/22 Pages) PHILIPS

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SSL21081AT

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Product data sheet

Rev. 6 — 3 October 2013

7 of 21

NXP Semiconductors

SSL21081AT

Compact non-dimmable LED driver IC

8.2 Valley detection

A new cycle is started when the primary switch is switched on (see Figure 4). Following

time t1, when the peak current is detected on the SOURCE pin, the switch is turned off

and the secondary stroke starts (3). When the secondary stroke is complete and the coil

current at t3 equals zero, the drain voltage starts to oscillate around the VIN  VOUT level.

The amplitude equals VOUT. A special feature, called valley detection is an integrated part

of the SSL21081AT circuitry. Dedicated built-in circuitry connected to the DRAIN pin,

senses when the voltage on the drain of the switch has reached its lowest value. The next

cycle is then started and as a result the capacitive switching losses are reduced.

If both the frequency of the oscillations and the voltage swing are within the range

specified (fring and ∆Vvrec(min)) for detection, a valley is detected and accepted. If a valid

valley is not detected, the secondary stroke is continued until the maximum off-time

(toff(high)) is reached. Then the next cycle is started.

Fig 4.

Buck waveforms and valley detection

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