SSL2109T datasheet(6/19 Pages) NXP

6 / 19 page

SSL2109T

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Product data sheet

Rev. 2 — 26 April 2012

6 of 19

NXP Semiconductors

SSL2109T

GreenChip controller for LED lighting

8.5 Valley detection

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

following sections on represents the conductive state and off the non-conductive state.

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

turned off and the secondary stroke starts at t2. When the secondary stroke is completed

with the coil current at t3 equaling zero, the drain voltage starts to oscillate at

approximately VIN  VOUT level. The peak to peak amplitude equals 2 × VOUT. A special

feature, called valley detection is an integrated part of the SSL2109T 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 at t00 and as a result

the capacitive switching losses are reduced. A valley is detected and accepted if both the

frequency of the oscillations and the voltage swing are within the range specified (fring and

∆Vvrec(min)) for detection. ∆Vvrec(min) is the voltage differential between the HV (pin) in and

the DRAIN pin. 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

aaa-001744

demagnetization

VIN

valley

VGATE

VDRAIN

VOUT

magnetization

t00

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