SSL2129AT datasheet(6/21 Pages) NXP

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SSL2129AT

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Product data sheet

Rev. 2 — 23 October 2012

6 of 21

NXP Semiconductors

SSL2129AT

Dimmable LED controller IC

8.4 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 the VIN  VOUT level. The peak to peak amplitude equals 2  VOUT. In

tapped buck topology, this amplitude is multiplied by the ratio of the windings.

A special feature, called valley detection is an integrated part of the SSL2129AT 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

When 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.

A series resistance can be included at the drain sensing pin for flyback mode to remove

the high-frequency ringing caused by the transformer leakage inductance.

Fig 4.

Waveforms and valley detection

aaa-001744

demagnetization

VIN

valley

VGATE

VDRAIN

VOUT

magnetization

t00

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