SSL21081AT Datasheet, PDF (7/22 Pages) NXP Semiconductors – Compact non-dimmable LED driver IC

NXP Semiconductors
SSL21082AT
Mains dimmable LED driver IC
8.3 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 V_i  V_O level. The peak-to-peak amplitude equals 2  V_O. In a 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 SSL21082AT circuitry.
Dedicated built-in circuitry connected to the DRAIN pin, senses when the voltage on the
drain of the switch reaches its lowest value. The next cycle is then started at t₀₀ 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 (f_ring and V_vrec(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 (t_off(high)) is reached. Then the next cycle is started.
V_GATE
V_DRAIN
V_IN
0
magnetization
demagnetization
I_L
0
1
2
3
t₀
t₁ t₂
t₃
T
Fig 4. Waveforms and valley detection
V_OUT
valley
4
t₀₀
aaa-001744
SSL21082AT
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 6 — 3 October 2013
© NXP B.V. 2013. All rights reserved.
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