AN11136_15 Datasheet, PDF (6/22 Pages) NXP Semiconductors – SSL2109T/AT/SSL2129AT controller for SSL applications

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2. Application diagram
A typical buck application for the SSL2109T/AT and the SSL2129AT, driving a single LED chain, is shown in Figure 3. The
mains voltage is rectified, filtered and buffered in the input section. From there, the current flows through the LED string and
the inductor to pin DRAIN of the external MOSFET Q1. During switching of the external MOSFET, the amount of energy
stored in L2 determines the current through the LED chain. During the primary stage when the MOSFET is ON, the current
through the inductor is sensed via resistors R4/R5. When 0.5 V (V_{th(ocp)SOURCE}) is reached at pin SOURCE, the MOSFET is
switched off.
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Fig 3. SSL2109T/AT application diagram
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The external MOSFET is switched on again (start of new primary stage), when the secondary stage has ended (no current
through the inductor). However, this state only occurs after a valley is detected on pin DRAIN. Calculations of most
components are described in the application note “SSL21081, SSL21083, and SSL2109 non-dimmable buck converter in
low ripple configurations” (AN11041).