I have 2 shematics of circuits in order to drive a LED I want to know how exactly these circuits work and what is the reason of using each components and why we need a dc-dc converter Tnx very much the link of the image is http://www.infineon.com/dgdl/LED+Driving+Concepts+and+Infineon+Basic+LED+Drivers_V1.1.pdf?folderId=db3a304314dca389011561889ef01fe7&fileId=db3a30432f91014f012f96084ec339a3
What you have in the first schematic is a buck-converter. This is a special kind of switched-mode power supply, intended to regulate a higher voltage to a lower one. But instead of regulating a constant output voltage, it is using the resistor Rsense to regulate a constant current (when the voltage across Rsense is constant, so is the current through it). This ensures that the current through the LEDs is always the same, regardless of possible losses in the cables, or changing input voltage.
The second circuit is nearly the same, but uses a separate constant-current regulator after the buck converter. So the converter operates in constant voltage mode, and the separate regulator is responsible for creating a constant current flow.
Using a switching regulator, instead of a linear one, leads to reduced power losses, and reduces the created heat.
Most of the LEDs' brightness/luminance depend on its current.
Having constant current flowing through the LED will maintain the brightness consistency but it's not a good idea both in terms of power consumption (P=IV) and LED life span.
Hence, circuit are designed to drive/pulse the LED at high frequency (higher than human eye can perceive), higher current to bring out the max value of an LED. The extra components you see in the block diagram is to drive LEDs with optimal result (lower power consumption, consistent brightness if not brighter).
SMPS still have regulation losses that are significant for small strings. Maximizing the string size improves efficiency but increases risk of exposure to parasitic effects of polarity reversal on rapid turn-off so leakage R ladders are suggested in parallel and used by Sharp recently in their Hybrid arrays of LEDs.
You can use CV mode on LED strings if you can guarantee thermal conductance is high so that thermal runaway does not occur. This is done by minimizing Tja and adding enough Rs to compensate for reduction in Vf vs T. This can be very inefficient or very efficient depending on design of operating point, heatsink and ambient.
p.s. ( I successfully employ this design on 85 Watts of LEDs in strings of 6s x 13p using 1W LEDs. Each string of 6 LEDs on Alum substrates with Alum heat spreader strip 6 ft apart using speaker wire parallel, 19.5V no load & 19.0V rated @ 4.5A max , intended as universal laptop charger) All PCA's are equal brightness due to my choice of LEDs. Your mileage will vary.