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december 2009 doc id 15679 rev 1 1/32 AN2983 application note constant current inverse buck led driver using l6562a introduction whenever a lighting application, such as street lighting for example, requires an elevated number of leds, there are basically two solutions : the first is to connect all the diodes in series in a single "string"; the second is to place several strings in parallel with fewer elements in each one. the first solution, even if simpler, poses stringent safety requirements due to the high supply voltage. the latter needs a lower input voltage but the current through each string has to be independently controlled. since, from a system point of view, the seco nd solution seems more viable, we have developed an application to investigate the possibility of employing an l6562a to implement such a constant current controller. this document describes the evl6562a-led demonstration board and summarizes the relevant results obtained. figure 1. evl6562a-led: l6562a constant current inverse buck driver module ! - v www.st.com
contents AN2983 2/32 doc id 15679 rev 1 contents 1 main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 system configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 power section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 l6562a controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3 fot (fixed off time) delay circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4 current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.5 led number compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.6 shutdown/dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.7 auxiliary power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.8 open-/short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3 measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.1 led voltage dependency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.2 input voltage dependency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4 electrical schematic and bill of materials . . . . . . . . . . . . . . . . . . . . . . . 27 5 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 AN2983 list of figures doc id 15679 rev 1 3/32 list of figures figure 1. evl6562a-led: l6562a constant current inverse buck driver module . . . . . . . . . . . . . . . . 1 figure 2. system configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 figure 3. module schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 4. standard buck converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 5. inverse buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 6. controller diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 7. simulated waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 8. fot net. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 9. toff delay (actual) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 10. toff delay (nominal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 11. current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 12. i_led/v_trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 13. i_led/vled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 14. enable/dimming detailed view (turn-on) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9 figure 15. enable/dimming detailed view (turn-off) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 16. dimming 1% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 17. dimming 10% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 18. dimming 50% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 19. dimming 90% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 20. dimming 99% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 21. auxiliary power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 22. short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 23. short-circuit application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 24. short-circuit removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 25. short-circuit detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 26. load current decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 27. led current (average, maximum, minimum) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 28. led current (ripple) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 29. switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 30. efficiency [%] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 31. led current (average) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 32. switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 33. efficiency [%] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 34. evl6562a-led electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 7 main characteristics AN2983 4/32 doc id 15679 rev 1 1 main characteristics 1.1 system configuration figure 2 shows a possible system configuration, in which the key modules are: the main power supply, which converts the ac input voltage to an internal power bus. the cc drivers, which are the modules that implement the constant current sources. the controller that configures the cc current drivers through a dedicated bus. the leds strings. the cc drivers are the modules considered in this application note and implemented in the evl6562a-led demonstration board. figure 2. system configuration 1.2 requirements the board?s design takes into account the following key points. input voltage: 48 v (+/- 20%) output current (average): 0.35 a output ripple current < 140 ma (+/- 20%) output current setting/calibration digital dimming open-/short-circuit protection absence of electrolytic capacitors the design can be easily changed to adapt to different needs. ! - v ? - a i n 0 o w e r 3 u p p l y ? 0 o w e r " u s ? , % $ s 3 t r i n g s ? # o n t r o l l e r ? # # $ r i v e r s ? # o n t r o l " u s AN2983 main characteristics doc id 15679 rev 1 5/32 1.3 interface ideally the module should have only two pins a nd behave as a constant current sink, but for practical reasons, and in order to gain a highe r degree of flexibility, its connector has the following pinout. d_dimm is a digital (ttl) input for the module . a high level shuts off the circuit. a low level enables the nominal load current and a square wave with variable duty cycle can be used as a dimming control c_set is an analog input (0...12 v). a voltage applied to this pin is used to set the load current to the required value gnd pins 3 and 4 are the ground return for the controller?s power supply (vcc). d_dimm and c_set are referred to this ground vcc is the connection to the controller?s power supply input vin_gnd is the main power supply return, internally connected to the gnd pins vin is the main power supply input (48 v) leds_a is the connection to the anode of the diode string leds_k is the connection to the cathode of the diode string table 1. evl6562a-led interface pin number signal name connection notes 1 d_dimm shutdown / digital dimming digital input 2 c_set load current setting 0.. 12 v input 3 gnd auxiliary power (gnd) 4 gnd auxiliary power (gnd) 5 vcc auxiliary power (vcc) (18 v) input/output 6n.c. 7 vin_gnd main power (gnd) 8 vin main power (vin) (48 v nom.) input 9 leds_a leds anode interconnection output 10 leds_k leds cathode interconnection output circuit description AN2983 6/32 doc id 15679 rev 1 2 circuit description the following is a list of the main components that form the module evl6562a-led. power section l6562a controller fixed off time (fot) delay current setting led number compensation shutdown/dimming auxiliary power open-/short-circuit protection figure 3. module schematic 2.1 power section the topology of this stage is the so-called inverse buck (also referred to as modified or low- side buck). simply stated, it is a standard buck converter with the power and ground connections interchanged, as shown in figure 4 and figure 5 . ! - v , ! # o n t r o l l e r ? & |