EVL6562A-LED

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 second 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 December 2009 Doc ID 15679 Rev 1 1/32 www.st.com Contents AN2983 Contents 1 2 3 Main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 System configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 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 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 2/32 Doc ID 15679 Rev 1 AN2983 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Figure 30. Figure 31. Figure 32. Figure 33. Figure 34. EVL6562A-LED: L6562A constant current inverse buck driver module . . . . . . . . . . . . . . . . 1 System configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Module schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Standard buck converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Inverse buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Controller diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Simulated waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 FOT net. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 toff delay (actual) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 toff delay (nominal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 I_Led/V_trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 I_Led/VLed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Enable/dimming detailed view (turn-on) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Enable/dimming detailed view (turn-off) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Dimming 1% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Dimming 10% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Dimming 50% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Dimming 90% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Dimming 99% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Auxiliary power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Short-circuit application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Short-circuit removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Short-circuit detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Load current decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 LED current (average, maximum, minimum) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 LED current (ripple) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Efficiency [%] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 LED current (average) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Efficiency [%] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 EVL6562A-LED electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Doc ID 15679 Rev 1 3/32 Main characteristics AN2983 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 ? 0OWER"US ? -AIN0OWER3UPPLY ? ,%$S3TRINGS ##$RIVERS ? ? #ONTROLLER ? #ONTROL"US !-V 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. 4/32 Doc ID 15679 Rev 1 AN2983 1.3 Main characteristics Interface Ideally the module should have only two pins and behave as a constant current sink, but for practical reasons, and in order to gain a higher degree of flexibility, its connector has the following pinout. 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) 6 n.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 (18 V) input/output ● 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 Doc ID 15679 Rev 1 5/32 Circuit description 2 AN2983 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. 6IN Module schematic !UX ?  0OWER ? $ ELAY &/4 ? ,%$S ? ? ! ,%$S 6CC IMM $? $ 0OWER ,  ! #ONTROLLER 3ECTION $? $IMM ? 3RING ,%$S ? + ? #URR 3ETT  3HORT0ROT ET # ? 3 !-V 2.1 Power section The topology of this stage is the so-called inverse buck (also referred to as modified or lowside buck). Simply stated, it is a standard buck converter with the power and ground connections interchanged, as shown in Figure 4 and Figure 5. 6/32 Doc ID 15679 Rev 1 AN2983 Figure 4. 9 Circuit description Standard buck converter 9LQ 9LQ / ' Figure 5. 9 6 9RXW 5/ 9RXW Inverse buck converter 6IN 6IN $ 2, 6OUT , 6IN?6OUT !-V 6 !-V Other than the power and ground connections being exchanged, there are no differences between the two configurations; the behavior and dimensioning of the inverse buck are the same as that of the standard buck. Refer to application note AN2928(a) for a detailed description and design rules. The following are a reminder of the fundamental applicable equations. Inductor current variation (charging period) Equation 1 ⎡ Vin − VLed ⎤ ΔI _ ton = ⎢ ⎥ ∗ ton L ⎣ ⎦ Inductor current variation (discharging period) Equation 2 ⎡ VLed ⎤ ΔI _ toff = ⎢ ⎥ ∗ toff ⎣ L ⎦ The circuit works in continuous conduction mode, then in steady-state the current variations during ton and toff are the same (in module) and equivalent to the ripple current I_rip. Ripple current Equation 3 I _ rip = ΔI _ ton = ΔI _ toff Duty cycle Equation 4 ⎡ ton ⎤ ⎡ VLed ⎤ D=⎢ ⎥=⎢ ⎥ ⎣ T ⎦ ⎣ Vin ⎦ Additionally, the average current of the LED can be expressed as: a. See Chapter 5: References on page 30. Doc ID 15679 Rev 1 7/32 Circuit description AN2983 Equation 5 I _ Led _ avg = I _ Led _ pk − 1 I _ rip 2 Equation 6 ⎡ VLed ⎤ I _ Led _ avg = I _ Led _ pk − 1 ⎢ ∗ toff 2 ⎣ L ⎥⎦ 2.2 L6562A controller The L6562A is used in a "fixed off time" and "peak current mode" topology. Figure 6 represents the controller with its main functional blocks, the FOT_Delay circuitry and the power section. Figure 6. Controller diagram ? ?$ELAY 4OFF ? 6IN ,%$S?3TRING ? $ ? 3TARTER ? , ? ? && :#$ ? ? ?$RIVER 'ATE 3 1 ? ? '$ ? -/3 ? ? 2 ? 1 ? 4$ #3 ? ? 23ENSE ? 0OWER3ECTION 6TH ? #ONTROLLER ? !-V At power-on, the "starter" sets the flip-flop, whose output (Q) goes high activating the gate driver (GD). The power MOSFET transistor is turned on and the load current (I_Led) flows through the LED diodes, inductor, power MOSFET transistor and sense resistor. The load current develops a voltage on the sense resistor: VS = I_Led * RSense. This voltage is applied to the CS input of the controller where it is compared to the reference voltage Vth = 1.08 V (nom.) When Vs becomes higher than Vth, the comparator's output goes high, activating the reset input of the FF. The FF Q output is set to low and the gate driver output voltage goes to gnd. The power MOSFET is turned off, Vs goes to zero and the I_Led current decreases, flowing through the LEDs, inductor and Flywheel diode. 8/32 Doc ID 15679 Rev 1 AN2983 Circuit description The falling edge of the gate driver starts the toff delay (see Section 2.3 on page 9). At the end of the toff delay the set input of the FF is activated and a new cycle begins. Figure 7. Simulated waveforms  ,B/HG /ͺ>ĞĚсůŽĂĚĐƵƌƌĞŶƚ      9' sсDK^ƌĂŝŶsŽůƚĂŐĞ       9BVHW sͺƐĞƚсĨůŝƉͲĨůŽƉƐĞƚŝŶƉƵƚ͕ dŚĞŝŶŝƚŝĂůŚŝŐŚůĞǀĞůŝƐĚƵĞƚŽƚŚĞ ƐƚĂƌƚĞƌĐŝƌĐƵŝƚ       9BUHVHW sͺƌĞƐĞƚсĨůŝƉͲĨůŽƉƌĞƐĞƚŝŶƉƵƚ        2.3   7LPHXV   !-V FOT (fixed off time) delay circuit Figure 8. FOT net 'ATE?$RIVER '$ &&?1 $ 2 # 2 # :#$#OMPARATOR :#$ &&?3 6 6 4OFF?$ELAY #ONTROLLER !-V The fixed off time delay circuit is implemented by connecting the gate drive (GD) output to the ZCD input by means of a diode D5 in series with R11 and C7 (in parallel), and ZCD to ground with R18 and C8 in parallel. There is a clamp circuit behind the ZCD pin of the controller that limits the maximum voltage to 5.7 V, and a comparator whose output goes high if the input voltage falls below the threshold level of 0.7 V. Hence when GD is high (10 V nominal), ZCD is forced to the clamp level of 5.7 V, but as soon as the gate driver goes low, the diode D5 turns off and the capacitor C8 discharges through R18 until the voltage reaches 0.7 V. At this point the comparator switches on and triggers the set input of the flip-flop, whose output goes high. Doc ID 15679 Rev 1 9/32 Circuit description AN2983 This causes the gate driver to go high again and then the power MOSFET to conduct. The time delay toff is simply governed by the equation of the discharge of the capacitor C8 through the resistor R18 with the boundary conditions V (t0) = 5.7 V, V (t1) = 0.7 V. Equation 7 V(t1) = V(t 0 ⎡ (toff) ⎤ −⎢ ⎥ ) * e ⎣ R18 *C8 ⎦ Equation 8 ⎡ V(t1) ⎤ ⎡ toff ⎤ ln⎢ ⎥ = −⎢ ⎥ ⎣ V(t 0 ) ⎦ ⎣ R18 * C 8 ⎦ Equation 9 ⎡ V(t ) ⎤ toff = (R18 * C 8 ) * ln⎢ 0 ⎥ ⎣ V(t1) ⎦ With our values this gives: Equation 10 ⎡ 5 .7 ⎤ toff = (R18 * C 8 ) * ln⎢ ⎥ = 2.1 * (R18 * C 8 ) = 1.17μS ⎣ 0 .7 ⎦ Refer to AN2782(b) for a detailed description of the FOT controller. Figure 9 shows the key waveforms with the LEDs’ current (I_LED), capacitor (C8) voltage (V_C8) and gate drive output voltage (V_GD). Figure 9. toff delay (actual) 4R ACES #( ;GREEN= )?,%$ #( ;BLUE= 6?# #( ;RED= 6?'$ !-V It is worth noting that the real toff time is 1.57 µS and that the low threshold is 0.44 V. This is due to the components’ tolerance and to the propagation delay from the comparator input to the gate driver output. b. See Chapter 5: References on page 30. 10/32 Doc ID 15679 Rev 1 AN2983 Circuit description If we consider the time at which the capacitor voltage crosses the 0.7 V threshold, the equivalent toff time is 1.32 µS (+150 nS compared to the nominal value, due to the tolerance of C8 and stray capacitance) while the remaining 250 nS are related to the propagation delay (during which the voltage falls from 0.7 V to 0.44 V). Figure 10. toff delay (nominal) Traces: CH(4) green=I_LED; CH(2) blue=V_C8; CH(1) red=V_GD; AM04261v1 2.4 Current setting As already indicated, the CS pin of the L6562A controller is internally connected to the noninverting input of the CS comparator, whose threshold (Vth) is fixed to 1.08 V (nom.) Tying it to the sense resistor forces the comparator’s output high when I_Led = Vth / RS. This is a very simple way to detect – and limit – the peak load current to a fixed value. In order to make this value adjustable, we can introduce an auxiliary voltage source (Va), connected to the CS pin through a resistor (Ra) and another resistor (Rb) between CS and the sense resistor, as indicated in Figure 11. In this way the auxiliary source can be used to modify the threshold at which the (peak) load current triggers the comparator. Figure 11. Current setting !-V Doc ID 15679 Rev 1 11/32 Circuit description AN2983 Equation 11 Vth = Vs − Ib * Rb Equation 12 Vth = Va − Ia * Ra Neglecting the current that flows though the CS pin (1 µA max): Equation 13 Ib = −Ia And then: Equation 14 Vth = Vs − Ib * Rb Equation 15 Vth = Va + Ib*Ra Then: Equation 16 Vs = Vth + Ib*Rb and: Equation 17 ⎛ 1 ⎞ Ib = ⎜ ⎟ * (Vth − Va) ⎝ Ra ⎠ Equation 18 ⎛ 1 ⎞ Vs = Vth + ⎜ ⎟ * (Vth − Va) * Rb ⎝ Ra ⎠ Equation 19 ⎛ Rb ⎞ ⎛ Rb ⎞ Vs = Vth + ⎜ ⎟ * Vth − ⎜ ⎟ * Va ⎝ Ra ⎠ ⎝ Ra ⎠ Equation 20 ⎛ Ra + Rb ⎞ ⎛ Rb ⎞ Vs = Vth * ⎜ ⎟ − Va * ⎜ ⎟ ⎝ Ra ⎠ ⎝ Ra ⎠ Equation 21 ⎛ Ra + Rb ⎞ ⎛ Rb ⎞⎤ ⎛ 1 ⎞ ⎡ I _ Led _ pk = ⎜ ⎟ − Va * ⎜ ⎟⎥ ⎟ * ⎢Vth * ⎜ Rs Ra ⎝ ⎠ ⎝ Ra ⎠⎦ ⎝ ⎠ ⎣ If we consider the limit condition Va = 0, which is equivalent to connecting Ra directly to ground, the maximum peak LED current I_Led_pk (max) can be obtained by: 12/32 Doc ID 15679 Rev 1 AN2983 Circuit description Equation 22 ⎛ Ra + Rb ⎞ I _ Led _ pk(max) = I _ Led _ pk(nom) * ⎜ ⎟ ⎝ Ra ⎠ Doc ID 15679 Rev 1 13/32 Circuit description AN2983 Where, as already seen, the (nominal) LED peak current is: Equation 23 ⎛ 1 ⎞ I _ Led _ pk(nom) = ⎜ ⎟ * Vth ⎝ Rs ⎠ On the other hand, if we fix I_Led_pk = 0 we can estimate the value of Va for which the LED current is reduced to zero. Equation 24 ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ ⎛ Rb ⎞⎤ 0=⎜ ⎟ * ⎢Vth * ⎜ ⎟ − Va * ⎜ ⎟⎥ Rs Ra ⎝ ⎠ ⎣ ⎝ Ra ⎠⎦ ⎝ ⎠ Equation 25 ⎛ Rb ⎞ ⎛ Ra + Rb ⎞ Va * ⎜ ⎟ = Vth * ⎜ ⎟ ⎝ Ra ⎠ ⎝ Ra ⎠ Equation 26 ⎛ Ra + Rb ⎞ Va = Vth * ⎜ ⎟ ⎝ Rb ⎠ With our values of Ra = R19 = 10 kΩ, and Rb = R14 = 1 kΩ, Equation 21 can be rewritten as: Equation 27 ⎛ 1 ⎞ I _ Led _ pk = ⎜ ⎟ * [(1.1) * Vth − (0.1) * Va ] ⎝ Rs ⎠ Equation 22 becomes: Equation 28 I _ Led _ pk(max) = I _ Led _ pk(nom) * (1.1) And Equation 25: Equation 29 Va = Vth * (11) In the plot shown in Figure 12 the LED current (I_Led) is expressed as a function of the trim voltage V_trim (= Va) applied at the C_set module input, with the supply voltage (Vin) as parameter (fixed at 36, 48 and 60 V). As you can see, the linearity is quite good for currents in the range of 400 to 75 mA and voltages in the range 0... 10 V, while with V_trim from 10 to 12 V the current still decreases down to zero, but with some non-linearity. This is due to the fact that the LED diodes are a non-linear load and their behavior at low current levels may change from one device to another. Another point that has to be taken into account is that when the average current is reduced, the minimum LED current is reduced to zero, causing the converter to change from continuous conduction mode (CCM) to discontinuous conduction mode (DCM) operation. 14/32 Doc ID 15679 Rev 1 AN2983 Circuit description Figure 12. I_Led/V_trim !-V 2.5 LED number compensation The average output current depends on the number of LEDs connected to the module, or to be more precise, on the voltage (VLed) developed across them. This is due to the fact that: Equation 30 I _ Led _ avg = I _ Led _ pk − 1 I _ rip 2 and: Equation 31 ⎛ toff ⎞ I _ rip = ⎜ ⎟ * VLed ⎝ L ⎠ and then: Equation 32 ⎛ toff ⎞ I _ Led _ avg = I _ Led _ pk − 1 ⎜ * VLed 2 ⎝ L ⎟⎠ With a constant I_Led_pk, toff and L, it appears that if VLed increases, the average current decreases, and vice versa. Usually the number of LEDs is determined in the early stages of the design phase, but if required, it is possible to make the load current almost independent of the number of LEDs employed by connecting two resistors (R17 and R20) from the LEDs’ cathode to the CS pin of the controller. With a technique similar to that of the current setting, the circuit senses the voltage [Vin - VLed] that depends on the number of LEDs and corrects the voltage applied to the current sense pin. In this way the circuit behaves as if the internal comparator triggers when the average current – instead of the peak current – exceeds the threshold value. Doc ID 15679 Rev 1 15/32 Circuit description AN2983 The drawback of this configuration is that since the circuit is now more sensitive to input voltage variations, Vin has to be more tightly regulated. The following equations demonstrate what has been previously asserted. Equation 33 ⎛ toff ⎞ I _ Led _ avg = I _ Led _ pk − 1 ⎜ * VLed 2 ⎝ L ⎟⎠ Equation 34 ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ ⎛ Rb ⎞⎤ I _ Led _ pk = ⎜ ⎟ * ⎢Vth * ⎜ ⎟ − Va * ⎜ ⎟⎥ ⎝ Rs ⎠ ⎣ ⎝ Ra ⎠⎦ ⎝ Ra ⎠ Where: I_Led_avg is the LEDs’ average current, I_Led_pk is the LEDs’ peak current and Va = (Vin - VLed) is the LEDs’ cathode voltage. Equation 35 ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ ⎛ Rb ⎞⎤ 1 ⎛ toff ⎞ I _ Led _ avg = ⎜ ⎟ * ⎢Vth * ⎜ ⎟ − (Vin − VLed ) * ⎜ ⎟⎥ − 2 ⎜ ⎟ * VLed ⎝ Rs ⎠ ⎣ ⎝ Ra ⎠ ⎝ Ra ⎠⎦ ⎝ L ⎠ Equation 36 ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ ⎛ Rb ⎞⎤ I _ Led _ avg = ⎜ ⎟ * ⎢Vth * ⎜ ⎟ − Vin * ⎜ ⎟⎥ + ⎝ Rs ⎠ ⎣ ⎝ Ra ⎠ ⎝ Ra ⎠⎦ ⎡⎛ 1 ⎞ ⎛ Rb ⎞ 1 ⎛ toff ⎞⎤ + VLed * ⎢⎜ ⎟*⎜ ⎟ − 2⎜ ⎟⎥ ⎝ L ⎠⎦ ⎣⎝ Rs ⎠ ⎝ Ra ⎠ To make I_Led_avg independent of VLed, the VLed coefficient has to be reduced to zero. Equation 37 1 ⎛ Rb ⎞ 1 ⎛ toff ⎞ *⎜ ⎟ − 2⎜ ⎟=0 Rs ⎝ Ra ⎠ ⎝ L ⎠ That is to say, setting: Equation 38 ⎛ Rb ⎞ 1 ⎛ toff ⎞ ⎜ ⎟ = 2⎜ ⎟=0 ⎝ Ra ⎠ ⎝ L / Rs ⎠ reduces Equation 36 to: Equation 39 ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ ⎛ Rb ⎞⎤ I _ Led _ avg = ⎜ ⎟ * ⎢Vth * ⎜ ⎟ − (Vin) * ⎜ ⎟⎥ ⎝ Rs ⎠ ⎣ ⎝ Ra ⎠ ⎝ Ra ⎠⎦ In other words, connecting Ra to the cathode of the LEDs’ string, the average LED current depends only on Vth and Vin. 16/32 Doc ID 15679 Rev 1 AN2983 Circuit description Going into further detail, we also have to consider the delay of the current sense comparator (tdel) and modify Equation 33 as follows. Equation 40 I _ Led _ avg = I _ Led _ pk − 1 I _ rip + I _ del 2 With: Equation 41 ⎛ tdel ⎞ I _ del = ⎜ ⎟ * (Vin − VLed ) ⎝ L ⎠ Equation 42 ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ ⎛ Rb ⎞⎤ ⎛ tdel ⎞ I _ Led _ avg = ⎜ ⎟ * ⎢Vth * ⎜ ⎟ − Vin * ⎜ ⎟⎥ + Vin * ⎜ ⎟+ Rs Ra Ra ⎝ ⎠ ⎣ ⎝ ⎠ ⎝ ⎠⎦ ⎝ L ⎠ ⎡⎛ 1 ⎞ ⎛ Rb ⎞ 1 ⎛ toff ⎞⎤ ⎛ tdel ⎞ + VLed * ⎢⎜ ⎟*⎜ ⎟ − 2⎜ ⎟⎥ − VLed * ⎜ ⎟ Rs Ra L ⎠ ⎝ ⎠ ⎝ ⎠⎦ ⎝ L ⎠ ⎣⎝ Equation 43 ⎡⎛ Rb ⎞ ⎛ tdel ⎞⎤ ⎤ ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ I _ Led _ avg = ⎜ ⎟ * ⎢Vth * ⎜ ⎟ − Vin * ⎢⎜ ⎟−⎜ ⎟⎥ ⎥ + ⎝ Rs ⎠ ⎢⎣ ⎝ Ra ⎠ ⎣⎝ Ra ⎠ ⎝ L / Rs ⎠⎦ ⎥⎦ ⎡⎛ 1 ⎞ ⎛ Rb ⎞ 1 ⎛ toff ⎞ ⎛ tdel ⎞⎤ + VLed * ⎢⎜ ⎟*⎜ ⎟ − 2⎜ ⎟−⎜ ⎟⎥ ⎝ L ⎠ ⎝ L ⎠⎦ ⎣⎝ Rs ⎠ ⎝ Ra ⎠ And with Equation 38 that now becomes: Equation 44 ⎛ Rb ⎞ 1 ⎛ toff ⎞ ⎛ tdel ⎞ ⎜ ⎟ = 2⎜ ⎟+⎜ ⎟ ⎝ Ra ⎠ ⎝ L / Rs ⎠ ⎝ L / Rs ⎠ And Equation 40: Equation 45 ⎡⎛ Rb ⎞ ⎛ tdel ⎞⎤ ⎤ ⎛ 1 ⎞ ⎡ ⎛ Ra + Rb ⎞ I _ Led _ avg = ⎜ ⎟ * ⎢Vth * ⎜ ⎟ − Vin * ⎢⎜ ⎟−⎜ ⎟⎥ ⎥ ⎝ Rs ⎠ ⎣⎢ ⎝ Ra ⎠ ⎣⎝ Ra ⎠ ⎝ L / Rs ⎠⎦ ⎦⎥ With the values of our application: Equation 46 toff = 1.57 μ S, tdel = 0.2 μ S, L = 470 μ H, Rs = 2.8 Ω Equation 47 ⎛ L ⎞ ⎛⎜ 470 * 10 −6 ⎜ ⎟= 2 .8 ⎝ Rs ⎠ ⎜⎝ ( ) ⎞ ⎟ = 1.68 * 10 − 4 S ⎟ ⎠ Equation 48 1 (toff ) + (tdel) = (0.985 * 10 −6 )S 2 Doc ID 15679 Rev 1 17/32 Circuit description AN2983 And then: Equation 49 ( ) 1.68 * 10 −4 ⎛ Ra ⎞ = 170 ⎜ ⎟= ⎝ Rb ⎠ 0.985 * 10 −6 ( ) On the demonstration board several measures have been taken with Vin = 48 V, and Rb = 1 kΩ. Figure 13 is the plot of the LED current as a function of the voltage on the LED string (VLed) with Ra (actually R17 + R20) as a parameter. Figure 13. I_Led/VLed !-V 2.6 Shutdown/dimming The D_Dim input (pin 1 of the J1 connector) has a dual function. ● Module enable ● Digital (PWM) dimming Forcing it low, or leaving it floating, enables the controller's normal activity (current sink), while pulling it high (> 2.7 V) causes the module to shut down. Therefore, this input can be used to switch the load on and off. Figure 14 and Figure 15 highlight the turn-on and turn-off with an input voltage (Vin) of 48 V and an equivalent output voltage (VLed) of 20 V. In these conditions, the rise and fall times of the load current are lower than 10 µS. Applying a square wave to this pin forces the module to 'work and stop' at the input signal frequency; in this way the average load current can be modified by simply changing the duty cycle of the PWM control. Supposing a dimming frequency of 200 Hz is used, and taking into account the 10 µS of rise/fall time of the LED current, we can estimate the minimum dimming as (20 uS)*(200 Hz) = 0.4%. Figure 16, 17, 18, 19, and 20 show the LED current for different dimming factors between 1% and 99%. 18/32 Doc ID 15679 Rev 1 AN2983 Note: Circuit description R1 = 10 kΩ, C9 = 100 pF, and D1= 5.6 V Zener diode. All these devices have been introduced to protect the controller from excessive input voltages or noise; in a complete system, with a driver directly connected, they can be avoided. Figure 14. Enable/dimming detailed view (turn-on) #(GREEN)?,%$#( REDGATEDRIVE#( PURPLE$?$)-- !-V Figure 15. Enable/dimming detailed view (turn-off) #(GREEN)?,%$#( REDGATEDRIVE#( PURPLE$?$)-- Doc ID 15679 Rev 1 !-V 19/32 Circuit description AN2983 Figure 16. Dimming 1% Figure 17. Dimming 10% !-V !-V CH(4) green: LED current [mA] CH(1) red: gate drive output [V] CH(3) purple: D_Dimm input [V] CH(4) green: LED current [mA] CH(1) red: gate drive output [V] CH(3) purple: D_Dimm input [V] Figure 18. Dimming 50% !-V CH(4) green: LED current [mA] CH(1) red: gate drive output [V] CH(3) purple: D_Dimm input [V] 20/32 Doc ID 15679 Rev 1 AN2983 Circuit description Figure 19. Dimming 90% Figure 20. Dimming 99% !-V !-V CH(4) green: LED current [mA] CH(1) red: gate drive output [V] CH(3) purple: D_Dimm input [V] 2.7 CH(4) green: LED current [mA] CH(1) red: gate drive output [V] CH(3) purple: D_Dimm input [V] Auxiliary power The components for the auxiliary power supply are Q1, D2, C2, R2, R3, and R5. This section is included as a commodity to reduce the main voltage (48 V nominal) to the 18 V required from the Vcc input of the L6562A controller, thus avoiding the use of another power supply. It has been designed to work with input voltages in the range of 33 to 65 V, but can be easily resized for different ranges. Note that it is a linear regulator and that its efficiency is not very good. For this reason, in real applications where multiple modules are employed, it is better to consider a solution with an external supply common to all the modules. Simply removing R2 and R5 disconnects the entire block and allows the power supply (Vcc) to be provided from pin 5 of the header connector. Figure 21. Auxiliary power 6IN 2 2 1 2 # 6CC Doc ID 15679 Rev 1 $ !-V 21/32 Circuit description 2.8 AN2983 Open-/short-circuit protection As indicated, one of the requirements is that the module can sustain open and short circuits indefinitely and restart the correct functionality as soon as the fault is removed. From the "open circuit" point of view the module is intrinsically safe: if the load is disconnected, no current runs through the sense resistor and the controller drives the power MOSFET transistor in conduction and pin 10 of the J1 connector to ground. Whenever the load is reconnected, the current restarts to flow and normal operation is resumed. The short-circuit condition is more critical. At the end of the toff delay, the power MOSFET is turned on, the current starts to flow and charges the inductor, and the voltage on the sense resistor quickly reaches the threshold level. However, because of the controller’s internal delay (175 nS nom.) the power MOSFET does not shut down immediately; as such, a minimal amount of energy is still transferred to the inductor, but during the toff time – since the load is a short-circuit – this energy is NOT dissipated. The result is that the load current rises abnormally, leading to catastrophic failure unless the cycle is blocked. For this reason the circuit outlined in Figure 22 has been introduced. Figure 22. Short-circuit protection 6CC 2 2 $ $ ).6 1 # 23ENSE 2 !-V In normal conditions the voltage developed across the sense resistor is not sufficient to turn on Q3. The INV pin of the controller is then at: Equation 50 ⎛ ⎞ 22K R9 ⎛ ⎞ ⎟⎟ = 1.44V (Vcc) * ⎜ ⎟ = (18V) * ⎜⎜ + + ( 220 K 33 K 22 K ) R 6 R 7 R 9 + + ⎝ ⎠ ⎝ ⎠ On the other hand, in the case of a short-circuit, the load current increases until Q3 goes in conduction, C5 is discharged and the INV pin goes to: Equation 51 ⎛ ⎞ 22K ⎛ R9 ⎞ ⎟⎟ = (0.25) * (0.4)V = 0.1V (Q3 _ Vce _ sat) * ⎜ ⎟ = (0.25V) * ⎜⎜ ⎝ R7 + R9 ⎠ ⎝ (33K + 22K ) ⎠ The controller then shuts down, the power MOSFET stops conducting, the inductor discharges and the load current decays to zero. Since no current flows through the sense 22/32 Doc ID 15679 Rev 1 AN2983 Circuit description resistor, Q3 turns off allowing C5 to charge through R6, and the voltage of the INV pin rises again. When it reaches 0.45 V the disable condition is removed and the controller restarts. If the short-circuit condition is removed, the circuit restores its normal functionality. If the short-circuit condition persists, the hiccup cycle is repeated. Figure 23. Short-circuit application Figure 24. Short-circuit removal #( 'REEN)?,ED#( "LUE6CE?1 #( 2ED62SENSE !-V #( 'REEN)?,ED#( "LUE6CE?1 #( 2ED62SENSE !-V Figure 25. Short-circuit detection Figure 26. Load current decay #( 'REEN)?,ED #( 2ED62SENSE &+*UHHQ,B/HG &+5HG95VHQVH !-V Doc ID 15679 Rev 1 !-V 23/32 Measurements AN2983 3 Measurements 3.1 LED voltage dependency The first set of measures was taken at a nominal input voltage with Vin = 48 V and with the output voltage (Vled) as a parameter. Figure 27. LED current (average, maximum, minimum) !-V Figure 28. LED current (ripple) !-V 24/32 Doc ID 15679 Rev 1 AN2983 Measurements Figure 29. Switching frequency !-V Figure 30. Efficiency [%] !-V Doc ID 15679 Rev 1 25/32 Measurements 3.2 AN2983 Input voltage dependency A second set of measures was taken varying the input voltage from 36 to 60 V with several load conditions as parameters (VLed from 15 to 45 V). M! Figure 31. LED current (average) ;6= !-V Figure 32. Switching frequency ;6= !-V Figure 33. Efficiency [%] ;6= !-V 26/32 Doc ID 15679 Rev 1 Doc ID 15679 Rev 1  5 %$6 ' %$6 ' 5 N 5 N  S) & S) &  ' ' N 5 ' 006=7* 4 %&& Q) & X) %$6 %$6 5 N 5 N N 5 &B6HW 'B',00     S) & - +($'(5 327N (;7B3 /('6B. =&' *1' *' 9FF     BRKP BRKP 9FF Q) & Q) %&; 5 & 4  9,1B*1' 9,1 /('6B$ 5 /$ &6 08/7 &203 ,19 8 9FFB*1' (;7B& 9FF             . 5 QP  & 006=7* ' (;7B/('V 0/('V S) & S) 5 N & %$6  5 . 5  5 BRKP 5 %$6 ' X)9/ & N 5 ' 9,1 9,1B*1' 5 N . 5 5  5  4 6711) X+ / 6736+$ ' 5 . 4  AN2983 Electrical schematic and bill of materials Electrical schematic and bill of materials Figure 34. EVL6562A-LED electrical schematic !-V 27/32 Electrical schematic and bill of materials Table 2. Item Qty AN2983 EVL6562A-LED BOM Reference Part PCB footprint Notes 1 1 C1 0.22 µF 100VL 1812 2 3 C2,C3,C5 220 nF 805 3 1 C4 Not mounted 4 3 C6,C8,C9 100 pF 805 5 1 C7 220 pF 805 6 1 C10 820 pF 7 1 D1 MMSZ4690T1G SOD-123 8 1 D2 MMSZ18T1G SOD-123 9 1 D3 STPS2H100A SMA 10 6 D4,D5,D6,D7, D8,D9 BAS316 SOD-323 11 1 EXT C 0.22 µF External jig 12 1 EXT LEDs MLEDs LEDs string 13 1 EXT P POT 10 kΩ multiturns External jig 14 1 J1 Strip header, 10-pin, 90°, 2.54 mm pitch 15 1 L1 470 µH 16 1 Q1 BCX56 SOT-89 17 1 Q2 STN3NF06 SOT-223 18 1 Q3 BC846C SOT-23 19 3 R1,R12,R19 10 kΩ 805 20 1 R2 1 kΩ 1206 21 1 R3 18 kΩ 1206 22 3 R4,R5,R13 0Ω 805 23 1 R6 220 kΩ 805 24 1 R7 33 kΩ 805 25 1 R8 1Ω 805 26 1 R9 22 kΩ 805 27 1 R10 10 Ω 805 28 1 R11 1.5 kΩ 805 29 1 R14 1.0 kΩ 805 30 2 R15,R16 5.6 Ω 1210 31 1 R17 100 kΩ 805 32 1 R18 5.6 kΩ 805 33 1 R20 68 kΩ 805 28/32 Doc ID 15679 Rev 1 STMicroelectronics Coilcraft MMS1260-474KLB STMicroelectronics AN2983 Table 2. Item Qty Electrical schematic and bill of materials EVL6562A-LED BOM (continued) Reference Part PCB footprint 34 1 R21 470 805 35 1 U1 L6562A SO-8 Doc ID 15679 Rev 1 Notes STMicroelectronics 29/32 References 5 Note: 30/32 AN2983 References 1. AN2928 2. AN2782 3. L6562A datasheet These references are available on the STMicroelectronics web site at www.st.com. Doc ID 15679 Rev 1 AN2983 6 Revision history Revision history Table 3. Document revision history Date Revision 16-Dec-2009 1 Changes Initial release. Doc ID 15679 Rev 1 31/32 AN2983 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. 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