High Efficiency Universal LED Driver Solution for Emergency Lights Background In a LED emergency light application, a 12V, 18V or 24 battery pack powers the LED driver. The LED’s can be connected in series with 5~12 pieces per string, with the total LED voltage of 16V to 48V. Because the battery pack voltage can be lower or higher than the LED voltage, buck-boost topology will be used for some of the combinations of supply voltage range and a number of LED’s. The cases of LED voltage always higher than supply voltage will use boost topology because of the better efficiency and lower component voltage rating compared to buck-boost topology. This application note provides a simple universal circuit that can be easily configured in the field with optimum efficiency and minimum required component rating for LED emergency lights. The most popular 1W LED with 350mA current is used in this application note as an example. The LED current can be set by adjusting ACTL voltage with a potentiometer for varieties of LED current ratings. Introduction A basic buck-boost LED drive circuit diagram with RT8482 is shown below. L1 D1 Vin Vout Cin 10uF Q1 RT8482 15 VCC 13 5V Analog Dimming 9 GATE EN ISW ACTL ISN 10 8 Rvc 10k Cvc 3.3nF 11 2 VC SS GBIAS 1 Css 0.1uF LED's Rsense R1 Cb 1uF GND 16, Exposed Pad (17) NC 3 5 12 Revision: February 16, 2010 Cout 7 ISP 6 14 OVP DCTL Rsw 0.05 4 R2 A basic boost LED drive circuit diagram with RT8482 is shown below. L1 D1 Rsense Vin Vout Cin 10uF Q1 RT8482 15 VCC 13 5V GATE EN 9 Analog Dimming ISW ACTL ISP 10 8 11 Rvc 10k 2 Rsw 0.05 4 LED's 6 VC ISN 7 14 OVP SS GBIAS 1 DCTL Css 0.1uF Cvc 3.3nF Cout 1uF R1 Cb 1uF R2 GND 16, Exposed Pad (17) NC 3 5 12 A universal high efficiency LED emergency light application diagram using RT8482 with three configurable switches is shown below. It’s recommended to use a 3-in-1 interlocked switch (also known as 3P2T switch) for mode-selection. Vin S1, S2, S3: Cin 30uF =boost 15 D1 R29 ISW 10 0.2V ~ 1.2V S1 Vout Cout 4.7uF GATE 2 C2 0.47uF 9 =buck-boost Q1 1 GBIAS 27k 5.0k VCC RT8482 13 EN 5V R30 Analog Dimming L1 Rsw 0.05 ACTL DCTL ISP S2 6 Rsense R31 680 LED's 4 S3 ISN 7 8 OVP 14 R2 10k C5 3.3nF VC C11 10n 11 SS Css 10nF GND 16, Exposed Pad (17) NC 3 5 12 R32 390k R14 10k R21 Vout 169k Evaluation Board (EVB) Description RT8482 EVB provides a convenient mode configuration for LED emergency lighting application accepting wide range of input voltages and flexible number of LED to the light fixtures. A buck-boost or boost mode can be easily configured by flipping a 3P2T switch in the field depending on the condition of input voltage range and desired LED amounts for the lighting area. This 3P2T switch physically changes all three contact points as shown above simultaneously. Buck-boost mode should be used when input supply voltage is higher or lower than the total LED forward voltage. Boost mode should be used when input supply voltage is lower than the total LED forward voltage. Although buck-boost mode itself can cover all application corners, the efficiency for boost mode is higher than buck-boost’s efficiency, due to the effective power to the LED in boost mode is higher 2 than that of buck-boost mode. For example in a 5-LED design with 18V voltage input in buck-boost mode, the LED sees the effective 16.5V (5*3.5V) , but the total switch node voltage equals 34.5V (18V+16.5V) with respect to ground. It’s the same VIN, VOUT condition as an 18V to 34.5V boost mode conversion. They have similar conversion losses and the buck-boost delivers less output power than the corresponding boost, which explains the efficiency difference between the buck-boost and the boost. Another benefit of boost design over buck-boost is the lower requirement in component voltage rating, especially for the MOSFET Q1 and Schottky diode D1. VOUT of boost is roughly the total forward voltage of the LED string, while VOUT of buck-boost is the total forward voltage of the LED string plus VIN. Higher VOUT requires higher component rating of Q1 and D1. RT8482 EVB runs in high efficiency. All the components of this design generate very little heat, thus enhance the reliability and safety of the lighting fixture. This design for universal applications minimizes the inventory and management effort for both manufacturer and installer. Evaluation Board Features Application example: LED emergency lights LED quantities: 5 to 12 pieces DC VIN source: 12V, 18V, or 24V Topology: configurable with a 3P2T switch for either buck-boost or boost High efficiency: 84%~87% for buck-boost mode; 91%~93% for boost mode LED current: default at 0.35A (Note 1), can be adjusted up to 1A with a potentiometer Note 1. Different LED current can be set by adjusting different ACTL voltage (0.2V~1.2V) with a potentiometer (R30 on the schematic) and/or by setting different Rsense (R22 on the schematic) value according to the equation of I LED = 190mV * V ACTL 1.2 * R SENSE Bill of Materials (BOM) For cost effectiveness, two component tables are presented in scenarios of 1. Dedicated for models with 1W LED (300~350mA), which is the most popular LED type. 2. Universal design covering 0.3~1A LED current range. 3 The following BOM is based on Scenario-1: 1W LED. This BOM is used for data measurement in this application note. Reference Q t y Part Number Description Package Manufacturer U1 1 RT8482 1 1 1 1 3 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 GRM188R71C474K 445-1450-1 GRM188R71H333K GRM188R71H103K GRM32ER71H475KA88 SR28 BAV70 NR8040T470M SUD40N08-16 WR06X0103F WR06X1003F WR06X0000F CS12FTER050 CS12FTER200 WR06X1693F 3299W-502 WR06X0273F WR06X0681F WR06X3903F 35-024 WQFN-16L, SOP-16 0603 1206 0603 0603 1210 DO-214C SOT-23-3 8x8x4 mm TO252 0603 0603 0603 2512 2512 0603 3.8” square 0603 0603 0603 Richtek C2 C4 C5 C6 C8,C9,C10 D1 DN1 L1 Q2 R2,R14 R5 R8 R10 R22 R12 R29 R30 R31 R32 SW1 High voltage multi topology LED driver 0.47uF/10V/X5R 4.7uF/100V/X7R 3.3nF/50V/X7R 10nF/50V/X7R 10uF/50V/X7R Schottky diode 2A 80V Dual diode common cathode 47u 1.4A inductor 80V N-ch MOSFET 16mR 10K ohm resistor 100K ohm resistor 0 ohm resistor 0.05 ohm resistor 1% 0.2 ohm resistor 1% 169K ohm resistor 5.0K ohm potentiometer 27K ohm resistor 680 ohm resistor 390K ohm resistor Switch 3P2T 5A 125VAC 17x13x20mm GC Electronics Murata TDK Murata Murata Murata Panjit Diodes Taiyo Yuden Vishay Walsin Walsin Walsin Viking Viking Walsin Bourns Walsin Walsin Walsin The following BOM recommendation is based on Scenario-2: 0.3A~1A LED. Cx, Cy, D1, L1 and R10 are upgraded to support 1A LED current. Reference Q t y Part Number Description Package Manufacturer U1 1 RT8482 1 3 1 1 3 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 GRM188R71C474K 445-1450-1 GRM188R71H333K GRM188R71H103K GRM32ER71H475KA88 B380-13-F BAV70 744 770 9330 SUD40N08-16 WR06X0103F WR06X1003F WR06X0000F CS12FTER015 CS12FTER200 WR06X1693F 3299W-502 WR06X0273F WR06X0681F WR06X3903F 35-024 WQFN-16L, SOP-16 0603 1206 0603 0603 1210 DO-214AB SOT-23-3 Richtek C2 C4, Cx, Cy C5 C6 C8,C9,C10 D1 DN1 L1 Q2 R2,R14 R5 R8 R10 R22 R12 R29 R30 R31 R32 SW1 High voltage multi topology LED driver 0.47uF/10V/X5R 4.7uF/100V/X7R 3.3nF/50V/X7R 10nF/50V/X7R 10uF/50V/X7R Schottky diode 3A 80V Dual diode common cathode 33uH 5.5A inductor 80V N-ch MOSFET 16mR 10K ohm resistor 100K ohm resistor 0 ohm resistor 0.015 ohm resistor 1% 0.2 ohm resistor 1% 169K ohm resistor 5.0K ohm potentiometer 27K ohm resistor 680 ohm resistor 390K ohm resistor Switch 3P2T 5A 125VAC Murata TDK Murata Murata Murata Diodes Diodes 12x12x10 mm Wurth Elektronik TO252 0603 0603 0603 2512 2512 0603 3.8” square 0603 0603 0603 Vishay Walsin Walsin Walsin Viking Viking Walsin Bourns Walsin Walsin Walsin GC Electronics 17x13x20mm 4 Evaluation Board Schematic 5 Efficiency Performance LED Qty 5 6 7 8 9 10 11 12 V(LED) 16~20V 19~24V 22~28V 25~32V 28~36V 32~40V 35~44V 38~48V VIN 24V (20~28V) Bk-Bst (84%) Bk-Bst (85%) Bk-Bst (86%) Bk-Bst (87%) Boost (93%) Boost (93%) Boost (93%) Boost (93%) VIN 18V (15~21V) Bk-Bst (86%) Bk-Bst (87%) Boost (93%) Boost (93%) Boost (93%) Boost (93%) Boost (93%) Boost (93%) VIN 12V (10~14V) Boost (91%) Boost (92%) Boost (92%) Boost (92%) Boost (92%) Boost (92%) Boost (92%) Boost (92%) Case Temperature on Major Components VIN=12VDC, 5 LED, boost mode, ambient temperature ﹫25°C RT8482 Inductor MOSFET Schottky diode Rsense Output capacitor 36.0 33.8 32.5 35.6 31.2 26.1 VIN =24VDC, 5 LED, buck-boost mode, ambient temperature ﹫25°C RT8482 Inductor MOSFET Schottky diode Rsense Output capacitor 46.1 42.4 38.5 41.5 34.5 36.9 VIN =24VDC, 8 LED, buck-boost mode, ambient temperature ﹫25°C RT8482 Inductor MOSFET Schottky diode Rsense Output capacitor 47.6 48.2 41.6 44.8 36.7 39.9 VIN =24VDC, 12 LED, boost mode, ambient temperature ﹫25°C RT8482 Inductor MOSFET Schottky diode Rsense Output capacitor 46.2 42.3 38.5 41.9 34.6 37.3 LED Current Regulation (mA, rms) LED Qty 5 6 7 8 9 10 11 12 V(LED) 16~20V 19~24V 22~28V 25~32V 28~36V 32~40V 35~44V 38~48V VIN 24V 20~28V 356 355 355 355 358 358 357 357 VIN 18V 15~21V 349 348 350 350 350 350 349 349 VIN 12V 10~14V 339 339 338 336 336 335 335 335 Note: Above efficiency and thermal data is based on 350mA LED current setting. For 1A LED current condition, component power dissipation will be higher at certain VIN and V(LED) conditions. This extra component dissipation must be considered during PCB thermal design. 6 Efficiency Calculation & Waveform LED No: 5 buck-boost topology LED No: 12 boost topology Vin 24.1100 Vout 16.4240 Vin 24.0500 Vout 39.4500 Iin 0.2895 Iout 0.3556 Efficiency 0.8367 Ch1: Gate; Ch4: LED current LED No: 5 boost topology Vin 12.1120 Vout 16.3000 Iin 0.4987 Iout 0.3390 Ch1: Gate; Ch4: LED current Iin 0.6283 Iout 0.3572 Efficiency 0.9326 Ch1: Gate; Ch4: LED current Efficiency 0.9148 LED No: 12 boost topology Vin 11.9650 Vout 39.1000 Iin 1.1883 Iout 0.3348 Efficiency 0.9207 Ch1: Gate; Ch4: LED current Component Selection and Calculations GBIAS Regulator and Bypass Capacitor The GBIAS pin requires a capacitor for stable operation and to store the charge for supplying Gate switching currents. A 0.47uF 10V X5R MLCC is used for best performance. Loop Compensation A typical compensation RC value for this application, 10kohm resistor and 3.3nF capacitor, are used for covering all application corners. Soft Start The soft start interval is set by the soft start capacitor, according to the equation TSS = 2.4 * CSS ISS 7 TSS = 2.4 * CSS 2.4 *10n = = 3ms ISS 6uA where CSS = 10nF, ISS=6uA LED Output Current LED output current can be set by the sense resistor RSENSE and ACTL voltage. RSENSE sets the adjustable range and ACTL voltage adjusts LED current within the range. The equation for the LED current is 190mV * VACTL 1.2 * R SENSE 190mV *1.2V = = 0.95 A 1.2 * 0.2 = 0A I LED = when 0.2V < VACTL < 1.2V I LED when VACTL > 1.2V I LED when VACTL < 0.2V In this project, a 0.2 ohm (R22 on the schematic) is used. This allows the maximum of ~1A LED current when ACTL voltage is set higher than 1.2V with a potentiometer (R29 on the schematic). The most popular LED current 350mA is used in this application note to demonstrate the performance. Dimming Control RT8482 offers three dimming modes (analog, PWM digital, PWM converting to analog) to be performed according to user preference. However, no dimming control is required for LED emergency light application in general. This project makes use of the analog dimming pin ACTL for LED current adjustment. The theory is illustrated as the conceptual circuit diagram below Vin Universal LED driver Module RT8482 I-LED Boost / Buck-Boost 27k GBIAS max Analog P1 Dimming 5.0K 0 0.2 ~ 1.2V 1.2V ACTL Rsense ISN V-ACTL 0.2V ISP 6 7 680 Set I-LED max Over Voltage Protection (OVP) In the buck-boost and boost applications, OVP is a necessary function to protect components from over voltage damage at fault conditions. OVP of RT8482 can be set by the ratio of a voltage divider with the following equation VOVP = 1.2V * ( 1 + R1 ) R2 where Vovp is the voltage limited by the OVP circuit OVP trip point should be set between the maximum operational voltage and the limit of component rating. In this project, the condition with 24V VIN and 8 LEDs in buck-boost topology has the maximum VOUT, which is VIN + 8xVF. VOUT should be limited below component rating of MOSFET Q2, Schottky diode D1, and the output capacitor C4. 8 The conditions with 12 LEDs in boost topology have maximum voltage at ISP and ISN pins of RT8482, which is 12xVF, and should not exceed the maximum allowable operational voltage of ISP/ISN pins. To protect both ISP/ISN pins and VOUT-related components, the OVP circuit shown below can be used. VOUT1 and ISP are wired OR with a common-cathode diode to the OVP pin. This OVP circuit sets the OVP voltage on VOUT1 according to the following calculation VOVP = 1.2v * ( 1 + 390k + 169k ) + 0.7v = 69V 10k The maximum rated voltage on ISP and ISN pins of RT8482 is 54V. This OVP circuit sets the OVP voltage on ISP/ISN pins at the same time according to the following calculation VOVP = 1.2v * ( 1 + 390k ) + 0.7v = 48.7V 10k In addition, adding a 10nF capacitor in parallel with the bottom resistor (R14 on the schematic) can reject spike voltage to prevent false triggering of OVP. VOUT1 ISP R12 169k DN1 BAV70 R32 390k OVP C13 10nF R14 10k ISW Sense Resistor Selection To leave at least 30% margin from the 110mV trip voltage for MOSFET current limit protection, the maximum allowable Rsw value in buck-boost application can be calculated, according to R SW = VIN * 0.08 (VIN + VOUT) * IOUT Based on Scenario-1 350mA LED current requirement, the maximum allowable Rsw value can be calculated as the following R SW = 15 * 0.08 = 0.09ohm (15 + 24) * 0.35 when VIN=18V, 6 LED R SW = 20 * 0.08 = 0.09ohm (20 + 32) * 0.35 when VIN =24V, 8 LED For boost application, the maximum allowable resistor value can be calculated, according to R SW = VIN * 0.08 VOUT * IOUT The corresponding calculation for boost topology for 350mA LED current is as the following 9 R SW = 10 * 0.08 = 0.05ohm 48 * 0.35 when VIN =12V, 12 LED A 0.05 ohm resistor (R10 on the schematic) is used in this project for data collection, which sets 0.11/0.05 = 2.2A MOSFET current limit value. The lab data shows very safe and stable operational performance covering the entire application range. If 0.3A~1A LED current needs to be covered, please change Rsw value to 0.015 ohm, which sets 0.11/0.015 = 7.3A MOSFET current limit value. Because the current loop gain is deceased and overall loop gain is increased, the compensation network may need some fine tune to optimize the best performance for all range of application. Inductor Selection A 47uH inductor is used in this project for covering all corner conditions with 350mA LED current. In general, 22uH~68uH is the reasonable range from which the optimal inductor value can be decided according the VIN, VOUT, IOUT conditions and the criterion of ripple current. Care should be taken that the inductor’s saturation current is higher than the maximum inductor peak current that can occur in the application. See BOM for recommended types. Power MOSFET Selection The general selection criteria are Vds with enough rating beyond the OVP setting and the worst condition VOUT of buck-boost application, in case of an open-load fault condition. Low Rds(on) is usually a secondary consideration due to switching loss dominates power loss. The switching losses can be minimized by using strong gate drive, low drain-source capacitance, and fast Schottky diode. Vgate voltage of RT8482 is 7.2V typical and the MOSFET should have a low Rds(on) at Vgate=6.5V. The maximum voltage of this application at SW note is about 70V, as calculated in the previous OVP section. As a simple guide line for 350mA LED current application a MOSFET with Vds 80V, Rds(on) less than 0.1 ohm at 6V Vgs, Drain current rating greater than 5A, and power dissipation greater than 2W, is a good selection for this application. An 80V 40A 16mohm rated MOSFET SUD40N08-16 is selected to cover both 350mA and 1A LED current application conditions. Schottky Diode Selection Choose a suitable Schottky diode whose reverse voltage rating is greater than the maximum output voltage. The diode’s average current rating must exceed the average output current. Choosing Panjit International SR28 2A 80V rated Schottky diode for Scenario-1 or Diodes Inc B380 3A 80V rated Schottky diode for Scenario-2 meets requirements of this project. Capacitor Selection The minimum value of the output capacitor Cout is given by this equation ILED * VOUT VIN * Fsw * VRIPPLE 0.35 * 50 = = 4.2uF 24 * 350k * 0.5 C OUT = C OUT C OUT = 1 * 50 = 12uF 24 * 350k * 0.5 for 350mA LED Current for 0.3~1A LED Current A 4.7uF 100V X7R MLCC is selected for 350mA applications and three pieces of 4.7uF 100V X7R MLCC is selected for 0.3A~1A applications. 10 RT8482 EVB Operating Guide RT8482 EVB provides an easy way to evaluate a number of LED driven by RT8482 for LED emergency light application. An add-on board with a switch is wired to the RT8482 EVB for configuring the topology either buck-boost or boost. This application note shows RT8482 EVB driving 5 through 12 LED. The input voltage can be 12, 18, or 24VDC. Function Switch LED Connection Power Source Potentiometer Label / Silkscreen buck-boost: switch toward left boost:: switch toward right POS: LED anode NEG: LED cathode VIN: 12VDC ~ 24VDC GND: ground Clockwise: Counter clockwise: increase LED current decease LED current Operating Procedures 1. 2. 3. 4. 5. Connect LED anode to POS terminal and cathode to NEG terminal Configure buck-boost (left) or boost (right) mode with a 3P2T switch Connect power source to VIN and GND terminals Turn on the power supply Tune the potentiometer if LED current other than 350mA is desired The default LED current is set at 350mA. The voltage at ACTL for 350mA LED current is ~0.58V. It is very important to note that the topology can be changed by the 3P2T switch only when the EVB has no power applied. 11 Evaluation Board Picture & Illustration VIN Terminal GND Terminal Potentiometer RT8482 LED Anode Terminal 3P2T Switch LED Cathode Terminal 12