February 2011 As the luminous efficiency of LED continues to grow, and the awareness of energy saving raises up within recent years, the LED lighting applications are getting more and more popular. After successful delivery of LED backlight solutions, Richtek launches a series of high power LED driver ICs for LED lighting. These products have been widely designed in different applications, such as MR16, AR111, E27, T8, Par lamps, Light Bars, Ceiling lights, Street lights, and Car lights. For DC input to constant current applications, Richtek introduces RT8450/2/3 and RT8480/2/8 series. These LED driver ICs provide customers several different design options, including buck, boost, and buck-boost topology for different loading currents. The current-mode constant current design which is different from the hysteretic solution, delivers stable and precise LED current with small LC values. True current sense technology will maintain the current accuracy regardless the change of input voltage, output voltage or inductance tolerance. This improves LED luminous consistency and design flexibility. The ability of using multi-topology and wide input/output voltage range allow customers to use the same IC or even same power boards for different applications. RT8450 is typical out of this product line. With a built-in NPN power switch and being able to support Buck, Boost and Buck-Boost modes, RT8450 is suitable for many types of applications. On the other hand, RT8482 is for high LED current designs and is functionally compatible with RT8450 but has to be combined with an external MOSFET. These two chips can cover most of the single string LED lighting applications. RT8452 is another good product which can provide sharp PWM dimming support and operate with high speed PWM dimming signal up to 50KHz, and it is a good fit for high resolution LED projector applications. RT8470 and RT8471 provide very low cost MR16 solutions with high electronic transformer compatibility. A simple circuit design can eliminate most of the flickers and provide very high efficiency. For AC input to constant current applications, Richtek offers RT8454/6/7 secondary-side constant current controllers to generate highly accurate current over wide input/output voltage range and also wide temperature range without suffering from the transformer accuracy. RT8456 can work with many different primary side topologies to improve the output current accuracy. This will unify the luminance of E27 LED light bulbs when a lot of them line up together. RT8454/7 support different kinds of dimming functions including Triac Dimming. The secondary side modulation provides a more precise current control than the primary side dimming scheme. The secondary-side dimming chip, RT8457, is used widely not only for Triac dimming but also many of other dimming technologies. The multi-channel designs of RT8454 allow a single chip to provide well-balanced constant current to up to 6 strings of LED. This will benefit the high power applications such as street lights to save cost. RT8454 also provide interfaces to link multi-chip together. This design makes single primary side can control up to hundreds of LED strings. As the understanding of LED lighting regulation is improved, the non-isolated design starts to be popular. A good isolation design on mechanical makes the non-isolated design on power module become possible. This will bring the system higher efficiency and lower cost and these are two key points to decide whether a product can be successful or not. RT8458 is announced to introduce a high efficiency and low cost PAR lights or T5, T8 solution. A special floating-buck topology improves the efficiency and also provides very simple methods of OVP and OCP. The true current sense technology keeps precision current over universal input range. RT8459 is a PFC version of RT8458. With proper output, designing PAR lights or T8 with RT8458 and RT8459 can keep efficiency > 90% and PF>0.9 easily. (PS: RT8459 is expected to be announced on March 2011.) High efficiency, low cost PAR lights, T8 design with RT8458 and RT8459 The Richtek RT8458/9 PWM controllers employ an The RT8459 adds power and eliminates the need for any complex external circuitry to optimize AC power innovative self-biasing design that reduces power loss transformers or start-up circuitry in non-isolated applications. Instead, these devices require only a few simple low-cost components. An integrated high-side floating gate driver controls an external MOSFET that may be selected to support all popular AC line voltages. The programmable LED DC current is regulated by a precise averaging current feedback loop that keeps LED current constant over large line voltage variations, and will compensate for poorly controlled system component tolerances. The RT8458/9 integrate protection for fault conditions, including open and shorted LED strings, Under Voltage Lockout (UVLO), Over Current Protection circuit (OCP) and Over Voltage Protection (OVP). factor (PF) compensation utilization. Features-RT8458, RT8459 ■ Low Cost and Efficient Buck Converter Solution ■ Universal Input Voltage Range with Off-Line Topology ■ Programmable Constant LED Current ■ Dimmable LED Current by ACTL ■ Output LED String Open Protection ■ Output LED String Short Protection ■ Output LED String Over Current Protection ■ Built-in Thermal Protection ■ Built-in High Power Factor Correction Circuit (RT8459) RT8458 efficiency measurement Eff. vs. LED pcs (350mA) 100% 95% Eff. (%) 90% 85% 80% 110VAC 75% 70% 220VAC 3 4 5 6 7 8 9 10 LED pcs With the RT8458 you get precise, constant LED current An efficiency >90%, PF>0.9 PAR light design with RT8459 8459 Vout=44V//200mA/15LEDs Vin.ac (V) 90 110 130 180 220 264 Vo (V) 43.33 43.37 43.35 43.31 43.36 43.41 Io (mA) 194 196 195 191 194 196 Pin (W) 9.3 9.26 9.19 9.03 9.23 9.42 Po (W) 8.41 8.50 8.45 8.27 8.41 8.51 PF 0.980 0.981 0.985 0.972 0.955 0.932 Eff. (%) 90.39 91.80 91.98 91.61 91.14 90.32 current offset (%) -1.02% 0.00% -0.51% -2.55% -1.02% 0.00% RT8456, RT8457 Secondary Side Constant Current LED Driver and Dimming Controller The RT8456 is an isolated secondary LED current controller designed specifically for lighting fixtures such as E27, GU10, T5 and T8. The IC is suitable for applications that require power line isolation for safety and reliability, high conversion efficiency, as well as high LED current accuracy. and an opto-coupler driver to control the primary side of the transformer to complete the system loop. RT8457 provides dimming function on secondary side. Precise dimming is achieved by controlling the LED current at the secondary side via analog dimming. Applying a voltage greater than 1.1V at the ACTL pin will set the LED current sense threshold between the LED and GND pins at the highest level. The maximum LED current is programmed by the sense resistor between the LED and GND pins. Lowering the ACTL pin voltage will reduce the LED current sense threshold. The RT8456 contains 1) a constant current regulating amplifier with 60mV threshold used to program the LED string current with a simple resistor to within 5% LED current accuracy; 2) an over voltage comparator to protect the output when LED string is open or broken; 3) Features-RT8456 ■ Secondary Side LED Current Regulation ■ 4.75V to 48V Operation Range ■ High Accuracy 60mV LED Current Threshold Control ■ 0.6mA Operating Current ■ Programmable LED Current ■ Output Over Voltage Protection ■ Small SOT-23-6 Package Features-RT8457 Secondary Side LED Current Regulation ■ High Accuracy 58.5mV LED Current Threshold Control ■ 0.5mA Operating Current ■ Programmable LED Current ■ Precision Secondary Side LED Current Dimming ■ Precision Analog Dimming at Secondary Side ■ Output Over Voltage Protection ■ SOP-8 Package ■ RT8456 application circuit RT8457: Linear LED current mapping between LED current and ACTL voltage VOUT Opto LED String OUT - CV + OVP 60mV - CC CN RLED VCC OUT C2 RT8457 0.2 0.4 0.6 0.8 1 1.2 1.4 OVP R1 3 Optional CIC2 RIC2 LED String 5 CIC1 7 ACTL 0 VOUT Opto 4 GND ILED GND 1 2 2V2 100 VACTL(V) RT8457 application circuit C1 150 0 GND Rs Primary side 200 50 + CP 250 I LED (mA) Primary side 300 RT8456 2.5V ILED vs. VACTL 350 Vcc R2 RIC1 LED 6 RS RT8456 and RT8457 work with most of the primary side topologies to provide constant current LED driver and dimming function. Typical DC to CC LED driver design with RT8450/RT8482 The RT8450 is a current mode constant current LED driver with 1.5A switching MOS inside, supporting multi-topology includes buck, boost, and buck-boost. It provides very wide input and output range. Input voltage works from 4.5V to 40V, and this can be extended by zener diode added. Output voltage can be up to 50V. RT8482 is an external MOS version of RT8450 to support higher power applications. A higher voltage product – RT8462 is ready soon. VIN 4.5V to 40V C1 RT8450/B VCCx ISP PWM Dimming control ACTL ISN 5V EN High Voltage : VIN Up to 40V, VOUT Up to 50V ■ Buck, Boost or Buck-Boost Operation ■ Easy Dimming : Analog, PWM Digital or PWM Converting to Analog with One External Capacitor ■ Programmable Soft Start to Avoid Inrush Current ■ Programmable Over Voltage Protection to Limit ■ Output Voltage ■ VIN Under Voltage Lockout and Thermal Shutdown ■ RoHS Compliant and Halogen Free R4 C4 GND SS L 20µH SW C3 10nF C2 3nF R3 OVP VC R1 10k ■ RT8450 buck L 20uH VIN 4.5V to 40V The average current sense constant current controller, different from peak sense, provide a stable and high precise LED current. Regardless of the input voltage change or LED number change, the variation of LED 5V current is very small. It is very easy to design a general PWM purpose LED power module with RT8450 or RT8482 for Dimming control flexible input and output. C1 SW EN ISP DCTL ISN R2 190mV VC C3 10nF VISP – VISN Threshold(mV) 194 192 VCCx SS C2 3nF C5 1µF RT8450/B R1 10k VISP – VISN Threshold vs. Input Voltage 196 DCTL R2 190mV R3 OVP ACTL GND C4 0.5µF VOUT R4 RT8450 boost 190 188 C5 186 4 13 22 31 40 L VIN 4.5V to 40V C1 Input Voltage(V) RT8450/B VCCx LED Current vs. ACTL 2500 5V LED Current(mA) PWM Dimming control 2000 1500 1000 C2 3nF 500 0 0.4 0.6 0.8 ACTL(volt) 1 1.2 1.4 SW EN ISP DCTL ISN R2 190mV VC R1 10k 0.2 VOUT 20uH C3 10nF SS OVP ACTL GND C4 0.5µF RT8450 buck-boost R3 R4 VOUT Design challenges for MR-16 retrofit LED drivers. halogen ET. On first sight, it looks straightforward: The ET 12Vac waveform is rectified and smoothed by the bulk cap. It appears that the rectified voltage should be sufficient to use a standard BUCK converter to drive the LED string. With the continuous drive for energy saving and green products, many traditional lighting applications are being replaced by LED’s. One of these “retrofit” applications is the low voltage halogen lamp, generally called “MR-16” after its low voltage lamp connector. In practice, it’s a bit different: Most halogen ET’s use a self-oscillating half-bridge converter with proportional base drive, which modulates the mains sine wave with a 30 ~ 50khz switching waveform. Designed to drive 20~50W resistive loads, ET’s need a certain minimum load for continuous switching throughout the mains cycle. A standard BUCK LED driver circuit poses two problems: Low voltage halogen lamps generally range from 20 ~ 50 Watts, and are powered from 12Vac. In many cases, an “Electronic Transformer” (ET) is used to efficiently convert the mains AC voltage to 12Vac. ET output power ranges from 50 ~ 150W, so it’s possible to connect several halogen lamps in parallel to one ET. With some ET’s it is even possible to use phase-cut dimmers to regulate the light output. ● The input bridge and bulk cap are not resistive Replacing a standard halogen lamp with a LED light is not as straightforward as one would think. When trying to design a LED driver that works with the ET 12Vac output, it quickly becomes clear that there are several compatibility issues that can be quite tricky to solve. This article describes some background and possible solutions. ● LED’s are more efficient than halogen lamps so they consume much less power than the ET minimum load. As a result ET switching can become erratic over the mains cycle. Depending on the ET type and the LED driver configuration, LED flicker can result. The block diagram below shows an example of an LED light connected to a Several solutions can improve the compatibility between ET’s and a LED light. LED lamp AC mains ~ LED driver Electronic Transformer Cin Designed for 20 - 50W Resistive load Power ~ 4.5W Bridge rectifier with bulk capacitor Constant current driver Detailed schematics Figure 1 RT8453B MR-16 standard application D2 SR26 + 12Vac from Electronic Transformer D3 SR26 C4 1u R3 100k R2 0.5R Vcc C3 220u/25V ISN R1 EN 0.5R D1 SR26 C5 RT8453BGSP D4 SR26 4.7u ACTL VC R5 10k C6 3n3 C7 10n SW SS D6 SR26 L1 22uH GND Figure 2 RT8453B MR-16 dimmable application + D2 SR26 12Vac from Electronic Transformer C1 10u C3 100u 0.68R C4 1u R6 100k R2 0.5R Vcc ISN EN R1 D1 SR26 Z1 18V R3 180k C2 10u C5 RT8453BGSP 4.7u ACTL C8 10u R4 27k VC R5 10k C6 3n3 C7 10n SS SW GND L1 22uH 2.Use a small bulk capacitance to increase the input ripple for the LED driver. A high performance BUCK converter like the RT8453B or RT8470 (see figure 1), can smoothly drop out when the input voltage is lower than LED string voltage, resulting in some 120Hz ripple in the LED current As long as this 120Hz ripple is steady, the flicker will not be noticeable to the human eye. 3.To obtain a more stable LED current even with a large input ripple, use a buck-boost configuration (i.e. by using RT8450 in buck-boost mode). This will provide a more stable LED current with minor 120Hz ripple. A drawback is the inherently lower efficiency of a buck-boost topology. LED string + ~ 1.Add some series damping resistance in the rectifier circuit. Tests with various ET’s has shown that for the standard 4.5W MR-16 LED application, ~0.5 Ohm is often enough to get a sufficiently stable ET output to avoid flicker. Don’t make the bulk capacitance too large, as it will result in very short conduction time of the ET, increasing peak inrush current and making the ET’s more prone to instability. D6 SR26 4.When using a dimmable ET with compatible dimmers, the ET output voltage will be phase cut, depending on the dimmer setting. However, since most ET’s will not switch continuously over the full mains range, accurate phase cut detection is problematic. Adopting a circuit that detects the average voltage of the rectified ac, and reduces LED current when the input voltage is reduced can improve dimming performance. See figure 2 for an example.. Designing LED drivers for an MR-16 retrofit application is possible, although halogen Electronic Transformers and LED driver circuits are hardly made for each other. By using some circuit tricks and correctly choosing an LED driver IC, a relatively simple and low cost solution can be designed that works with most ET’s. For more information about Richtek’s broad line of LED driver solutions, visit www.richtek.com/LED/ High Efficiency Universal LED Driver Solution for Emergency Lights with RT8482 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 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. 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 Vin S1, S2, S3: =boost Cin 30uF 15 13 5V R30 5.0k GATE ISW R29 Analog Dimming EN C2 0.47uF 9 10 0.2V ~ 1.2V R31 680 C5 3.3nF Cout 4.7uF DCTL ISP S2 R32 390k C11 10n GND S3 Rsense 14 11 SS Css 10nF LED's 6 ISN 7 VC S1 =buck-boost 4 Rsw 0.05 OVP R2 10k Vout 2 ACTL 8 D1 Q1 1 GBIAS 27k L1 VCC RT8482 R14 10k R21 169k Vout 16, Exposed Pad (17) NC 3 5 12 Efficiency Performance LED Qty V(LED) VIN 24V (20~28V) 5 6 7 8 9 10 11 12 16~20V 22~28V 25~32V 25~32V 28~36V 32~40V 35~44V 38~48V Bk-Bst Bk-Bst Bk-Bst Bk-Bst Boost Boost Boost Boost (84%) (86%) (87%) (87%) (93%) (93%) (93%) (93%) VIN 18V Bk-Bst Boost Boost Boost Boost Boost Boost Boost VIN 12V Boost Boost Boost Boost Boost Boost Boost Boost (15~21V) (10~14V) (86%) (91%) (93%) (92%) (93%) (92%) (93%) (92%) (93%) (92%) (93%) (92%) (93%) (92%) (93%) (92%) 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 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 Picture & Illustration VIN Terminal Case Temperature on Major Components VIN=12VDC, 5 LED, boost mode, ambient temperature ﹫25°C RT8482 36.0 Inductor 33.8 MOSFET 32.5 Schottky diode 35.6 Rsense 31.2 Output capacitor 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 GND Terminal LED Anode Terminal Potentiometer RT8482 3P2T Switch LED Cathode Terminal LED Lighting Selection Guide Sorted by Application Application Feature Solid-State Lighting AC or DC Input MR16, AR111 Buck, Buck-Boost, Boost GU10, E27,PAR30,PAR38 Flyback, non-isolated T5, T8 Light Bar Flyback, non-isolated Office Lamp Boost, Flyback, non-isolated Decoration Light Boost Street Light, Solar Light Buck, Boost, flyback Emergency Light Boost RGB Light Buck Freeze Light Buck, Flyback, non-isolated Projector Synchronous Buck Automotive Application Interior Light Buck, SEPIC, Buck-Boost Exterior Light Buck, SEPIC, Buck-Boost Automotive Light Buck, SEPIC, Buck-Boost Richtek Solution RT8450B, RT8482, RT8453, RT8470, RT8471 RT8456, RT8457, RT8458, RT8459 RT8456, RT8457, RT8458, RT8459 RT8450B, RT8482, RT8456, RT8458, RT8459 RT8450B, RT8482, RT8480 RT8480, RT8482, RT8488, RT8470 RT8450B, RT8482 RT8453, RT8470 RT8450B, RT8453, RT8456, RT8470, RT8458 RT8452 RT8450B, RT8482, RT8488, RT8480 RT8450B, RT8482, RT8488, RT8480 RT8450B, RT8482, RT8488, RT8480 About Richtek Richtek Technology Corporation is one of the world’s leading analog IC companies. The company consistently delivers inventive power management solutions that improve the performance of consumer electronics, computers, and communications equipment. Richtek adds value to end equipment by synthesizing technological innovation, uncompromised quality, and devotion to customer service. Founded in 1998, the Company is headquartered in Taiwan with additional offices in Asia, the U.S., and Europe. Richtek is publicly traded on the Taiwan Stock Exchange under the ticker symbol 6286. For more information about Richtek and its analog IC solutions, please visit the Company’s Web site at www.richtek.com or www.richtek.com/LED. US Office Tel: 1-408-9690226 Europe Office Tel: 31-40-8515520 Taipei Office Tel: 886-2-86672399 [email protected] www.richtek.com/LED/