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1、RT8450 Buck-Boost MR16 LEDs Lamp ER-RT8450-0308-A Nov 2008 RichTek Conceptual LED MR16 Lamp Solution Using the Richtek RT8450 Chipset Patrick Huang Introduction This article describes a driver solution developed using the Richtek RT8450 Chipset to drive up to three high brightness LEDs as a replace
2、ment of a conventional halogen based MR16 light bulb. It is capable of providing 350 mA constant current to drive High-Brightness LEDs from a 12V AC source. The design has been optimized for part count and thermal performance. For 5W LED in the Lens section and wider input voltage range, this design
3、 can be arranged to suit the luminary requirements. Features: ? Input Voltage:DC12V/DC24V(up to 40V) or AC 540V ? Buck-Boost Topology, the input voltage can lower or higher than output LED voltage. ? Output Current:350mA ? Output Current Accuracy:5% ? High efficiency:80% ? Brightness control: DC vol
4、tage or PWM ? Switching frequency:1MHz ? Quiescent current with output off:100uA ? Working Environment: -40 to 85 ? Inherent open-circuit LED protection ? Soft-Start function(SS) The device encompasses a full bridge and a freewheeling diode realized using extremely low leakage 1A, 20V schottky diode
5、s to allow a nominal 12VAC input operations. The reference design has solder tags pins to bypass the bridge rectifier should the final lamp design to be used for purely DC operation. RT8450 Buck-Boost MR16 LEDs Lamp ER-RT8450-0308-A Nov 2008 RichTek Conceptual The RT8450 is a new space saving and t
6、hermal efficient device specifically designed for thermal and space critical requirements of MR16 application, As the input voltage of MR16 is 12 VAC, a bridge rectifier is required to rectify the AC input to DC level in order to provide supply to RT8450 LED driver. With wide input operating voltage
7、 range of the RT8450 (from 4.5 VDC to 40 VDC), a small input capacitor can maintain continuous operation. The switching frequency is internally set to 1 MHz, allowing the use of extremely small surface mount inductors and chip capacitors. To avoid the flick happened when the lower rectified voltage
8、from AC input, the buck-boost topology is designed for MR16 application. The input voltage can be either higher or lower than LED output voltage. The LED current can be maintained its setting and kept the brightness in application by current senses control topology. The smaller sense resistance can
9、be set the accurate output current easily. The circuit diagram of the MR16 lamp solution with RT8450 is shown in Figure 1, and Table 1 provides the listed component values and specific part numbers. Figure 1 System diagram of RT8450 MR16 Lamp Solution Quantity Reference Part Footprint Vendor 1 U1 RT
10、8450 WDFN_12L 3X RICHTEK 1 C1 4.7u/25V/X5R C1206 TDK RT8450 Buck-Boost MR16 LEDs Lamp ER-RT8450-0308-A Nov 2008 RichTek Conceptual 1 C2 FCA 220F/35V CAP-AEC-8X11.5 1 C4 3.3nF/25V C0603 1 C5 10nF/25V C0603 1 C6 68F/35V C2512 TDK 1 D1, SCD110 SOD-123 ZOWIE 5 D2,D3,D4,D5 SS12 SOD-123 PANJIT 1 L1 47uH/
11、1.3A L-SH8018 TDK 2 R1,R2 0.5R(2R*4) R0805 2 R3,R5 10K R0603 1 R4 100K R0603 1 R6 300K R0603 Table 1 Bill of Materials Figure 2 shows the circuit layout, both bottom layer and top layer are provided to display how effective devices are arranged. TOP COPPER AND SILKSCREEN BOTTOM COPPER AND SILK SCREE
12、N Figure 2: Circuit Layout of RT8450 MR16 Lamp The main layout design constraints are: All components are on the top layer. The bottom layer only provides connection to ground. On the board, there are two ac input connection pads and two dc current output connection pads for the LED. All thin device
13、s on one side employ a star RT8450 Buck-Boost MR16 LEDs Lamp ER-RT8450-0308-A Nov 2008 RichTek Conceptual connection for ground tracks. The Tracks connecting R1 and R2 to RT8450 are as short as possible (being sense traces). The filter capacitor C6 is connected as close as possible to the Vin pin.
14、The freewheeling current path is as short as possible to ensure system precision and efficiency. For a switching regulator the quality of the layout is crucial to minimize radiated EMI. This reference design keeps the critical track lengths to a minimum. Ground areas have been maximized around criti
15、cal areas. The figure 3 shows the real circuit board views and the PCB size. Module Configuration:（PCB Size:L=20mm*W=15mm*H=1mm） Figure 3 Circuit Board Views Circuit Performance Operation of a 5-W white-LED-based MR16 light fixture that uses the Figure 1 circuit together with the listed component va
16、lues (for specific part numbers, see the Table 1).The circuit performance has been evaluated taking into account two main parameters, the system voltage input range and the precision of the LED current. The total capacitance should be at least 200F. The capacitors can be either two 100-F tantalums or a single low-cost 220-F electrolytic, all
