EL7513 ® Technical Brief November 25, 2003 White LED Step-Up Regulator TB414.1 The brightness of the LEDs is adjusted through a voltage level on the CNTL pin. When the level falls below 0.1V, the chip goes into shut-down mode and consumes less than 1µA of current. The EL7513 is a constant current boost regulator specially designed for white LEDs. It can drive up to 5 LEDs in series or 12 LEDs in parallel/series configuration. Up to 91% of efficiency can be achieved. EL7513 is available in a thin SOT-23 package with maximum height of 1mm vs 1.45mm for a regular SOT-23 package. The complete demo board schematic diagram is shown in Figure 1. Only D2-D5 is stuffed. VOUT CS3 D11 D12 D13 D14 D15 WHITE WHITE WHITE WHITE WHITE D6 D7 D8 D9 D10 WHITE WHITE WHITE WHITE WHITE D1 D2 D3 D4 D5 WHITE WHITE WHITE WHITE WHITE R3 5.0 CS2 R2 5.0 C2 1uF/16V D0 L1 VIN C1 4.7uF 5 GND 6 7 8 22uH or 33uH PGND U1 LX EL7513CR SGND VOUT CS CNTL VIN COMP R1 BAT54HT1 4 5.0 GND LX 3 CNTL 2 1 C4 0.1uF C5 OPEN 0.1uF CS C3 0.1uF FIGURE 1. DEMO BOARD SCHEMATIC Demo Board Bill of Materials REFERENCE DESIGNATOR VALUE PACKAGE MAX HEIGHT (mm) MANUFACTURER MANUFACTURER’S PN MANUFACTURER’S PHONE NUMBER C1 4.7µF/6.3V 0805 1.25 TDK C2012X5R0J475K 847-803-6100 C2 1µF/16V 0805 1.25 TDK C2012X5R1C105K 847-803-6100 C3 0.1µF 0603 Any C4 OPEN 0603 Any C5 0.1µF 0603 Any D0 BAT54HT1 SOD323 R1, R2, R3 4.99Ω 0603 L1 33µH U1 EL7513 SOT23-8 1 1 Any 1 Coilcraft LPO1704-333M 847-639-6400 1 Intersil EL7513 888-ELANTEC CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright © Intersil Americas Inc. 2003. All Rights Reserved. Elantec is a registered trademark of Elantec Semiconductor, Inc. All other trademarks mentioned are the property of their respective owners. Technical Brief 414 Input Voltage This demo board is intended for VIN = 2.7 to 5.5V. However, with higher voltage rating input and output capacitors (C1 and C2), it can be used for higher input voltage up to 13.5V. Brightness Control The relationship between the LED current and CNTL voltage level is as follows: V CNTL I LED = ---------------------------13.33 × R1 White LED Connections One leg of LEDs connected in series will ensure all the uniformity of the brightness. 18V maximum voltage enables 4-5 LEDs can be placed in series. When placing 5 LEDs in series, make sure the worst total forward voltage does not exceed 18V. PCB Layout Considerations When R1 is 5Ω, 1V of VCNTL conveniently sets ILED to 15mA. The range of VCNTL is 250mV to 5.5V. Component Selection The input and output capacitors are not very important for the converter to operate normally. The input capacitance is normally 2.22µF - 4.7µF, output capacitance 0.22µF -1µF, depending on number of LEDs with 3 LEDs, 0.22µF is sufficient. Higher capacitances are allowed to reduce the voltage/ current ripple, but at added cost. Use X5R or X7R ceramic type (for its good temperature characteristics) capacitors with correct voltage rating and maximum allowed height. When choosing an inductor, make sure the inductor can handle the average and peak currents giving by following formulas: IO × VO I LAVG = ----------------------0.8 × V IN 1 I LPK = I LAVG + --- × ∆I L 2 V IN × ( V O – V IN ) ∆I L = -------------------------------------------L × VO Where ∆IL is the peak-to-peak inductor current ripple, L inductance in µH. A wide range of inductance (6.8µH - 68µH) can be used for the converter to function correctly. However, try different values of inductor in the same series to see which generates best efficiency. The diode should be Schottky type with minimum reverse voltage of 20V. The diode's peak current is the same as inductor's peak current, the average current is IO, and RMS current is: I DRMS = The compensation capacitor C4 can be any value from 4700pF to 1µF. C3 and C5 are for noise filtering; they may not be needed if there is no noise interference. I LAVG × I O Ensure the diode's ratings exceed these current requirements. 2 The layout is very important for the converter to function properly. Power Ground ( ) and Signal Ground ( ) should be separated to ensure the high pulse current in the power ground does not interference with the sensitive signals connected to Signal Ground. Both grounds should only be connected at one point right at the chip. The heavy current paths (VIN-L-LX pin-PGND, and VIN-L-D-Co-PGND) should be as short as possible. The trace connected to pin 7 (CS) is most important. The current sense resister R1 should be very close to the pin and when the trace is long, use a small filter cap (C5) close to pin 7. The heat of the IC is mainly dissipated through the PGND pin. Maximizing the copper area around the plane is preferable. In addition, a solid ground plane is always helpful for the EMI performance. Technical Brief 414 Demo Board Layout EL7513 Evaluation Board GND D1 D6 D11 D2 D7 D12 D3 D8 D13 D4 D9 D14 D5 D10 D15 C2 L1 D0 C1 LX C4 C3 C5 EL7513 R1 VIN VOUT CS R2 CS2 CNTL Rev A R3 CS3 GND LY FIGURE 2. SILKSCREEN FIGURE 4. BOTTOM LAYER Demo Board Layout (continued) FIGURE 3. TOP LAYER All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. For information regarding Intersil Corporation and its products, see www.intersil.com 3