D NE W ® ESI G Using the EL7562 Demo Board NS FO R DED 532 N E M E L7 COM SEETechnical Brief E R NOT June 20, 2002 TB423 There are 2 demo boards, one for nominal 5V input and another for 3.3V input. This document outlines the design consideration and lists the bill of materials and the layout. Please also refer to the advanced data sheet of EL7562 for detailed applications of the features. The EL7562 is a Buck (Step Down) DC:DC controller with integrated synchronous MOSFETs in a 16-pin QSOP package. With very few external components, a 2A step-down DC:DC converter can be very easily built, resulting in saved board space (0.5in2), minimal design effort, and improved design time. EL7562 Demo Board Circuit Schematic for VIN = 5V Application R V O = 0.985 × 1 + ------2- R 1 C3 C4 1 SGND PGND 16 2 COSC VREF 15 C5 R3 R2 FB 14 3 VDD C1 C2 VIN 4 PGND VDRV 13 5 PGND LX 12 6 VIN LX 11 7 VIN VHI 10 8 EN PGND 9 R1 L1 VO C7 C6 EL7562 EL7562 Demo Board Bill of Material VIN = 5V, VOUT = 3.3V REFERENCE DESIGNATION VALUE MANUFACTURER MANUFACTURER’S PHONE NUMBER PART NUMBER C1 100µF Sprague 207-324-4140 293D107X0010D2 C2, C3, C5, C6 0.1µF, 0603 Any C4 270pF, 5%, 0603 Any C7 100µF Sprague 207-324-4140 293D107X0010D2 L1 4.7µH Coilcraft 847-639-6400 D01813472HC R1 1kΩ, 0603 Any R2 2370Ω, 0603 Any R3 39Ω, 0603 Any 1 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. 2004. 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 423 EL7562 Demo Board Circuit Schematic for VIN = 3.3V Application R V O = 0.975 × 1 + ------2- R 1 C3 C4 0.1µF 270pF 1 SGND PGND 16 2 COSC VREF 15 C5 0.1µF R3 FB 14 3 VDD 39Ω C1 C2 100µF 0.1µF VIN (3V-3.6V) 4 PGND VDRV 13 5 PGND LX 12 D2 D3 D4 C8 0.1µF C9 0.1µF VO (2.5V, 2A) L1 LX 11 6 VIN 7 VIN VHI 10 8 EN PGND 9 EL7562 Demo Board Bill of Material C6 C7 4.7µF 0.1µF 100µF R2 1.54kΩ R1 1kΩ VIN = 3.3V, VOUT = 2.5V REFERENCE DESIGNATION VALUE MANUFACTURER MANUFACTURER’S PHONE NUMBER PART NUMBER C1 100µF Sprague 207-324-4140 293D107X0010D2 C2, C3, C5, C6, C8, C9 0.1µF, 0603 Any C4 270pF, 5%, 0603 Any C7 100µF Sprague 207-324-4140 293D107X0010D2 L1 4.7µH Coilcraft 847-639-6400 D01813472HC R1 1kΩ, 0603 Any R2 1.54kΩ, 0603 Any R3 39Ω, 0603 Any D2, D3/4 Bat54S Vishay Telefunken 402-563-6863 Bat54S 2 Technical Brief 423 Design Considerations Choosing the Component Values The following requirements are specified for a DC:DC converter: • Input voltage range: VIN = 4.5V-5.5V • Output voltage: VO = 3.3V 4. Output capacitor C7. ∆VO and ∆IL normally decide C7 value. ∆VO requires ESR of C7 be less than: ∆V O ESR = --------------------- = 89mΩ ∆I LMAX Double-check the RMS current requirement of the output capacitor: • Max output voltage ripple: ∆VO = 50mV • Output max current: IO = 2A ∆I LMAX ∆I C7 = -------------------12 The following steps briefly outline the steps to choose components. 1. Choose the feedback resister divider. The output voltage is decided by: R V OUT = 0.985 × 1 + ------2- R 1 which is 0.16A. For a capacitor or combination of capacitors with 89mΩ parallel ESR, it is more than enough to handle this current. For VIN = 5V 5. Input capacitors C1 and C2. R V OUT = 0.975 × 1 + ------2- R 1 If all the AC current is handled by the input capacitors its RMS current is calculated as: For VIN = 3.3V 2. Choose the converter switching frequency FS. FS, inductor L1, output capacitor C7, and EL7562’s switching loss are closely related. many iterations (or thermal measurements) may be required before a final value can be decided. Please refer to the EL7562 data sheet for the FS vs COSC curve. 3. Inductor L1. The EL7562 is internally ramp-compensated. For optimal operation, the inductor current ripple should be less than 0.6A. If ∆IL = 0.5A, then: ( 1 – D ) × VO L = --------------------------------∆I L × F S I IN,rms = [ D × ( 1 – D ) ] × IO This gives almost 0.99A when D = DMAX. Therefore a cap with 0.99A current handling capability should be chosen. However, in case some other capacitor is sharing current with it, this current requirement can be reduced. Layout Considerations The layout is very important for the converter to function properly. Power Ground ( ) and Signal Ground ( ) should be separated to ensure that the high pulse current in the Power Ground never interferes with the sensitive signals connected to Signal Ground. They should only be connected at one point (normally at the negative side of either the input or output capacitor.) The trace connected to pin 14 (FB) is the most sensitive trace. It needs to be as short as possible and in a “quiet” place, preferably between the PGND and SGND traces. where: VO D = --------V IN Choosing L1 = 4.7µH yields ∆ILMAX = 0.56A. L1 should also be able to handle DC current of 2A and peak current of 2.3A at temperature range. 3 In addition, the bypass capacitor C3 should be as close to pins 1 and 3 as possible. The heat of the chip is mainly dissipated through the PGND pins. Maximizing the copper area around these pins is preferable. In addition, a solid ground plane is always helpful for the EMI performance. Technical Brief 423 Demo Board Layout for VIN = 5V 0.5” 1” FIGURE 1. TOP LAYER 0.5” 1” FIGURE 2. TOP SILKSCREEN 4 Technical Brief 423 Demo Board Layout for VIN = 5V (Continued) 0.5” 1” FIGURE 3. BOTTOM LAYER 0.5” 1” FIGURE 4. BOTTOM SILKSCREEN 5 Technical Brief 423 Demo Board Layout for VIN = 3.3V 0.6” 1.2” FIGURE 5. TOP LAYER 0.6” 1.2” FIGURE 6. TOP SILKSCREEN 6 Technical Brief 423 Demo Board Layout for VIN = 3.3V (Continued) 0.6” 1.2” FIGURE 7. BOTTOM LAYER 0.6” 1.2” FIGURE 8. BOTTOM SILKSCREEN 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 the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 7