ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter Preliminary Contents 1. Features 2. Introduction 3. Regulator Design Procedure 4. Design Example This application note contains new product information. Diodes, Inc. reserves the rights to modify the product specification without notice. No liability is assumed as a result of the use of this product. No rights under any patent accompany the sale of the product. 1/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter 1.0 Preliminary Features ◆ Small Board Size - Entire circuit can fit in less than 1 square inch of PCB space ◆ Low Implementation Cost - Fewer than 4 discrete components required ◆ ON /OFF Control - Be controlled by external logic level signal ◆ Thermal Shut-down and Current Limit - Built-in function ◆ Simple Feedback Compensation - Lead compensation using external capacitor ◆ Immediate Implementation - Schematic, board-of-materials and board layout available from Anachip 2.0 Introduction This application note discusses simple ways to select all necessary components to implement a step-down (BUCK) regulator and gives a design example. In this example, the AP1509 monolithic IC is used to design a cost-effective and high-efficiency miniature switching buck regulator. Please refer to the datasheet for more complete information, pin descriptions and specifications for the AP1509 will not be repeated here. This demonstration board allows the designer to evaluate the performance of the AP1509 series buck regulator in a typical application circuit. The user needs only to supply an input voltage and a load. The demonstration board can be configured to evaluate fixed output voltages of 3.3V, 5V, 12V, and an adjustable output version of the AP1509 series. Operation at other voltages and currents may be accomplished by proper component selection and replacement. 2/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter 3.0 Preliminary Regulator Design Procedure 3.0.1 Given Power Specification V V V V I I IN (max) IN (min) OUT = Maximum Input Voltage = Minimum Input Voltage = Regulated Output Voltage RIPPLE = Ripple Voltage (peak to peak), typical value is 1% of the Output Voltage LOAD(max) LOAD(min) = Maximum Load Current = Minimum Load Current before the circuit becomes discontinuous, typical value is 10% of the Maximum Load Current F = Switching Frequency (fixed at a nominal 150kHz) 3.0.2 Programming Output Voltage (refer to 4.0.4 Demo Board Schematic P7) The Output Voltage is programmed by selection of the divider R1 and R2. The designer should use resistors R1 and R2 with ±1% tolerance in order to obtain best accuracy of the Output Voltage. The Output Voltage can be calculated from the following formula: Vout = 1.23 x (1 + R1 / R2) Select a value for R2 between 240Ω and 1.5KΩ. The lower resistor values minimize noise pickup in the sensitive feedback pin. If the designer selects a fixed output version of the AP1509, the resistor R1 shall be short and R2 shall be open. 3/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter Preliminary 3.0.3 Inductor Selection A. The minimum inductor L (min) can be calculated from the following design formula table: Calculation T T L Step-down (buck) regulator (V ON OFF [V (min) V IN (min) IN (min) OUT +V F ) − V SAT − V OUT ] − V SAT − V OUT × T ON (max) 2 × I LOAD (min) V V SAT F = Internal switch saturation voltage of the AP1509 = 1.25V = Forward voltage drop of output rectifier D1 = 0.5V B. The inductor must be designed so that it does not saturate or significantly saturate at DC current bias of . ( PK = Peak inductor or switch current = − LOAD (min) ) LOAD (max) PK I I I I 3.0.4 Output Capacitor Selection A. The output capacitor is required to filter the output and provide regulator loop stability. When selecting an output capacitor, the important capacitor parameters are; the 100kHz Equivalent Series Resistance (ESR), the RMS ripples current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter. The ESR can be calculated from the following formula: ⎞ ⎛ ESR = ⎜ V RIPPLE ⎟ ------------------------ (3) ⎟ ⎜ 2× I LOAD (min) ⎠ ⎝ An aluminum electrolytic capacitor's ESR value is related to the capacitance and its voltage rating. In most cases, higher voltage electrolytic capacitors have lower ESR values. Most of the time, capacitors with much higher voltage ratings may be needed to provide the low ESR values required for low output ripple voltage. If the selected capacitor's ESR is extremely low, it results in an oscillation at the output. It is recommended to replace this low ESR capacitor by using two general standard capacitors in parallel. B. The capacitor voltage rating should be at least 1.5 times greater than the output voltage, and often much higher voltage ratings are needed to satisfy the low ESR requirements needed for low output ripple voltage. 4/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter Preliminary 3.0.5 Output Rectifier Selection A. The Output Rectifier D1 current rating must be greater than the peak switch current IPK. The reverse voltage rating of the output rectifier D1 should be at least 1.25 times the maximum input voltage. B. The output rectifier D1 must be fast (short reverse recovery time) and must be located close to the AP1509 using short leads and short printed circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency, and should be the first choice, especially in low output voltage applications. 3.0.6 Input Capacitor Selection A. The RMS current rating of the input capacitor can be calculated from the following formula table. The capacitor manufacturers datasheet must be checked to assure that this current rating is not exceeded. Calculation Step-down (buck) regulator Ton/(Ton+Toff) I LOAD (max) + I LOAD (min) δ I I ΔI I PK I m LOAD (max) − I LOAD (min) 2 × I LOAD(min) L δ × ⎢(I PK × I m ) + ⎡ ⎣ IN ( rms ) 1 (Δ I L )2 ⎤⎥ 3 ⎦ B. This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage. 3.0 Design Example 4.0.1 Summary of Target Specifications Input Power Regulated Output Power Output Ripple Voltage Output Voltage Load Regulation Efficiency Switching Frequency V V V IN (max) OUT = +12V; = + 3.3V; RIPPLE V I IN (min) = +12V LOAD (max) = 2A; I LOAD (min) = 0.2A ≤ 50 mV peak-to-peak 1% (1/2 full load to full load) 75% minimum at full load f = 150kHz ± 15 % 5/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter Preliminary 4.0.2 Calculating and Components Selection Calculation Formula Vout = Vref x ((R1/R2) + 1) L(min) ≥ I PK = I [V IN (min) Select Condition 240Ω ≤ R2 ≤ 1.5KΩ ] − V SAT − V OUT × T ON (max) 2 × I LOAD (min) LOAD (max) − I LOAD (min) RRM PK I LOAD (max) ≥ I PK Select L1 = 39uH = 1.8A ESR ≤ 125mΩ V ≥ 1.25 ×V IN (max) = ≥ 38uH (min) rms ⎞ ⎛ ESR = ⎜ V RIPPLE ⎟ ⎟ ⎜ 2× I LOAD (min) ⎠ ⎝ V WVDC ≥ 1.5 ×V OUT V I L I + I LOAD (min) V I I RRM PK 1 2⎤ ⎡ I IN ( rms ) = δ × ⎢⎣(I PK × I m ) + 3 (Δ I L ) ⎥⎦ V WVDC ≥ 1.5 ×V IN (max) Select D1: 20V/2A ≥ 15V = 1.8A ripple V Select C3: 1000uF/10V*1pcs ≥ 7.5V WVDC ≥ I Component spec. R1 = 560Ω; R2 = 330Ω IN ( rms ) = 1A Select C1: 330uF/35V*1pcs ≥ 18V WVDC 4.0.3 Parts List (Board of Materials) Item Part Number C1 C2 0805 cap C3 C4 0805 cap D1 220 MFG/Dist. Description Value Quantity OST Aluminum Electrolytic 330uF, 35V 1 Viking Ceramic Capacitor 0.1uF, 25V 1 OST Aluminum Electrolytic 1000uF, 10V 1 Viking Ceramic Capacitor 0.1uF, 25V 1 Schottky Diode 20V, 2A 1 Pitch = 2.54mm, 3pin 1 J1 Jumper J2,J3 Pin L1 Inductor 39uH, 1.8A 1 2 U1 AP1509 Anachip PWM Buck Converter 150kHz, 2A 1 R1 0805 reg Viking Film Chip Resistor 560Ω 1 R2 0805 reg Viking Film Chip Resistor 330Ω 1 6/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter Preliminary 4.0.4 Demo Board Schematic 4.0.5 Demo Board Efficiency and Temperature 7/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter 4.0.6 Preliminary Typical PC Board Layout (1). Component Placement Guide 8/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated ANP013 Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter Preliminary (2). Component Side PC Board Layout (3). Solder Side PC Board Layout 9/9 ANP013 – App. Note 1 May 2006 www.diodes.com © Diodes Incorporated