ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter Contents 1. Features 2. Introduction 3. Pin Functions 4. Internal Block Diagram 5. Regulator Design Procedure 6. Design Example This application note contains new product information. Diodes, Inc. reserves the right 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. 1/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 1.0 Features ◆ Small Board Size - Entire circuit can fit on 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, bill-of-materials and board layout available from DIODES 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 AP1512/A monolithic IC is used to design a cost-effective and high-efficiency miniature switching buck regulator. For more complete information, pin descriptions and specifications for the AP1512/A will not be repeated here. Please refer to the datasheet when designing or evaluating with the AP1512/A. This demonstration board allows the designer to evaluate the performance of the AP1512/A 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 a fixed output voltage of 3.3V, 5V, 12V, and an adjustable output version of the AP1512/A series. Operation at other voltages and currents may be accomplished by proper component selection and replacement. 2/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 3.0 Pin Functions Number Name Function 1 2 3 4 5 +VIN Output GND FB SD Operating Voltage Input Switching Output Ground Output Feedback Control ON/OFF Shutdown Control +VIN (Pin 1): This pin is the main power input to the IC. The range of operating voltage is from +4.5V to +60V. A suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the switching current’s needs by the regulator. Output (Pin 2): Internal switch. The voltage at this pin switches between ( + V IN − VSAT ) and approximately -0.55 V, with a duty cycle of approximately VOUT V IN . To minimize coupling to sensitive circuitry, the PC board copper area connected to this pin should be kept at a minimum. GND (Pin 3): Circuit ground for the IC. FB (Pin 4): Senses the regulated output voltage to complete the feedback loop. SD (Pin 5): Allows the switching regulator circuit to be shutdown using logic level signals thus dropping the total input supply current to approximately 350uA. Pulling this pin below a threshold voltage of approximately 1.3V turns the regulator on, and pulling this pin above 1.3V (up to a maximum of 40V) shuts the regulator down. If this shutdown feature is not needed, the SD pin can be wired to the ground pin or it can be left open, in either case the regulator will be in the ON condition. 3/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 4.0 Internal Block Diagram SD V IN 200mV Current Source bias 2.5V Regulator 1.235V Reference 220mV Start up + Comp _ _ Comp + FB Frequency compensation _ + Amp _ Comp Pre-driver + 2A Switch Output 50KHz OSC. 5.0 Thermal Limit Gnd Regulator Design Procedure 5.0 .1 Given Power Specification V V V V I I IN (max) IN (min) OUT RIPPLE = Maximum Input Voltage = Minimum Input Voltage = Regulated Output Voltage = 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 50KHz) 4/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 5.0.2 Programming Output Voltage The output voltage is programmed by selection of the divider R2 and R3. The designer should use resistors R2 and R3 with ±1% tolerance in order to obtain the best accuracy of the output voltage. The output voltage can be calculated from the following formula: Vout = 1.23 x (1 + R2 / R3) Select a value for R3 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 AP1512/A, the resistor R2 shall be short and R3 shall be open. 5.0.3 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 SAT V of I = Internal switch saturation voltage of the AP1512/A = 1.3V F = 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 . ( PK = Peak inductor or switch current = + LOAD (min) ) LOAD (max) PK I I I 5.0.4 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 5/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 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. 5.0.5 Compensation Capacitor Selection For a stable application circuit, an additional capacitor C5 is required. The compensation capacitor C5 provides additional stability for high output voltages, low input-output voltages, and/or very low ESR output capacitors. 5.0.6 Output Rectifier Selection A. The output rectifier D1 current rating must be at least 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 AP1512/A 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. 5.0.7 Input Capacitor Selection A. The RMS current rating of the input capacitor can be calculated from the following formula table. The capacitor manufacturer’s data sheet must be checked to assure that this current rating is not exceeded. Calculation δ I I ΔI I Step-down (buck) regulator Ton/(Ton+Toff) I I PK m LOAD (max) + I LOAD (min) LOAD (max) − I LOAD (min) 2 × I LOAD(min) L IN ( rms ) δ × ⎢(I PK × I m ) + ⎡ ⎣ 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. 6.0 Design Example 6/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 6.0.1 Summary of Target Specifications Input Power V V V Regulated Output Power Output Ripple Voltage Output Voltage Load Regulation Efficiency Switching Frequency IN (max) OUT I = + 5V; RIPPLE V = +12V; IN (min) LOAD (max) = +12V = 2A; I LOAD (min) = 0.6A ≤ 50 mV peak-to-peak 1% (1/2 full load to full load) 75% minimum at full load F = 50KHz ± 15 % 6.0.2 Calculating and Components Selection Calculation Formula Vout = Vref x ((R2/R3) + 1) L I (min) PK = [V ≥ I IN (min) LOAD (max) ] − V SAT − V OUT × T ON (max) 2 × I LOAD (min) + I LOAD (min) ⎞ ⎛ ESR = ⎜ V RIPPLE ⎟ ⎟ ⎜ 2× I LOAD (min) ⎠ ⎝ V WVDC ≥ 1.5 ×V OUT V I RRM PK = ≥ 1.25 ×V IN (max) I LOAD (max) + I LOAD (min) 1 2⎤ ⎡ I IN ( rms ) = δ × ⎢⎣(I PK × I m ) + 3 (Δ I L ) ⎥⎦ V WVDC ≥ 1.5 ×V IN (max) Select Condition 240Ω ≤ R3 ≤ 1.5KΩ L I ≥ 47uH (min) rms ≥ I PK Select = 47uH/3A = 2.6A ESR ≤ 62.5mΩ V V I I WVDC RRM PK V Select C3 from "JACKCON" 470uF/16V*1pcs ≥ 7.5V ≥ 15V Select D1:SR360 60V/3A = 2.6A ripple ≥ WVDC I Component spec. R2 = 3KΩ; R3 = 1KΩ IN ( rms ) = 1.74A Select C1 from "ELCON" 110uF/100V*1pcs ≥ 18V 7/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 6.0.3 Parts List (Board of Materials) Item Description Value Quantity C1 Part Number MFG/Dist. Aluminum Electrolytic 110uF, 100V 1 C2 Ceramic Capacitor 1uF, 25V 1 C3 Aluminum Electrolytic 470uF, 16V 1 C5 Ceramic Capacitor 1nF, 25V 1 Ceramic Capacitor 0.1uF, 25V 1 Schottky Rectifier 60V, 3A 1 Inductor 47 uH, 3.8A 1 PWM Buck Converter 50KHz, 2A 1 C7 D1 B360A/B360B/B360 PDS360 L1 7447709470 U1 AP1512/AD R2 Std Film Chip Resistor 3KΩ±5%, 1/8W 1 R3 Std Film Chip Resistor 1KΩ±5%, 1/8W 1 DIODES Wurth Electronik DIODES 6.0.4 Demo Board Schematic VIN VOUT ON/OFF C1 C2 100uF/100V 1u SD 2 1 D1 47uH GND 5 Vin OUTPUT 3 1 L1 7447709470 2 FB 4 R2 3K B360A / B360B / B360 / PDS360 AP1512 C5 1n C7 C3 0.1u 470uF/16V R3 1K 6.0.5 Demo Board Efficiency at Vin 60V Load Vout = 5V 1A 70.83% 2A 76.84% 3A 74.25% 8/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 6.0.6 Typical PC Board Layout (1). Component Placement Guide 9/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter (2). Component Side PC Board Layout (3). Solder Side PC Board Layout 10/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated ANP022 Application Note AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter 6.0.7 Heatsink Layout Guide Line The heatsink is dependent on the maximum power dissipation and maximum ambient temperature of the application. For example: 12V to 5V/2A Top Copper Layout Area (mm*mm) 2 15*10 Bottom Copper Area (mm*mm) 12*20 Through Hole 1mm*48 IC Body Temperature 67 Gnd Pin Temperature 75 12V to 3.3V/2A Top Copper Layout Area (mm*mm) 2 15*10 Bottom Copper Area (mm*mm) 12*20 Through Hole 1mm*48 IC Body Temperature 61 Gnd Pin Temperature 68 60V to 5V/2A Top Copper Layout Area (mm*mm) 2 15*10 Bottom Copper Area (mm*mm) 12*20 Through Hole 1mm*48 IC Body Temperature 84 Gnd Pin Temperature 90 60V to 3.3V/2A Top Copper Layout Area (mm*mm) 2 15*10 Bottom Copper Area (mm*mm) 12*20 Through Hole 1mm*48 IC Body Temperature 78 Gnd Pin Temperature 84 11/11 ANP022 – App. Note 1 April 2008 www.diodes.com © Diodes Incorporated