ANP004 Application Note AP2001 Buck + Boost Converter Contents 1. AP2001 Specifications 1.1 Features 1.2 General Description 1.3 Pin Assignments 1.4 Pin Descriptions 1.5 Block Diagram 1.6 Absolute Maximum Ratings 2. Hardware 2.1 Introduction 2.2 Typical Application 2.3 Input / Output Connections 2.4 Schematic 2.5 Board of Materials 2.6 Board Layout 3. Design Procedure 3.1 Introduction 3.2 Operating Specifications 3.3 Design Procedures 3.3.1 Buck Converter 3.3.1.1 Selection of the Buck Inductor (L) 3.3.1.2 Selection of the Output Capacitor (Cout) 3.3.1.3 Selection of Power Switch (MOSFET) 3.3.1.4 Selection of Power Rectifier (D) 3.3.1.5 Selection of the Input Capacitor (Cin) 3.3.2 Boost Converter 3.3.2.1 Selection of the Boost Inductor (L) 3.3.2.2 Selection of the Output Capacitor (Cout) 3.3.2.3 Selection of Power Switch (MOSFET) 3.3.2.4 Selection of Power Rectifier (D) 3.3.2.5 Selection of the Input Capacitor (Cin) 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. 1/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 1. AP2001 Specification 1.1 Features - Dual PWM Control Circuitry - Operating Voltage can be up to 50V - Adjustable Dead Time Control (DTC) - Under Voltage Lockout (UVLO) Protection - Short Circuit Protection (SCP) - Variable Oscillator Frequency…500kHz max. - 2.5V Voltage Reference Output - 16-pin PDIP and SOP Packages 1.2 General Description The AP2001 integrates Pulse-Width-Modulation (PWM) control circuit into a single chip, mainly designed for a power-supply regulator. All the functions include an on-chip 2.5V reference output, two error amplifiers, an adjustable oscillator, two dead-time comparators, UVLO, SCP, DTC circuitry, and dual Common-Emitter (CE) output transistor circuit. Recommend the output CE transistors as pre-driver for driving externally. The DTC can provide from 0% to 100%. Switching frequency can be adjustable by trimming RT and CT. During low VCC situation, the UVLO makes sure that the outputs are off until the internal circuit is operating normally. 1.3 Pin Assignments ( Top View ) CT RT EA1+ EA1FB1 1 16 2 15 3 14 4 13 5 12 DTC1 OUT1 GND 6 11 7 10 8 9 REF SCP EA2+ EA2FB2 DTC2 OUT2 VCC PDIP/SOP 2/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 1.4 Pin Descriptions Name Description CT Timing Capacitor RT Timing Resistor EA+ Error Amplifier Input (+) EA - Error Amplifier Input (-) FB Feedback Loop Compensation DTC Dead Time Control OUT Pre-driver Output GND Ground VCC Supply Voltage SCP Short Circuit Protection REF Voltage Reference 1.5 Block Diagram VCC SCP RT Bandgap Reference REF CT DTC1 Oscillator MAX.500KHz + + - EA1 + EA1 - OUT1 VREF Error Amplifier 1 PWM Amplifier 1 170K FB1 1.18V + + UVLO R R S + + EA2+ EA2 Error Amplifier 2 OUT2 PWM Amplifier 2 FB2 GND DTC2 3/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 1.6 Absolute Maximum Ratings Symbol Rating Unit Supply Voltage 40 V VI Amplifier Input Voltage 20 V VO Collector Output Voltage 40 V Io Collector Output Current 21 mA VCC TOP TST TLEAD Parameter Operating Temperature Range Storage Temperature Range Lead Temperature 1.6mm (1/16 inch) from Case for 10 Seconds -20 to +85 o -65 to +150 o 260 o C C C 2. Hardware 2.1 Introduction The Buck + Boost demo board supplies two constant DC output voltages that are 3.3V and 12V. This board can supply output power up to 10W for buck output (3.3V / 3A) and up to 3.6W for boost output (12V / 0.3A). Using a DC input voltage of 5V to 7V, full load efficiency varies from 80 percent to 86 percent depending on the input voltage. This type of converter converts an unregulated input voltage to 2 regulated output voltages where one is always lower than the input voltage and the other is always higher than the input voltage. The control method used in the board is fixed frequency, variable on-time Pulse-Width-Modulation (PWM). The feedback method used is voltage-mode control. Other features of the board include Under Voltage Lockout (UVLO), Short-Circuit Protection (SCP), and adjustable Dead Time Control (DTC). 2.2 Typical Application The AP2001 may operate in either the CCM (Continuous Conduction Mode) or the DCM (Discontinuous Conduction Mode). The following applications are designed for CCM (Continuous Conduction Mode) operation. That is, the inductor current is not allowed to fall to zero. To compare the disadvantages and advantages for CCM and DCM, the main disadvantage of CCM is the inherent stability problems (caused by the right-half-plane zero and the double pole in the small-signal control to output voltage transfer function). However, the main disadvantage of DCM is that peak currents of switch and diode are larger than CCM when converting. Using power switch and output diode with larger current and power dissipation ratings should solve this issue of large peak current. The designer has to use larger output capacitors, and take more effort on EMI/RFI solution also. The designer could make a choice for each mode. For a low loading current, DCM is preferred for buck and CCM is preferred for boost. If the load current requirement is high, CCM is preferred for buck and DCM is preferred for boost. 4/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter Buck (Step Down) The Buck or Step-down converter converts a DC voltage to a lower DC voltage. Figure 1 shows the basic buck topology. When the switch SW is turned on, energy is stored in the inductor L and it has constant voltage “VL = VI – Vo”, the inductor current iL ramps up at a slope determined by the input voltage. Diode D is off during this period. Once the switch, SW, turns off, diode D starts to conduct and the energy stored in the inductor is released to the load. Current in the inductor ramps down at a slope determined by the difference between the input and output voltages. VS iS SW L iD Vi IO VL iL iC RL C D VD VO Figure 1. Typical Buck Converter Topology Boost (Step-up) The Boost or Step-up converter converts a DC voltage to a higher DC voltage. Figure 2 shows the basic boost topology. When the switch SW is turned on, energy is stored in the inductor L and the inductor current iL ramps up at a slope determined by the input voltage. Diode D is off during this period. Once the switch, SW, turns off, diode D starts to conduct and the energy stored in the inductor is released to the load, it has constant voltage “Vo = Vl + VL”. Current in the inductor ramps down at a slope determined by the difference Between the input and output voltages. iin vL iL L VIN iD vD SW D vS iO iC C iS RL VO Figure 2. Typical Boost Converter Topology 5/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 2.3 Input / Output Connections + Vin = 4.5V ~ 6V Vo2 = 12V / 0.3A Vo1 = 3.3V / 3A + - - + - + + - Figure 3. I/O Connections 6/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 2.4 Schematic Figure 4. Demo Board Schematic 7/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 2.5 Board of Materials No Value Qty Part Reference Manufacturers Part Number 1 0.1uF 1 C1 2 330pF 2 C2, C3 3 10nF 2 C4, C7 4 1uF C5, C6, C8, C11, C12, C13, 10 C14, C15, C16, C17, C18 5 1000pF 1 C9 Ceramic Chip CAP. 1000pF 25V ±10% K X7R 0805 6 short 1 C10 Short 7 RB160L-40 1 D1 Schottky Diode 1A 40V DIODES ROHM DIODES B140 RB160L-40 B340A 8 B340A 1 D2 Schottky Diode 3A 40V 9 470uF 3 EC1, EC2, EC3 Electrolysis Capacitors 10 CON2 3 J1, J2, J3 2P PCB Terminal Block DINKLE ELK508V-02P 11 100uH/1A 1 L1 TOROID COILS 10uH 1A Star Electronics 12 33uH/3A 1 L2 TOROID COILS 33uH 3A Star Electronics 13 NMOS_SOP8 1 Q1 N-Channel MOSFET 30V 1A↑ 14 MMBT4403 2 Q2, Q6 PNP BJT -40V -0.6A SOT-23 15 MMBT4401 2 Q3, Q4 NPN BJT 40V 0.6A SOT-23 16 PMOS_SOP8 1 Q5 P-Channel MOSFET -30V -3A↑ 17 470 1 R1 Chip Resistance 470 1/8W ±10% J 0805 18 0 4 R2, R10, R18, R20 Chip Resistance 0 1/8W ±10% J 0805 19 18K 1 R3 20 39K 1 R4 Description Ceramic Chip CAP. 0.1uF 25V ±10% K X7R 0805 Ceramic Chip CAP. 330pF 25V ±10% K X7R 0805 Ceramic Chip CAP. 2200pF 25V ±10% K X7R 0805 Ceramic Chip CAP. 1uF 25V ±10% K X7R 0805 Toshiba CET APEC ROHM DIODES ROHM DIODES Toshiba CET APEC TPC8005 CEM9426 AP4410M SST2907A MMBT4403 SST2222A MMBT4401 TPC8104-H CEM9435 AP9435M Chip Resistance 18K 1/8W ±10% J 0805 Chip Resistance 39K 1/8W ±10% J 0805 8/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 2.5 Board of Materials (continued) No Value Qty Part Reference 21 15K 1 R5 22 2.2K 1 R6 23 33K 5 R7 R11 R12 R16 R17 24 2K 1 R8 25 10K 1 R9 26 open 1 R13 27 4.7K 2 R14 R23 28 4.3K 2 R15 R19 29 5.6K 1 R21 30 8.2K 1 R22 31 AP2001 1 U1 Description Manufacturers Part Number Anachip AP2001S Chip Resistance 15K 1/8W ±10% J 0805 Chip Resistance 2.2K 1/8W ±10% J 0805 Chip Resistance 33K 1/8W ±10% J 0805 Chip Resistance 2K 1/8W ±10% J 0805 Chip Resistance 22K 1/8W ±10% J 0805 Open Chip Resistance 4.7K 1/8W ±10% J 0805 Chip Resistance 4.3K 1/8W ±10% J 0805 Chip Resistance 5.6K 1/8W ±10% J 0805 Chip Resistance 8.2K 1/8W ±10% J 0805 Monolithic Dual Channel PWM Controller 2.6 Board Layout Board size is 80mm(W) x 50mm(L) Figure 5. Silkscreen Layer 9/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 2.6 Board Layout (continued) Figure 6. Top Layer Figure 7. Bottom Layer 10/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 3. Design Procedure 3.1 Introduction The AP2001 integrated circuit is dual PWM controller. It operates over a wide input voltage range. This together with its low cost makes it a very popular choice for use in PWM controllers. This section will describe the AP2001 design procedure. The operation and the design of the buck + boost converter will also be discussed in detail. 3.2 Operating Specifications Specification Input Voltage Range Output (Buck/Boost) Voltage Range Output Power Range Output Current Range Operating Frequency Output Ripple Efficiency Min. Typ. Max. Units 5 3 / 11 0 300 / 50 100 6 3.3 / 12 10 / 3.6 3000 / 300 110 50 80 7 3.6 / 13 18 / 6.5 5000 / 500 120 V V W mA kHz mV % 74 86 Table 1. Operating Specifications 3.3 Design Procedures This section describes the steps to design continuous-mode buck and boost converter, and explains how to construct basic power conversion circuits including the design of the control chip functions and the basic loop. A switching frequency of 110 kHz was chosen. 3.3.1 Buck Converter Example calculations accompany the design equations. Since this is a fixed output converter, all example calculations apply to the converter with an output voltage of 3.3V and input voltage set to 6V, unless specified otherwise. The first quantity to be determined is the converter of the duty cycle value: Vo + Vd Ton = Ts , 0 ≤ D ≤ 1 Vin – Vds(sat) Assuming the commutating diode forward voltage Vd = 0.5V and the power switch on voltage Vds(sat) = 0.1V, the duty cycle for Vin = 5, 6 and 7 is 0.78, 0.64 and 0.55, respectively. Duty ratio D = 3.3.1.1 Selection of the Buck Inductor (L) A buck converter uses a single-stage LC filter. Choose an inductor to maintain continuous-mode operation down to 10 percent (Io(min)) of the rated output load: ΔIL = 2 x 10% x Io = 2 x 0.1 x 3 = 0.6A The inductor “L” value is: (Vin - Vds(sat) – Vo) x Dmin L≥ = ΔIL x fs (7 – 0.1 – 3.3) x 0.55 0.6 x (110 x 10^3) = 30μH So we can choose 33μH. 11/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 3.3.1.2 Selection of the Output Capacitor (Cout) Assuming that all of the inductor ripple current flows through the capacitor and the effective series resistance (ESR) is zero, the capacitance needed is: Cout ≥ ΔIL 8 x fs x ΔVo = 0.6 8 x (110 x 10^3) x 0.05 = 13.6μF Assuming the capacitance is very large, the ESR needed to limit the ripple to 50 mV is: ESR ≤ 0.05 ΔVo = ΔIo 0.6 = 0.083Ω The output filter capacitor should be rated at least ten times the calculated capacitance and 30–50 percent lower than the calculated ESR. This design used a 470-μF/25V OS-Con capacitor in parallel with a ceramic to reduce ESR. 3.3.1.3 Selection of the Power Switch (MOSFET) Based on the preliminary estimate, RDS(on) should be less than 0.10 V ÷ 3A = 33mΩ. The CEM4435 (CET) is a -30V p-channel MOSFET with RDS(on) = 35mΩ. Power dissipation (conduction + switching losses) can be estimated as: PMOSFET = Io^2 x Rds(on) x Dmax + [0.5 x Vin x Io x (tr + tf) x fs] Assuming total switching time (tr + tf) is 300 ns, a 55°C maximum ambient temperature, and thermal impedance RθJA = 50°C/W, thus: PMOSFET = (3 x 3 x 0.035 x 0.78) + [0.5 x 5 x 3 x (0.3 x 10^(-6)) x (110 x 10^3) = 0.5W TJ = TA+ (RθJA x PMOSFET) = 55 + (50 x 0.5) = 80°C 3.3.1.4 Selection of the Rectifier (D) The catch rectifier conducts during the time interval when the MOSFET is off. The B340 (DIODES) is a 3A, 40V Schottky Rectifier in an SMC power surface-mount package. The power dissipation is: PD = Io x Vd x (1 – Dmin) = 3 x 0.5 x (1 – 0.55) = 0.675W Assuming a 55°C maximum ambient temperature, and thermal impedance RθJA = 15°C/W, thus: TJ = TA+ (RθJA x PD) = 55 + (15 x 0.675) = 65.125°C 12/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 3.3.1.5 Selection of the Input Capacitor (Cin) The RMS current rating of the input capacitor can be calculated from the following formula. The capacitor manufacturers datasheet must be checked to assure that this current rating is not exceeded. Iin(rms) = √ [D x (Io(max) + Io(min)) x (Io(max) - Io(min)) + (ΔIL^2)/3] = √[0.78 x (3 + 0.3) x (3 – 0.3) + 0.36/3] = 2.67A This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 2 times the maximum input voltage. The input capacitor value is “470UF/25V”. 3.3.2 Boost Converter Example calculations accompany the design equations. Since this is a fixed output converter, all example calculations apply to the converter with output voltage 12V and input voltage set to 6V, unless specified otherwise. The first quantity to be determined is the converter duty cycle value: Duty ratio D = Vo + Vd- Vin(min) Vo + Vd- Vds(sat) = Ton Ts , 0≤D≤1 Assuming the commutating diode forward voltage Vd = 0.5 V and the power switch on voltage Vds(sat) = 0.1V, the duty cycle for Vin = 5, 6 and 7 is 0.60, 0.52 and 0.44, respectively. 3.3.2.1 Selection of the Boost Inductor (L) The boost inductor, converter switching frequency, input and output voltages, and output power determine a boost converter’s operating mode. This converter operates in the CCM (Continuous Conduction Mode). In continuous mode, the inductor current is not allowed to fall to zero. The peak-to-peak inductor current ripple is listed below: ΔIL = 2 x Io(min) x Vo Vin(min) = 2 x 0.05 x 12 5 = 0.24 The inductor “L” value is: L≥ Vin(min) – Vds(sat) x Dmax ΔIL x fs = (5 – 0.1) x 0.6 0.24 x (110 x 10^3) = 111.36μH So we can choose 120μH. 13/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 3.3.2.2 Selection of the Output Capacitor (Cout) Assuming that all of the inductor ripple current flows through the capacitor and the effective series resistance (ESR) is zero, the capacitance needed is: 0.3 x 0.6 = 32.73μF = 110k x 0.05 Io(max) x Dmax fs x ΔVo Cout ≥ Assuming the capacitance is very large, the ESR needed to limit the ripple to 50 mV is: Ipk = Io(max) 1 - Dmax + Vin(max) x D 2 x fs x L ESR ≤ ΔVo Ipk = = 0.3 1 – 0.6 + 7 x 0.6 = 0.91A 2 x 110k x 120μ 0.05 = 55mΩ 0.91 The output filter capacitor should be rated at least two to three times the calculated capacitance and 30 to 50 percent lower than the calculated ESR. This design used a 470-μF/25V OS-Con capacitor in parallel with a ceramic to reduce ESR. 3.3.2.3 Selection of Power Switch (MOSFET) The design uses an n-channel power MOSFET to simplify the drive-circuit design and minimize component count. Based on these calculations, the drain current rating should be chosen for 0.91A. The drain-to-source breakdown rating should be appropriate for the 20V applied to the device during the off time. A surface mount packaging is also recommended. The CEM9426 (CET) power MOSFET is a 20V n-channel MOSFET in a power surface mount package (SO-8) with an ID(MAX) rating of 10A. PMOSFET = Ipk^2 x Rds(on) x Dmax + [0.5 x Vin(max) x Ipk x (tr + tf) x fs] Assuming total switching time (tr + tf) is 300 ns, a 55°C maximum ambient temperature, and thermal impedance RθJA = 50°C/W, thus: PMOSFET = 0.91 x 0.91 x 0.0135 x 0.6 + [0.5 x 7 x 0.91 x 0.3μx 110k = 0.112W TJ = TA+ (RθJA x PD) = 55 + (50 x 0.112) = 60.6°C 14/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated ANP004 Application Note AP2001 Buck + Boost Converter 3.3.2.4 Selection of Power Rectifier (D) The catch rectifier conducts during the time interval when the MOSFET is off. The B140(DIODES) is a 1A, 40V Schottky rectifier in an SMC power surface-mount package. The power dissipation is: PD = Ipk x Vd = 0.91 x 0.5 = 0.455W Assuming a 55°C maximum ambient temperature, and thermal impedance RθJA = 15°C/W, thus: TJ = TA+ (RθJA x PD) = 55 + (15 x 0.455) = 61.825°C 3.3.2.5 Selection of the Input Capacitor (Cin) In boost switching regulators, triangular ripple current is drawn from the supply voltage due to the switching action. This appears as noise on the input line. This problem is less severe in boost converters due to the presence of inductor in series with the input line. Select the input capacitor for: Iin(rms) = Ipk √ (12) = 0.91 √ (12) = 0.263A This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 2 times the maximum input voltage. We select an input capacitor value of “470UF/25V”. 15/15 ANP004 – App. Note 1 Jun 2006 www.diodes.com © Diodes Incorporated