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ANP007
Application Note AP2001 Dual Buck Converter
Contents
1. AP2001 Specification
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 2.2 2.3 2.4 Introduction Typical Application Input / Output Connections Schematic
2.5 Bill of Material 2.6 Board Layout
3. Design Procedure
3.1 Introduction 3.2 Operating Specifications 3.3 Design Procedures 3.3.1 Selection of the buck inductor (L) 3.3.2 3.3.3 3.3.4 3.3.5 Selection of the output capacitor (Cout) Selection of power switch (MOSFET) Selection of power Rectifier (D) Selection of the input capacitor (Cin)
4. Voltage monitor by AP434
This application note contains new product information. Anachip Corp. 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. Rev. A.0 Feb. 20, 2003 1/11
ANP007
Application Note AP2001 Dual Buck 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 designs for power-supply regulator. All the functions included 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 operational normally.
1.3 Pin Assignments
( Top View ) CT RT EA1+ EA1FB1 DTC1 OUT1 GND
1 2 3 4 5 6 7 8
16 15 14 13 12 11 10
9
REF SCP EA2+ EA2FB2 DTC2 OUT2 VCC
PDIP/SOP
1.4 Pin Descriptions
Name CT RT EA+ EA FB DTC OUT GND VCC SCP REF Description Timing Capacitor Timing Resistor Error Amplifier Input(+) Error Amplifier Input(-) Feedback Loop Compensation Dead Time Control Pre-driver Output Ground Supply Voltage Short Circuit Protection Voltage Reference
Anachip Corp. www.anachip.com.tw 2/11
Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter 1.5 Block Diagram
VCC SCP RT CT DTC1
REF
Bandgap Reference
Oscillator
MAX.500KHz
EA1 + EA1 VREF Error Amplifier 1 170K FB1 1.18V EA2+ EA2 Error Amplifier 2 FB2 + + UVLO R R S
+ + PWM Amplifier 1
OUT1
+ + PWM Amplifier 2
OUT2
GND
DTC2
1.6 Absolute Maximum Ratings
Symbol VCC VI VO Io TOP TST TLEAD Supply voltage Amplifier input voltage Collector output voltage Collector output current Operating temperature range Storage temperature range Lead temperature 1.6 mm(1/16 inch) from case for 10 seconds Parameter Rating 40 20 40 21 -20 to +85 -65 to +150 260 Unit V V V
mA
o o o
C C C
Anachip Corp. www.anachip.com.tw 3/11
Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter 2. Hardware 2.1 Introduction
The dual-buck demo board supply two constant dc output voltage that are 3.3V and 5V. This board can supply output power up to 15W for buck1 output (5V / 3A) and up to 10W for buck2 output (3.3V / 3A). Using a dc input voltage of 10.8 V to 13.2 V, full load efficiency up to 86 percent. This type of converter converts an unregulated input voltage to 2 regulated output voltage that they are always lower 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 undervoltage 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 disadvantage and advantage 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 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. Designer could make a choice for each mode. For a low loading current, DCM is preferred for buck. If the load current requirement is high, CCM is preferred for buck. 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.
iS Vi
VS
SW
VD iD
iL
VL
L
IO iC
C
RL
D
VO
Figure 1. Typical Buck Converter Topology
Anachip Corp. www.anachip.com.tw 4/11
Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter 2.3 Input / Output Connections
Anachip Corp. www.anachip.com.tw 5/11
V in = 1 2 V (1 0.8 ~ 1 3.2 V ) V
O2
= 3.3 V / 3 A
V
O1
= 5V/3A
Figure 3. I/O Connections
Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter 2.4 Schematic
Figure 4. 2_buck demo board schematic
Anachip Corp. www.anachip.com.tw 6/11 Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter 2.5 Board of Materials
No. Value 1 open 2 1uF 3 4 5 6 7 8 9 10 Q'ty Part Reference 1 C1 C5 10 C2 C6 C7 C10 C11 C12 C13 C14 C15 C16 10nF 2 C3 C4 1uF 1 C8 470pF 1 C9 B340 2 D1 D2 470uF 3 EC1 EC2 EC3 CON2 3 J1 J2 J3 33uH/3A 1 L1 L2 PMOS_SOP8 2 Q1 Q4 2 Q3 Q6 2 Q2 Q5 1 2 4 1 5 4 1 1 1 1 1 R1 R2 R11 R3 R4 R13 R14 R5 R6 R9 R10 R12 R18 R7 R8 R16 R17 R15 R19 R20 R21 U1 Description Don't install Ceramic Chip CAP. 1uF 25V 10% K X7R 0805 Ceramic Chip CAP. 10nF 25V 10% K X7R 0805 Ceramic Chip CAP. 1uF 25V 10% K X7R 0805 Ceramic Chip CAP. 470pF 25V 10% K X7R 0805 Schottky Diode 3A 40V Electrolysis Capacitors 2P PCB Terminal Block TOROID COILS 33uH 3A P-Channel MOSFET -30V -3A NPN BJT 40V 0.6A SOT-23 PNP BJT -40V -0.6A SOT-23 Chip Resistance 470 1/8W 10% J 0805 Chip Resistance 47K 1/8W 10% J 0805 Chip Resistance 0 1/8W 10% J 0805 Chip Resistance 15K 1/8W 10% J 0805 Chip Resistance 4.7K 1/8W 10% J 0805 Chip Resistance 33K 1/8W 10% J 0805 Chip Resistance 8.2K 1/8W 10% J 0805 Chip Resistance 5.6K 1/8W 10% J 0805 To be define (5K ~ 50K) Chip Resistance 22K 1/8W 10% J 0805 Monolithic Dual Channel PWM Controller Manufacturers Philips Philips Philips Philips Philips DIODES DINKLE Star Electronics CET APEC ROHM DIODES ROHM DIODES Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Yageo(RL Series) Anachip Part Number
B340A ELK508V-02P CEM4435 AP4435M SST2222A MMBT4401 SST2907A MMBT4403
11 MMBT4401 12 MMBT4403 13 14 15 16 17 18 19 20 21 22 23 470 47K 0 15K 4.7K 33K 8.2K 5.6K TBD 200K AP2001
AP2001S
Anachip Corp. www.anachip.com.tw 7/11
Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter 2.6 Board Layout
Board size is 80mm(W)x50mm(L)
Figure 5. Silkscreen layer
Figure 6. Top layer
Figure 7. Bottom layer
Anachip Corp. www.anachip.com.tw 8/11
Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck 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 controller. This section will describe the AP2001 to design procedure. The operation and the design of the dual-buck converter will also be discussed in detail.
3.2 Operating Specifications
Specification Input Voltage Range Output Buck1 Voltage Range Output Buck2 Voltage Range Output Current (Buck1) Range Output Current (Buck2) Range Operating Frequency Output Ripple Efficiency 0.3 0.3 180 Min 10.8 Typ 12 5 3.3 3 3 200 50 Max 13.2 Units V V V A A KHz mV %
220 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 200 kHz was chosen. Example calculations accompany the design equations. Since this is a fixed output converter, all example calculations apply to the converter with output voltage is 3.3V/5V and input voltage set to 12 V, unless specified otherwise. The first quantity to be determined is the converter the duty cycle value. Ton Vo + Vd , = Ts Vin - Vds(sat)
Duty ratio
D=
0D1
Assuming the commutating diode forward voltage Vd = 0.5 V and the power switch on voltage Vds(sat) = 0.1V, the duty cycle for Vo = 3.3V and Vin = 10.8, 12, 13.2 is 0.36, 0.32, 0.29 respectively; The duty cycle for Vo = 5V and Vin = 10.8, 12, 13.2 is 0.51, 0.46 ,0.42 respectively. 3.3.1 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:
Anachip Corp. www.anachip.com.tw 9/11 Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter
IL = 2 x 10% x Io = 2 x 0.1 x 3 = 0.6A (For 5V and 3.3V)
The inductor "L" value is: (Vin - Vds(sat) - Vo) x Dmin IL x fs (Vin - Vds(sat) - Vo) x Dmin IL x fs (13.2 - 0.1 - 3.3) x 0.29 = 23.7H 0.6 x (200 x 10^3) (13.2 - 0.1 - 5) x 0.42 0.6 x (200 x 10^3)
L
=
For 3.3V
L
=
= 28.4H
For 5V
So we can choose 33H for output voltage "3.3V" and "5V".
3.3.2 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: 0.6 IL = = 7.5F 8 x fs x Vo 8 x (200 x 10^3) x 0.05
Cout
Assuming the capacitance is very large, the ESR needed to limit the ripple to 50 mV is: Vo = Io 0.05 = 0.083 0.6
ESR
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 470F/25V OS-Con capacitor in parallel with a ceramic to reduce ESR. 3.3.3 Selection of the power switch (MOSFET) Based on the preliminary estimate, RDS(on) should be less than 0.10 V / 3A = 33m. The CEM4435(CET) is a -30V p-channel MOSFET with RDS(on) = 35m. 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 150 ns, a 55C maximum ambient temperature, and thermal impedance RJA = 50C/W , thus: PMOSFET = (3 x 3 x 0.035 x 0.36) + [0.5 x 12 x 3 x (0.15 x 10^(-6)) x (200 x 10^3) = 0.65W
Anachip Corp. www.anachip.com.tw 10/11 Rev. A.0 Feb. 20, 2003
ANP007
Application Note AP2001 Dual Buck Converter
TJ = TA+ (RJA x PMOSFET) = 55 + (50 x 0.65) = 87.5C................................................(For 3.3V) PMOSFET = (3 x 3 x 0.035 x 0.51) + [0.5 x 12 x 3 x (0.15 x 10^(-6)) x (200 x 10^3) = 0.7W TJ = TA+ (RJA x PMOSFET) = 55 + (50 x 0.7) = 90C...................................................(For 5V) 3.3.4 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 a SMC power surface-mount package. The power dissipation is: PD = Io x Vd x (1 - Dmin) = 3 x 0.5 x (1 - 0.29) = 1.065W..........................................(For 3.3V) PD = Io x Vd x (1 - Dmin) = 3 x 0.5 x (1 - 0.42) = 0.87W............................................(For 5V) Assuming a 55C maximum ambient temperature, and thermal impedance RJA = 15C/W , thus: TJ = TA+ (RJA x PD) = 55 + (15 x 1.065) = 70.975C............................................... (For 3.3V) TJ = TA+ (RJA x PD) = 55 + (15 x 0.87) = 68.05C............................................... (For 5V) 3.3.5 Selection of the input capacitor (Cin) The RMS current rating of the input capacitor can be calculated from the following formula. The capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded. Iin(rms) = 2[D x (Io(max) + Io(min)) x (Io(max) - Io(min)) + (IL^2)/3] = [0.36 x (3 + 0.3) x (3 - 0.3) + 0.36/3] = 2 x 1.8A = 3.6A 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 input capacitor value "470uF/25V".
4. Voltage monitor by AP434
In some applications, the output voltage is concerned too high to damage the IC. To avoid the IC being damaged, comparing output voltage with a reference could monitor the output voltage. AP434 is a monolithic IC that includes one independent OP-Amp and another OP-Amp, which the non-inverting input is wired to a fixed voltage reference. AP434 data sheet provides the low cost and space saving of voltage monitoring function.
Written by Cheng-Yu Chen()
Anachip Corp. www.anachip.com.tw 11/11
Rev. A.0 Feb. 20, 2003

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