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AP2001 Dual Buck Converter 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
Contents
- 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
- Hardware
2.1 Introduction
2.2 Typical Application
2.3 Input / Output Connections
2.4 Schematic
2.5 Bill of Material
2.6 Board Layout
- 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 Selection of the output capacitor (Cout) 3.3.3 Selection of power switch (MOSFET) 3.3.4 Selection of power Rectifier (D) 3.3.5 Selection of the input capacitor (Cin) 4. Voltage monitor by AP434
AP2001 Dual Buck Converter Anachip Corp. 1. AP2001 Specification
- 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) - 2.5V voltage reference Output - 16-pin PDIP and SOP packages
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. ( Top View ) PDIP/SOP
Description
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
AP2001 Dual Buck Converter Anachip Corp. R S VREF UVLO R MAX.500KHz GND 170K 1.18V V VI Amplifier input voltage V VO Collector output voltage V Io Collector output current mA TOP Operating temperature range -20 to +85 oC TST Storage temperature range -65 to +150 oC TLEAD Lead temperature 1.6 mm(1/16 inch) from case for 10 seconds 260 oC
(UVLO), short-circuit protection (SCP), and adjustable dead time control (DTC). capacitors, and take more effort on EMI/RFI solution also. Designer could make a choice for each mode. inductor ramps down at a slope determined by the difference between the input and output voltages. Figure 1. Typical Buck Converter Topology
Figure 3. I/O Connections
Figure 4. 2_buck demo board schematic
AP2001 Dual Buck Converter Anachip Corp.
2.5 Board of Materials
No. Value Q'ty Part Reference
1 C1 C5
Don't install Philips 2 1uF
10 C2 C6 C7 C10 C11
Ceramic Chip CAP. 1uF 25V ±10% K X7R 0805 Philips 3 10nF
2 C3 C4
Ceramic Chip CAP. 10nF 25V ±10% K X7R 0805 Philips 4 1uF 1 C8 Ceramic Chip CAP. 1uF 25V ±10% K X7R 0805 Philips 5 470pF 1 C9 Ceramic Chip CAP. 470pF 25V ±10% K X7R 0805 Philips
6 B340
2 D1 D2
3 EC1 EC2 EC3
8 CON2
3 J1 J2 J3
1 L1 L2
10 PMOS_SOP8
2 Q1 Q4
P-Channel MOSFET -30V -3A↑ CET APEC CEM4435 AP4435M
11 MMBT4401
2 Q3 Q6
NPN BJT 40V 0.6A SOT-23 ROHM DIODES SST2222A MMBT4401
12 MMBT4403
2 Q2 Q5
PNP BJT -40V -0.6A SOT-23 ROHM DIODES SST2907A MMBT4403 13 470 1 R1 Chip Resistance 470 1/8W ±10% J 0805 Yageo(RL Series) 14 47K
2 R2 R11
Chip Resistance 47K 1/8W ±10% J 0805 Yageo(RL Series) 15 0
4 R3 R4 R13 R14
Chip Resistance 0 1/8W ±10% J 0805 Yageo(RL Series) 16 15K 1 R5 Chip Resistance 15K 1/8W ±10% J 0805 Yageo(RL Series) 17 4.7K 5 R6 R9 R10 R12 R18 Chip Resistance 4.7K 1/8W ±10% J 0805 Yageo(RL Series) 18 33K
4 R7 R8 R16 R17
Chip Resistance 33K 1/8W ±10% J 0805 Yageo(RL Series) 19 8.2K
1 R15
Chip Resistance 8.2K 1/8W ±10% J 0805 Yageo(RL Series) 20 5.6K
1 R19
Chip Resistance 5.6K 1/8W ±10% J 0805 Yageo(RL Series)
21 TBD
1 R20
To be define (5K ~ 50K) Yageo(RL Series) 22 200K
1 R21
Chip Resistance 22K 1/8W ±10% J 0805 Yageo(RL Series)
23 AP2001
Monolithic Dual Channel PWM Controller Anachip AP2001S
Figure 5. Silkscreen layer Figure 6. Top layer Figure 7. Bottom layer
also be discussed in detail. Table 1. Operating Specifications functions and the basic loop. A switching frequency of 200 kHz was chosen. unless specified otherwise. The first quantity to be determined is the converter the duty cycle value.
3.3.1 Selection of the buck inductor (L)
AP2001 Dual Buck Converter Anachip Corp. ΔIL = 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 L ≧ ΔIL x fs 0.6 x (200 x 10^3) = 23.7μH For 3.3V (Vin - Vds(sat) – Vo) x Dmin L ≧ ΔIL x fs 0.6 x (200 x 10^3) = 28.4μH For 5V So we can choose 33μH 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: ΔIL 0.6 Cout ≧ 8 x fs x ΔVo = 8 x (200 x 10^3) x 0.05 = 7.5μF Assuming the capacitance is very large, the ESR needed to limit the ripple to 50 mV is: ΔVo 0.05 ESR ≦ 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 470μF/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 = 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 150 ns, a 55°C maximum ambient temperature, and thermal impedance RθJA = 50°C/W , thus:
AP2001 Dual Buck Converter Anachip Corp.
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: Assuming a 55°C maximum ambient temperature, and thermal impedance RθJA = 15°C/W , thus:
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)) + (ΔIL^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(陳政佑)