ANP004

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
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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
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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
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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
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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
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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
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2.4 Schematic
Figure 4. Demo Board Schematic
7/15
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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
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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
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AP2001 Buck + Boost Converter
2.6 Board Layout (continued)
Figure 6. Top Layer
Figure 7. Bottom Layer
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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
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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
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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
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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
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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
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