ANP014

ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
Contents
1.
Features
2.
Introduction
3.
Regulator Design Procedure
4.
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.
1/9
ANP014 – App. Note 1
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
1.0
Features
◆
Small Board Size
- Entire circuit can fit in 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
- Thermal shutdown function built in and current limit level can be set by outside resistor
◆
Simple Feedback Compensation
- Lead compensation using external capacitor
◆
Immediate Implementation
- Schematic, bill-of-materials and board layout available from Anachip
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 AP1513 monolithic IC is used
to design a cost-effective and high-efficiency miniature switching buck regulator. Please refer to the
datasheet for more complete information, as pin descriptions and specifications for the AP1513 will not be
repeated here.
This demonstration board allows the designer to evaluate the performance of the AP1513 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 adjustable output voltage settings by two resistors.
Operation at different voltages and currents may be accomplished by proper component selection and
replacement.
2/9
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
3.0 Regulator Design Procedure
3.0 .1 Given Power Specification
V
V
V
V
I
I
IN (max)
IN (min)
OUT
= Maximum Input Voltage
= Minimum Input Voltage
= Regulated Output Voltage
RIPPLE
= Ripple Voltage (peak-to-peak), typical value is 0.6% 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 300 kHz)
3.0.2 Programming Output Voltage (refer to 4.0.4 Demo Board Schematic P7)
The output voltage is programmed by selection of the divider R1 and R2. The designer should use
resistors R1 and R2 with ±1% tolerance in order to obtain best accuracy of output voltage. The output
voltage can be calculated from the following formula:
Vout = 0.8 x (1 + R1 / R2)
Select a value for R2 between 0.7KΩ and 5KΩ. The lower resistor values minimize noise pickup in the
sensitive feedback pin.
3.0.3 Programming Current Limit Level (refer to 4.0.4 Demo Board Schematic P7)
Select a value for R4 to set the current limit level by using this formula:
I
LOAD
× R DS ( on ) =
I
OCSET
× ROCSET
In this application we use R4 to be the ROCSET and in the example we use 3.0K resistor, the RDS(ON) is
100mΩ and the IOCSET is 90uA, so we limit the maximum load current to 2.7A.
3/9
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
3.0.4 Inductor Selection
A. The minimum inductor
L
(min)
can be calculated from the following design formula table:
Calculation
Step-down (buck) regulator
(V OUT + V F )
Duty
T
T
L
V
IN (min)
ON
[V
OFF
(min)
V
IN (min)
(V
IN (min)
− V SAT + V F
OUT
+V F )
− V SAT − V OUT
]
− V SAT − V OUT × T ON (max)
2 × I LOAD (min)
V
V
SAT
F
= Internal switch saturation voltage of the AP1513 =
I
LOAD
× R DS ( on ) V
= 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 of
IPK . ( PK = Peak inductor or switch current = I LOAD (max) + I LOAD (min) )
I
3.0.5 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
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.
4/9
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
3.0.6 Output Rectifier Selection
A. The current rating of the output rectifier D1 must be 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
AP1513 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.
3.0.7 Input Capacitor Selection
A. The RMS current rating of the input capacitor can be calculated from the following formula table. The
capacitor manufactured by the datasheet must be checked to assure that this current rating is not
exceeded.
Calculation
Step-down (buck) regulator
δ
I
I
ΔI
I
Ton/(Ton+Toff)
I
I
PK
m
LOAD (max)
+ I LOAD (min)
LOAD (max)
− I LOAD (min)
2 × I LOAD(min)
L
δ × ⎢(I PK × I m ) +
⎡
⎣
IN ( rms )
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.
4.0 Design Example
4.0.1 Summary of Target Specifications
Input Power
Regulated Output Power
Output Ripple Voltage
Output Voltage Load Regulation
Efficiency
Switching Frequency
V
V
V
IN (max)
OUT
= +12V;
= + 5V;
RIPPLE
I
V
IN (min)
LOAD (max)
= +12V
= 2A;
I
LOAD (min)
= 0.2A
≤ 50 mV peak-to-peak
0.6% (0.2A to 2A)
87% minimum at full load
F = 300kHz ± 15 %
5/9
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
4.0.2 Calculating and Components Selection
Calculation Formula
Vout = Vref x ((R1/R2) + 1)
L
(min)
[V
≥
IN (min)
]
− V SAT − V OUT × T ON (max)
2 × I LOAD (min)
Select Condition
0.7KΩ ≤ R2 ≤ 5KΩ
Component spec.
R2 = 1.3KΩ; R1 = 6.8KΩ
≥ 24uH
I rms ≤ I PK = 2.2A
Select L1 = 33uH
ESR ≤ 125mΩ
Select C4:
470uF/10V*1pcs
L
(min)
I PK = I LOAD (max) + I LOAD (min)
⎞
⎛
ESR = ⎜ V RIPPLE ⎟
⎟
⎜ 2× I
LOAD (min) ⎠
⎝
V WVDC ≥ 1.5 ×V OUT
V
RRM
≥ 1.25 ×V IN (max)
1
2⎤
⎡
I IN ( rms ) = δ × ⎢⎣(I PK × I m ) + 3 (Δ I L ) ⎥⎦
V WVDC ≥ 1.5 ×V IN (max)
I
LOAD
× R DS ( on ) =
I
OCSET
× ROCSET
V
WVDC
V
RRM
I
ripple
V
≥ 7.5V
Select D1:
20V/2A
≥ 15V
≥
WVDC
I
IN ( rms )
= 1.293A Select C2:
680uF/25V*1pcs
≥ 18V
2 A × 100mΩ = 90uA × R OCSET
Rocset ≥ 2.3k
Select:
R4 = 3.0K
6/9
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
4.0.3 Parts List (Board of Materials)
Item
Part Number
MFG/Dist.
Description
Value
Quantity
C1
0805 cap (optional)
Viking
Ceramic Capacitor
1nF, 25V
1
OST
Aluminum Electrolytic
470uF, 25V
1
Viking
Ceramic Capacitor
0.1uF, 25V
1
OST
Aluminum Electrolytic
470uF, 10V
1
Viking
Ceramic Capacitor
0.1uF, 25V
1
C2
C3
0805 cap
C4
C6
0805 cap
C7
Optional
C8
0805 cap
Viking
Ceramic Capacitor
0.1uF, 25V
1
C9
0805 cap
Viking
Ceramic Capacitor
0.1uF, 25V
1
D1
B220
Schottky Diode
20V, 2A
1
WE
Inductor
33uH, 2A
1
L1
U1
AP1513
Anachip
PWM Buck Converter
300kHz, 2A
1
R1
0805 reg
Viking
Film Chip Resistor
6.8KΩ
1
R2
0805 reg
Viking
Film Chip Resistor
1.3KΩ
1
R4
0805 reg
Viking
Film Chip Resistor
3KΩ
1
R5
0805 reg
Viking
Film Chip Resistor
100KΩ
1
4.0.4 Demo Board Schematic
U1
VIN+12V
ON/OFF
R5
100K
C2
C3
R4
470uF
0.1uF
3K
C7
Option
1
2
3
C6
0.1uF
4
AP1513
FB
VSS
EN
VSS
OCSET
Output
VCC
Output
8
7
6
1
5
D1
B220
C9
0.1uF
L1 33uH
VOUT = 5V/2A
2
R1
6.8K
C1
Option
C8
0.1uF
C4
470uF
R2
1.3K
7/9
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
4.0.5
Demo Board Efficiency and Temperature
Vin (V)
Iin (A)
Vout (V)
Iout (A)
Efficiency (%)
12.08
0.165
3.37
0.5
84.54
12.03
0.323
3.37
1
86.73
12.06
0.644
3.37
2
86.78
Vin (V)
Iin (A)
Vout (V)
Iout (A)
Efficiency (%)
12.09
0.245
5.33
0.5
89.97
12.01
0.483
5.33
1
91.88
12.07
0.962
5.32
2
91.63
Vin (V)
Iin (A)
Vout (V)
Iout (A)
Efficiency (%)
5.00
0.361
3.320
0.5
91.97%
5.00
0.722
3.318
1.0
91.90%
5.00
1.463
3.313
2.0
90.51%
AP1513 Temperature vs. Efficiency
Temperature (°C)
Parameter
-20
0
25
50
85
Vin (V)
12.03
12.06
12.07
12.14
12.16
Iin (A)
0.334
0.327
0.323
0.320
0.318
Vout (V)
3.43
3.41
3.39
3.37
3.34
Iout (A)
1
1
1
1
1
Efficiency (%)
85.37
86.47
86.95
86.75
86.37
8/9
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ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
4.0.6 Typical PC Board Layout (refer to ANP016)
(1). Top Side Layout Guide
Use vias to conduct the heat into the backside of PCB layer .
The PCB heat sink copper area should be solder-painted
(2). Bottom Side Layout Guide
without being masked. This approaches a “best case” pad heat
sink.
Written by Maverick Huang
9/9
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May 2006
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