ANP022

ANP022
Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
Contents
1.
Features
2.
Introduction
3.
Pin Functions
4.
Internal Block Diagram
5.
Regulator Design Procedure
6.
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.
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ANP022
Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
1.0
Features
◆
Small Board Size
- Entire circuit can fit on 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
- Built-in function
◆
Simple Feedback Compensation
- Lead compensation using external capacitor
◆
Immediate Implementation
- Schematic, bill-of-materials and board layout available from DIODES
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 AP1512/A monolithic IC is
used to design a cost-effective and high-efficiency miniature switching buck regulator. For more complete
information, pin descriptions and specifications for the AP1512/A will not be repeated here. Please refer to the
datasheet when designing or evaluating with the AP1512/A.
This demonstration board allows the designer to evaluate the performance of the AP1512/A 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 a fixed output voltage of 3.3V, 5V, 12V, and an adjustable
output version of the AP1512/A series. Operation at other voltages and currents may be accomplished by
proper component selection and replacement.
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Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
3.0
Pin Functions
Number
Name
Function
1
2
3
4
5
+VIN
Output
GND
FB
SD
Operating Voltage Input
Switching Output
Ground
Output Feedback Control
ON/OFF Shutdown Control
+VIN (Pin 1):
This pin is the main power input to the IC. The range of operating voltage is from +4.5V to
+60V. A suitable input bypass capacitor must be present at this pin to minimize voltage
transients and to supply the switching current’s needs by the regulator.
Output (Pin 2):
Internal switch. The voltage at this pin switches between ( + V IN − VSAT ) and approximately
-0.55 V, with a duty cycle of approximately VOUT V IN . To minimize coupling to sensitive
circuitry, the PC board copper area connected to this pin should be kept at a minimum.
GND (Pin 3):
Circuit ground for the IC.
FB (Pin 4):
Senses the regulated output voltage to complete the feedback loop.
SD (Pin 5):
Allows the switching regulator circuit to be shutdown using logic level signals thus dropping
the total input supply current to approximately 350uA. Pulling this pin below a threshold voltage
of approximately 1.3V turns the regulator on, and pulling this pin above 1.3V (up to a maximum
of 40V) shuts the regulator down. If this shutdown feature is not needed, the SD pin can be
wired to the ground pin or it can be left open, in either case the regulator will be in the ON
condition.
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Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
4.0
Internal Block Diagram
SD
V IN
200mV
Current
Source
bias
2.5V
Regulator
1.235V
Reference
220mV
Start
up
+
Comp
_
_
Comp
+
FB
Frequency
compensation
_
+
Amp
_
Comp
Pre-driver
+
2A
Switch
Output
50KHz
OSC.
5.0
Thermal
Limit
Gnd
Regulator Design Procedure
5.0 .1 Given Power Specification
V
V
V
V
I
I
IN (max)
IN (min)
OUT
RIPPLE
= Maximum Input Voltage
= Minimum Input Voltage
= Regulated Output Voltage
= Ripple Voltage (peak-to-peak), typical value is 1% 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 50KHz)
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AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
5.0.2 Programming Output Voltage
The output voltage is programmed by selection of the divider R2 and R3. The designer should use
resistors R2 and R3 with ±1% tolerance in order to obtain the best accuracy of the output voltage. The output
voltage can be calculated from the following formula:
Vout = 1.23 x (1 + R2 / R3)
Select a value for R3 between 240Ω and 1.5KΩ. The lower resistor values minimize noise pickup in the
sensitive feedback pin.
If the designer selects a fixed output version of the AP1512/A, the resistor R2 shall be short and R3 shall
be open.
5.0.3 Inductor Selection
A.
The minimum inductor
L
(min)
can be calculated from the following design formula table:
Calculation
T
T
L
Step-down (buck) regulator
(V
ON
OFF
[V
(min)
V
IN (min)
IN (min)
OUT
+V F )
− V SAT − V OUT
]
− V SAT − V OUT × T ON (max)
2 × I LOAD (min)
V
SAT
V
of
I
= Internal switch saturation voltage of the AP1512/A = 1.3V
F
= 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
. ( PK = Peak inductor or switch current =
+ LOAD (min) )
LOAD (max)
PK
I
I
I
5.0.4 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
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ANP022
Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
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.
5.0.5 Compensation Capacitor Selection
For a stable application circuit, an additional capacitor C5 is required. The compensation capacitor C5
provides additional stability for high output voltages, low input-output voltages, and/or very low ESR output
capacitors.
5.0.6 Output Rectifier Selection
A.
The output rectifier D1 current rating must be at least 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
AP1512/A 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.
5.0.7 Input Capacitor Selection
A. The RMS current rating of the input capacitor can be calculated from the following formula table.
The capacitor manufacturer’s data sheet must be checked to assure that this current rating is not exceeded.
Calculation
δ
I
I
ΔI
I
Step-down (buck) regulator
Ton/(Ton+Toff)
I
I
PK
m
LOAD (max)
+ I LOAD (min)
LOAD (max)
− I LOAD (min)
2 × I LOAD(min)
L
IN ( rms )
δ × ⎢(I PK × I m ) +
⎡
⎣
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.
6.0 Design Example
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AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
6.0.1 Summary of Target Specifications
Input Power
V
V
V
Regulated Output Power
Output Ripple Voltage
Output Voltage Load Regulation
Efficiency
Switching Frequency
IN (max)
OUT
I
= + 5V;
RIPPLE
V
= +12V;
IN (min)
LOAD (max)
= +12V
= 2A;
I
LOAD (min)
= 0.6A
≤ 50 mV peak-to-peak
1% (1/2 full load to full load)
75% minimum at full load
F = 50KHz ± 15 %
6.0.2 Calculating and Components Selection
Calculation Formula
Vout = Vref x ((R2/R3) + 1)
L
I
(min)
PK
=
[V
≥
I
IN (min)
LOAD (max)
]
− V SAT − V OUT × T ON (max)
2 × I LOAD (min)
+ I LOAD (min)
⎞
⎛
ESR = ⎜ V RIPPLE ⎟
⎟
⎜ 2× I
LOAD (min) ⎠
⎝
V WVDC ≥ 1.5 ×V OUT
V
I
RRM
PK
=
≥ 1.25 ×V IN (max)
I
LOAD (max)
+ I LOAD (min)
1
2⎤
⎡
I IN ( rms ) = δ × ⎢⎣(I PK × I m ) + 3 (Δ I L ) ⎥⎦
V WVDC ≥ 1.5 ×V IN (max)
Select Condition
240Ω ≤ R3 ≤ 1.5KΩ
L
I
≥ 47uH
(min)
rms
≥
I
PK
Select = 47uH/3A
= 2.6A
ESR ≤ 62.5mΩ
V
V
I
I
WVDC
RRM
PK
V
Select C3 from "JACKCON"
470uF/16V*1pcs
≥ 7.5V
≥ 15V
Select D1:SR360
60V/3A
= 2.6A
ripple
≥
WVDC
I
Component spec.
R2 = 3KΩ; R3 = 1KΩ
IN ( rms )
= 1.74A
Select C1 from "ELCON"
110uF/100V*1pcs
≥ 18V
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Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
6.0.3 Parts List (Board of Materials)
Item
Description
Value
Quantity
C1
Part Number
MFG/Dist.
Aluminum Electrolytic
110uF, 100V
1
C2
Ceramic Capacitor
1uF, 25V
1
C3
Aluminum Electrolytic
470uF, 16V
1
C5
Ceramic Capacitor
1nF, 25V
1
Ceramic Capacitor
0.1uF, 25V
1
Schottky Rectifier
60V, 3A
1
Inductor
47 uH, 3.8A
1
PWM Buck Converter
50KHz, 2A
1
C7
D1
B360A/B360B/B360
PDS360
L1
7447709470
U1
AP1512/AD
R2
Std
Film Chip Resistor
3KΩ±5%, 1/8W
1
R3
Std
Film Chip Resistor
1KΩ±5%, 1/8W
1
DIODES
Wurth
Electronik
DIODES
6.0.4 Demo Board Schematic
VIN
VOUT
ON/OFF
C1
C2
100uF/100V
1u
SD
2
1
D1 47uH
GND
5
Vin OUTPUT
3
1
L1 7447709470
2
FB
4
R2
3K
B360A /
B360B /
B360 /
PDS360
AP1512
C5
1n
C7
C3
0.1u
470uF/16V
R3
1K
6.0.5 Demo Board Efficiency at Vin 60V
Load
Vout = 5V
1A
70.83%
2A
76.84%
3A
74.25%
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Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
6.0.6 Typical PC Board Layout
(1). Component Placement Guide
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Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
(2). Component Side PC Board Layout
(3). Solder Side PC Board Layout
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Application Note
AP1512/A 50KHz, 2A/3A PWM Buck DC/DC Converter
6.0.7 Heatsink Layout Guide Line
The heatsink is dependent on the maximum power dissipation and maximum ambient temperature of the
application.
For example:
12V to 5V/2A
Top Copper
Layout
Area (mm*mm)
2
15*10
Bottom Copper
Area (mm*mm)
12*20
Through
Hole
1mm*48
IC Body
Temperature
67
Gnd Pin
Temperature
75
12V to 3.3V/2A
Top Copper
Layout
Area (mm*mm)
2
15*10
Bottom Copper
Area (mm*mm)
12*20
Through
Hole
1mm*48
IC Body
Temperature
61
Gnd Pin
Temperature
68
60V to 5V/2A
Top Copper
Layout
Area (mm*mm)
2
15*10
Bottom Copper
Area (mm*mm)
12*20
Through
Hole
1mm*48
IC Body
Temperature
84
Gnd Pin
Temperature
90
60V to 3.3V/2A
Top Copper
Layout
Area (mm*mm)
2
15*10
Bottom Copper
Area (mm*mm)
12*20
Through
Hole
1mm*48
IC Body
Temperature
78
Gnd Pin
Temperature
84
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