ANP012

ANP012
Application Note
AP2004 Buck Controller
Contents
1.
AP2004 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
2.2
2.3
2.4
2.5
3.
Introduction
Typical Application
Schematic
Board of Materials
Board Layout
Design Procedures
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)
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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ANP012
Application Note
AP2004 Buck Controller
1. AP2004 Specifications
1.1 Features
- Operating Voltage can be up to 27V
- Under Voltage Lockout (UVLO) Protection
- Short Circuit Protection (SCP)
- Soft-Start Circuit
- Variable Oscillator Frequency --- 300kHz Max
- 1.25V Voltage Reference Output
- 8-pin PDIP and SOP packages
1.2 General Description
The AP2004 integrates Pulse-Width-Modulation (PWM) control circuit into a single chip, mainly designed
for power-supply regulator. All the functions include an on-chip 1.25V reference output, an adjustable oscillator,
UVLO, SCP, soft-start circuitry, and a push-pull output circuit. Switching frequency is adjustable by trimming
the 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 )
OUT
1
8
VCC
COMP
FB
2
7
3
6
4
5
GND
CT
SS
SCP
PDIP/SOP
1.4 Pin Descriptions
Name
Description
CT
Timing Capacitor
FB
Voltage Feedback
SS
Soft-Start
COMP
Feedback Loop Compensation
OUT
PWM Output
GND
Ground
VCC
Supply Voltage
SCP
Short Circuit Protection
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AP2004 Buck Controller
1.5 Block Diagram
VCC
1.25V
Bandgap
Reference
CT
SS
2.5V
Internal use
Oscillator
UVLO
VCC
MAX.500KH
z
Iss
PWM Amplifier
FB
+
+
-
OUT
Error Amplifier
COMP
1.5V
0.7V
+
+
SCP
GND
1.6 Absolute Maximum Ratings
Symbol
Rating
Unit
Supply Voltage
27
V
VI
Amplifier Input Voltage
20
V
VO
Collector Output Voltage
VCC-1.0V
V
Source Current
200
ISINK
Sink Current
200
mA
mA
TOP
Operating Temperature Range
VCC
ISOURCE
TST
TLEAD
Parameter
Storage Temperature Range
Lead Temperature 1.6 mm (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 demo board supplies a constant DC output voltage of 3.3V, and supplies the output power up to 10W
(3.3V / 3A). Using a DC input voltage of 12V, 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 a regulated output voltage
that is always lower than the input voltage. The control method used in the board is a fixed frequency, variable
on-time pulse-width-modulation (PWM). The feedback method is used voltage-mode control. Other features of
the board include Under-Voltage Lockout (UVLO), Short-Circuit Protection (SCP), and Soft-Start.
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AP2004 Buck Controller
2.2 Typical Application
The AP2004 may operate in either the CCM (Continuous Conduction Mode) or the DCM (Discontinuous
Conduction Mode). The following applications are designed for CCM 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 a power switch and output
diode with larger current and power dissipation ratings should solve this issue of large peak currents. The
designer has to use larger output capacitors, and take more effort on EMI/RFI solution too. The designer could
make a choice for each mode. For a light load, DCM is preferred for a buck frame, but for a heavy load, CCM is
preferred.
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. The current in the inductor ramps down at a slope determined by the difference between
the input and output voltages.
iS
VS
iL
SW
Vi
L
iD
VD
IO
VL
iC
C
D
RL
VO
Figure 1. Typical Buck Converter Topology
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AP2004 Buck Controller
2.3
Schematic
Q1
D
S
1
8
2
7
3
6
G
PMOS
4
5
4435
J1
U1
1
Vin
2
Scp
2
5
ss
CON2
6
+
7
C2
0.1u
ct
C1
1000uF
C6
C7
220nF 50nF
VCC
SCP
SS
CT
OUT
FB
COMP
GND
1
out
R1
short
4
3
8
comp
Switch
C3
Short
R2
56K
AP2004
L1
33uH
Vout
1
J2
2
D1
5A
R3
5.6K
C8
330p
C9
CON2
+
NA
R4
3.3K
C5
0.1uF
C4
1000uF
Figure 2. Demo board schematic
2.4 Board of Materials
Part Reference
Value
Description
Manufactu Part
Q'ty
rers
Number
U1
Q1
D1
R1
L1
R2
R3
R4
C1
AP2004
30V, 8.8A
5A, 40V
0Ω ±5%
33uH, 3A
56KΩ ±1%
5.6KΩ ±1%
3.3KΩ ±1%
1000uF, 25V
PWM Buck Controller
PMOS
Schottky Diode
Resister 0805
Inductor
Resister 0805
Resister 0805
Resister 0805
Aluminum electrolytic
Anachip
Fairchild
C2, C5
C3
C4
0.1uF, ±10%
Short
1000uF, 16V
Ceramic, 50V, 0805 X7R
Aluminum electrolytic
C6
C7
C8
C9
J1, J2
220nF, ±10%
50nF, ±10%
330pF, ±10%
NA
Pitch = 5.08mm, 3pin
Ceramic, 50V, 0805 X7R
Ceramic, 50V, 0805 X7R
Ceramic, 50V, 0805 X7R
Option
Terminal Block
AP2004S
SI4435DY
B540A
AXIS Power
OST
OST
OST RLX
series
OST RLX
series
1
1
1
1
1
1
1
1
1
2
1
1
1
1
1
2
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AP2004 Buck Controller
2.5 Board Layout
Figure 3. Silkscreen Layer
Figure 4. Top Layer
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AP2004 Buck Controller
Figure 5. Bottom Layer
3. Design Procedure
3.1 Introduction
The AP2004 integrated circuit is a PWM controller, it operates over a wide input voltage range. This
section will describe the AP2004 to design procedure. The operation and the design of the Buck converter
will also be discussed in detail.
3.2 Operating Specifications
Specifications
Min.
Typ.
Max.
Units
Input Voltage Range
11.4
12
12.6
V
Output Voltage Range
3.3
V
Output Power Range
0
7
10
W
Output Current Range
0
2
3
A
Operating Frequency
194
215
263
kHz
Output Ripple
Efficiency
50
82.34
84.9
mV
84.6
%
Table 1. Operating Specifications
3.3 Design Procedures
This section describes the steps to design continuous-mode Buck 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 215 kHz was chosen.
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AP2004 Buck Controller
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 output voltage of 3.3V and input voltage set to 12 V,
unless specified otherwise. The first quantity to be determined is the duty cycle value.
Duty cycle =
VIN
Vo + VD
T
= ON , 0 ≤ D ≤ 1
TS
− ( I O × RDS ( ON ) ) + VD
Assuming the commutating diode forward voltage VD = 0.5 V, and the P-MOS RDS (ON ) = 30mΩ
When VIN = 12V , I O = 0.3~3A , and the duty cycle is equal to 0.32.
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 value “L” is:
L≥
[ VIN − ( I O × RDS (ON ) ) − VO ]× D [12 − (0.3 × 0.03 ) − 3.3]× 0.32
=
ΔI L × f S
0.6 × 215 × 103
= 22μH
So we can choose 33μH.
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 ΔV o
0.6
3
8 x (215 x 10 ) x 0.05
= 7μF
Assuming the capacitance is very large, the ESR needed to limit the ripple to 50 mV is:
ESR ≤
ΔV o
=
ΔI o
0.05
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 1000μF/16V OS-Con capacitor in
parallel with a ceramic to reduce ESR.
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AP2004 Buck Controller
3.3.1.3 Selection of the power switch (MOSFET)
Based on the preliminary estimate, the RDS(on) of MOSFET is 30mΩ. The SI4435DY 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 20 ns, a 55°C maximum ambient temperature, and
thermal impedance RθJA = 50°C/W, thus:
PMOSFET = (3 x 3 x 0.035 x 0.32) + [0.5 x 12 x 3 x (20 x 10
−9
3
) x (215 x 10 ) = 1.0854W
TJ = TA+ (RθJA x PMOSFET) = 55 + (50 x 1.0854) = 109.27°C
3.3.1.4 Selection of the Rectifier (D)
The catch rectifier conducts during the time interval when the MOSFET is off. The
B540A(DIODES) is a 5A, 40V Schottky Rectifier in a SOP-8 package. The power dissipation is:
PD = Io x Vd x (1 – Dmin) = 3 x 0.5 x (1 – 0.32) = 1.02W
Assuming a 55°C maximum ambient temperature, and thermal impedance RθJA = 15°C/W, thus:
TJ = TA+ (RθJA x PD) = 55 + (15 x 1.02) = 70.3°C
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 manufacturer’s datasheet must be checked to assure that this current rating is not
exceeded.
2
Iin(rms) = √ [D x (Io(max) + Io(min)) x (Io(max) - Io(min)) + (ΔIL )/3] = √[0.32 x (3 + 0.3) x (3 – 0.3) +
0.36/3] = 1.72A
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 “1000uF/25V”.
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