ANP019

ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
Contents
1.
AP1604 Specifications
1.1 Features
1.2 General Descriptions
1.3 Pin Assignments
1.4 Pin Descriptions
1.5 Block Diagram
1.6 Absolute Maximum Ratings
2. Design Procedures
2.1 Parameter Statement
2.2 Programming Output Voltage
2.3 Inductor Selection
2.4 Output Capacitor Selection
2.5 Compensation Capacitor Selection
2.6 Output Rectifier Selection
2.7 Input Capacitor Selection
3.
Design Examples
3.1 Summary of Target Specifications
3.2 Calculating and Components Selection
3.3 Demo Board Efficiency Calculation
4.
Hardware
4.1 Introduction
4.2 Demo Board Schematic
4.3 Board of Materials
4.4 Board Layout
4.5 PC Board Layout Guide Line
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.
1/13
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© Diodes Incorporated
ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
1. AP1604 Specifications
1.1
Features
- Input Voltage Range: 2.2V~5.5V(VOUT type)
- Oscillator Frequency: 600kHz(Typ.)
- Internal Reference: 1.0V (Typ.)
- High Efficiency: 93%(Typ.)
- Stand-by Capability: ISTB = 2µA (Typ.)
- Soft-start Time Set-up External Type Possible
- Current Limit and Thermal Shutdown Protection
- Pb-Free Package: SOT23-5
1.2
General Descriptions
The AP1604 series are multi-functional step-down DC/DC converters with built-in speed, low ON
resistance drivers. It is possible to deliver more than 800mA output current with external connecting coil,
diode and capacitor.
Output voltage is set-up by external resistors(±2.5% accuracy). AP1604 with 600kHz switching
frequency can work out with smaller value external component that produces a more compact board. The
device switches to and works under PFM mode with light loads. It remains at high efficiency for both light
loads and large output currents. There is a soft-start capability by connecting a proper external capacitor.
The stand-by current is lower than 2uA when the input voltage is below the stipulated voltage (CE/SS pin
is “LOW”) and the device is forced to switch off.
1.3
Pin Assignments
1.4
Pin Descriptions
(Top View)
CE/SS
4
FB
5
AP1604A
1
VOUT
2
GND
3
VCC
Pin Name
VOUT
Vcc
GND
CE/SS
FB
Function
Output Voltage
Input Supply
Ground
Chip Enable / Soft Start
Feedback Pin
SOT23-5
2/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
1.5
Block Diagram
FB
VDD
Phase
Compensation
Current Limit
+
-
+
ERR
AMP
-
Soft
Start
1.6
PWM
Comparator
Ramp Wave
Generator,
OSC
PWM/PFM
Controller
Vref with
CE
CE/SS
Vout
GND
Absolute Maximum Ratings
Symbol
Parameter
Ratings
Units
-03 ~ 6.5
V
VCC
VIN Pin Voltage
VOUT
VOUT Pin Voltage
-0.3 ~ VIN+0.3
V
VFB
FB Pin Voltage
-0.3 ~ VIN+0.3
V
VCE/SS
CE/SS Pin Voltage
-0.3 ~ VIN+0.3
V
Pd
Continuous Total
Power Dissipation
Internal limited
mW
Topr
Operating Ambient
Temperature
-25 ~ +80
°C
Tstg
Storage Temperature
-40 ~ +125
°C
3/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
2.
Design Procedure (refer to the “Demo Board Schematic”)
2.1
Parameter Statement
V
V
V
V
I
I
IN (max)
IN (min)
= Maximum Input Voltage
= Minimum Input Voltage
= Converter Output Voltage
OUT
RIPPLE
= 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
2.2
= Switching Frequency (fixed at a nominal 600KHz)
Programming Output Voltage
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 the Output
Voltage. The Output Voltage can be calculated from the following formula: Vout = 1.0 x (1 + R1 / R2),
select a value for R2 between 100K and 200KΩ. The higher resistor values minimize leakage current
pickup in the feedback pin.
2.3
Inductor Selection
A. The minimum inductor
L
(min)
Calculation
T
T
L
can be calculated from the following design formula table:
Step-down (buck) Converter
(V OUT + V F )
ON
OFF
[V
V
IN (min)
IN (min)
− V SAT − V OUT
]
− V SAT − V OUT × T ON (max)
2 × I LOAD (min)
(min)
V
= Internal Driver dropout Voltage of the AP1604 = ILoad * 350mΩ
SAT
V
F
= Forward voltage drop of output rectifier D1 = 0.4V
B. The inductor must be designed so that it does not saturate or significantly saturate at a DC current
bias of
.
PK
I
I
PK
= Peak inductor or switch current =
I
LOAD (max)
+ I LOAD (min)
4/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
2.4
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 ⎟
⎟
⎜ 2× I
LOAD (min) ⎠
⎝
B.
The ESR of the output capacitor puts a zero in the loop gain which can be used to reduce
excess negative phase shift. The phase margin can be system stabilized. If the phase margin is
less than 30°, the loop will either oscillate or ring severely. The effects of low and high ESR on
phase margin can be illustrated using the following example:
Phase Shift ( ° )
Loop Gain (dB)
We choose the 22uF output solid tantalum capacitor. The universal usolid tantalum
capacitors ESR is 0.3Ω @ 25°C (100kHz), a value that is almost perfectly centered in the stable
region (Figure 1). This system corresponds to a phase margin of 74°, which is extremely stable.
Frequency (kHz)
Figure 1. Usolid Tantalum Capacitor of ESR Causes Loop
5/13
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Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
Phase shift ( ° )
Loop Gain (dB)
We change conditions and assume the ESR of the 22uF output capacitor is increased to
10Ω. This appears to leave a phase margin of 62° (which is stable) (Figure 2), when ESR keeps
increasing, the phase margin can shift more and causes the system to be unstable.
Frequency (kHz)
Figure 2. High ESR Causes Unstable Loop
Phase shift ( ° )
Loop Gain (dB)
An output capacitor with a very low ESR value can cause the system to be unstable.
Generally, a multi-layer Ceramic Capacitor (MLCC) has very low ESR values (<20mΩ).
Continuing the example developed in the previous section, we will reduce the ESR of the 22uF
Output Capacitor to 10mΩ (Figure 3), the phase margin can be shifted to 2° and it is unstable. In
case of using MLCC capacitors, a compensation circuit is required for improving stability. The
compensation circuit will add a system zero pole and improve the phase margin, so the system
will be extremely stable (refer to 2.5 compensation capacitor selection).
Frequency (KHz)
Figure 3. Low ESR Causes Unstable Loop
C.
When selecting an output capacitor for AP1604, a solid tantalum capacitor is usually the best
choice. It is extremely stable on AP1604.
6/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
2.5
Compensation Capacitor Selection
The Compensation Capacitors for increasing the phase margin provide additional stability. It is
required if the output capacitor is MLCC, but not for solid tantalum, because the phase margin is perfect
(about 70°) on this condition (Figure 4). Refer to the MLCC model circuit in 4.0.4 Demo Board Schematic,
a 47pF capacitor C4 in parallel with the in series RB and CB are added in between Vout and FB for
compensation purposes. The optimum values for CB and RB are 47pF and 500K~1MΩ, respectively.
Figure 4. The addition of RB and CB Compensation Makes Loop stable
2.6
Output Rectifier Selection
A.
The current of output rectifier D1 must be greater than the peak switch current IPK. The
reverse voltage of the output rectifier D1 should be at least 1.25 times of the maximum input
voltage.
B.
The output rectifier D1 must be fast (short reverse recovery time) and is located close to the
AP1604 using short leads and short printed circuit traces. Because of the fast switching speed
and low forward voltage drop, Schottky diodes provide the best performance and efficiency. It
should be the first choice, especially in low output voltage applications.
7/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
2.7
Input Capacitor Selection
A. The RMS current rating of the input capacitor can be calculated from the following formula table.
The capacitor manufacturer’s datasheet must be checked to assure that this current rating is not
exceeded.
Calculation
Step-down (buck) Converter
Ton/(Ton+Toff)
I LOAD (max) + I LOAD (min)
δ
I
I
ΔI
PK
I
m
I
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 of the maximum input voltage.
3.
Design Example
3.1
Summary of Target Specifications
Input Power
Converter Output Power
Output Ripple Voltage
Efficiency
Switching Frequency
V
V
V
IN (max)
OUT
= +5.5V;
= +2V;
RIPPLE
I
V
IN (min)
LOAD (max)
= +2.5V
= 1A;
I
LOAD (min)
= 0.1A
≤ 50 mV peak-to-peak
85% minimum at full load.
f = 600kHz ± 15 %
8/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
3.2
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)
I
PK
=
I
LOAD (max)
+ I LOAD (min)
⎞
⎛
ESR = ⎜ V RIPPLE ⎟
⎟
⎜ 2× I
LOAD (min) ⎠
⎝
V WVDC ≥ 1.5 ×V OUT
I
I
IN ( rms )
V
RRM
PK
=
Select Condition
100KΩ ≤ R2 ≤ 200KΩ
I
(min)
rms
≥
≥ 6.6UH
I
PK
= 1.1A
200 mΩ ≤ ESR ≤ 10Ω
V
≥ 1.25 ×V IN (max)
V
I LOAD (max) + I LOAD (min)
1
2⎤
⎡
= δ × ⎢(I PK × I m ) + (Δ I L ) ⎥
3
⎣
⎦
V WVDC ≥ 1.5 ×V IN (max)
L
ripple
V
PK
≥
≥ 3V
≥ 6.875V
RRM
I
I
WVDC
= 1.1A
I
WVDC
IN ( rms )
= 1A
≥ 8.25V
Component spec.
R1 = 100KΩ; R2 = 100KΩ
Select L = 10uH / 1.2A
Select C5 from "Viking Tech"
68uF/6.3V*1pcs
Select D1 = 40V/2A
Select C1 from "Viking Tech"
68uF/16V*1pcs
9/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
3.3
4.
Demo Board Efficiency Calculation
Iout (mA) Efficiency (%)
Vcc (V)
Icc (mA)
Vout (V)
3.301
6.6
1.810
10
83.06
3.306
60.9
1.807
100
89.73
3.305
122.3
1.807
200
89.39
3.303
186.2
1.808
300
88.17
3.302
254
1.809
400
86.28
3.304
322
1.810
500
85.07
3.3
395
1.810
600
83.31
3.303
469
1.810
700
81.79
5.001
7.74
3.183
10
82.22
5.002
69
3.168
100
91.77
4.999
136
3.154
200
92.78
5.005
204
3.156
300
92.73
5.001
275
3.160
400
91.89
5.005
347
3.164
500
91.08
5.001
420
3.166
600
90.44
5.007
495
3.166
700
89.42
Hardware
4.1
Introduction
This application note discusses simple ways to select all necessary components to implement a
step-down (BUCK) DC/DC Converter and gives a design example. In this example, the AP1604
monolithic IC is used to design a cost-effective and high-efficiency miniature switching buck converter.
For more complete information, pin descriptions and specifications for the AP1604 will not be repeated
here, please refer to the datasheet when designing or evaluating with the AP1604.
This demonstration board allows the designer to evaluate the performance of the AP1604 series
buck converter in a typical application circuit. The user needs only to supply an input voltage and a load.
Operation at other voltages and currents may be accomplished by proper component selection and
replacement.
10/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
4.2
Demo Board Schematic
(1) Generality
U1
3
C2
0.1u
4
R3
1M
Vout
CE
FB
1
1
2
Vout
10uH
5
D1
B240A
AP1604
L1
R1
100K
2
C1
68u/16V
VDD
GND
VCC
C3
0.1u
C6
0.1u
R2
100K
C5
68u/6.3V
Vout=1.0*(1+R1/R2)
R2 = 100K~200K
(2) MLCC model
U1
3
VCC
4
C1
R3
1M
47uF/MLCC
C3
L1
VDD
CE
AP1604
2
Vout
D
N
G
FB
1
1
5
RB
1M
CB
47pF
1nF
2
Vout
10uH
D1
CDBM140L
R1
300K
R2
130K
C4
47pF
C5
22uF/MLCC
Vout=1.0*(1+R1/R2)
R2 = 100K~200K
11/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
4.3
Board of Materials
Item
Value
Q’ty
Description
MFG/Dist.
C1
68uF, 16V
47uF, 10V
0.1uF, 25V
0.1uF, 25V
1nF, 25V
47pF, 25V
68uF, 6.3V
22uF, 10V
0.1uF, 25V
47pF, 25V
40V, 2A
40V, 1A
1
1
1
Solid Tantalum Capacitor
MLCC
0805 Ceramic SMD Capacitor
Viking Tech
Viking Tech
Viking Tech
1
0805 Ceramic SMD Capacitor
Viking Tech
1
1
1
1
1
1
1
0805 Ceramic SMD Capacitor
Solid Tantalum Capacitor
MLCC
0805 Ceramic SMD Capacitor
0805 Ceramic SMD Capacitor
Viking Tech
Viking Tech
Viking Tech
Viking Tech
Viking Tech
C2
C3
C4
C5
C6
CB
D1
Schottky Diode
L1
10uH, 1.3A
1
SMD Inductance
Wurth Elektronik
U1
600kHz, 1A
100KΩ
300KΩ
100KΩ
130KΩ
1MΩ
1MΩ
1
Step-down DC/DC Converter
Anachip
1
1% 0805 SMD Resistor
Viking Tech
1
1% 0805 SMD Resistor
Viking Tech
1
1
1% 0805 SMD Resistor
1% 0805 SMD Resistor
Viking Tech
Viking Tech
R1
R2
R3
RB
Part
Number
B240A
CDBM140L
WE-TPC
744062100
AP1604
12/13
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ANP019
Application Note
AP1604 600KHz,1A PWM/PFM Dual Mode Step-down DC/DC Converter
4.4
PC Board Layout
(1) Top View
→ General Size (36*27 mm) ←
→ Small Size (23*18 mm)←
(2) Bottom View
→ General Size (36*27 mm) ←
4.5
→ Small Size (23*18 mm)←
PC Board Layout Guide Line
CMOS IC is sensitive to external noise. So the Component selection and PC Board Layout are
more important. It is most important that Input Capacitors must be close to IC, as it can reduce ripple
noise that affects IC stability. The power GND must connect directly to the input capacitor GND, to allow
the system to work in a more stable manner.
13/13
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