AN1063

Application Note 1063
Design Consideration with AP3211
Prepared by Ji Jin
System Engineering Dept.
1. Introduction
2. Function Block Description
The AP3211 is a 1.4MHz fixed frequency, current
mode, PWM buck (step-down) DC-DC converter,
capable of driving a 1.5A load with high efficiency,
excellent line and load regulation. The device
integrates N-channel power MOSFET switch with
low on-resistance. Current mode control provides fast
transient response and cycle-by-cycle current limit.
The pin configuration and the representative block
diagram of the AP3211 are respectively shown in
Figure 1.
Pin 1 Dot by Marking
A standard series of inductors are available from
several different manufacturers optimized for use
with the AP3211. This feature greatly simplifies the
design of switch-mode power supplies.
BS
1
6
SW
GND
2
5
IN
FB
3
4
EN
This IC is available in SOT-23-6 package.
Figure 1. Pin Configuration of AP3211 (Top View)
IN
5
SCHOTTKY
FB
SS
HICCUP
VREF
1
BOOSTRAP
REGULATOR
BS
CL
2
VDD/VDD1
REGULATOR
GND
2A/V
OSC
1.4MHz/
400KHz
RAMP
GENERATOR
CS
OSC
1pF
S
Q
VLIMIT
EN
4
3.3V
REFERENCE
VOLTAGE
SS
3
SWITCH/LDO
R
27pF
1M
FB
CL
220k
6
SW
R
ERR AMP
PWM
COMPARATOR
Figure 2. Functional Block Diagram of AP3211
Jul. 2012
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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Application Note 1063
3. Operation
3.2 Over Voltage Protection
The AP3211 has internal OVP circuits. When VOUT
is higher than the OVP threshold, the power
switching will be turned off. The AP3211 will restart
once released from OVP condition.
Operation can be best understood by referring to
Figure 2 and Figure 3. The current sense signal is
compared with the EA output signal to regulate the
output voltage and adjust the MOSFET’s duty cycle.
The AP3211 is also high reliability IC with
integrated OCP, OVP, OTP, UVLO circuit. For more
information please refer to the functional block
diagram (Figure 2).
3.3 Over Temperature Protection
The OTP circuitry is provided to protect the IC if the
maximum junction temperature is exceeded. When
the junction temperature exceeds 160ºC, it will shut
down the internal control circuit. The AP3211 will
restart automatically under the control of soft-start
circuit when the junction temperature decreases to
140ºC.
3.1 Over Current Protection
The AP3211 has internal over current protection
function to protect from catastrophic failure. The
AP3211 can monitor the drain-to-source current of
MOSFET. The peak current-limit threshold is
internally set at 2.4A. When the inductor current is
higher than the current limit threshold, OCP function
will be triggered, forcing MOSFET to turn off, and
working in the hiccup mode, which will turn on
MOSFET after a constant delay time to keep IC cool
when OCP happens.
3.4 Under Voltage Lock Out
The AP3211 provides an under voltage lockout
circuit to prevent it from undefined status when
startup. The UVLO circuit shuts down the device
when VIN drops below 3.8V. The UVLO circuit has
200mV hysteresis, which means the device starts up
again when VIN rises to 3.6V.
Figure 3. Typical Application of AP3211
Jul. 2012
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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Application Note 1063
4. Application
Where IPEAK is the peak inductor current.
Typical application circuit is shown in the Figure 3.
For the circuit parameters setting please refer to the
following descriptions.
The current rating of the selected inductor should be
ensured to be 1.5 times of the peak inductor current.
4.3 Input Capacitor Setting
A high-quality input capacitor with big value is
needed to filter noise at input voltage source and
limit the input ripple voltage while supplying most
of the switch current during ON time. For input
capacitor selection, a ceramic capacitor is highly
recommended due to its low impedance and small
size. However, tantalum or low electrolytic capacitor
is also sufficed.
4.1 Output Voltage Setting
The output voltage can be set using a voltage divider
from the output to FB pin. VOUT is divided by the
voltage divider as below:
⎛ R2
V FB = VOUT × ⎜⎜
⎝ R1 + R2
⎞
⎟⎟
⎠
Where VFB is the feedback voltage, and VFB=0.81V.
Thus, VOUT can be expressed as:
⎛ R + R2
VOUT = 0.81 × ⎜⎜ 1
⎝ R2
There are two important parameters of the input
capacitor: the voltage rating and RMS current rating.
The voltage rating should be at least 1.25 times
greater than the maximum input voltage, and the
RMS current of input capacitor can be expressed as:
⎞
⎟⎟
⎠
4.2 Inductor Setting
The inductor is used to supply smooth current to
output when driven by a switching voltage. Its value
relies on the operating frequency, load current, ripple
current and duty cycle. A higher-value inductor can
decrease the ripple current and output ripple voltage,
however usually with larger physical size. So some
compromise needs to be made when selecting the
inductor. The peak-to-peak inductor ripple current is
26% of the maximum output current when operating
in continuous mode (In most applications, a good
compromise is from 20% to 30% of the maximum
load current of the converter), and the inductor L1
can be selected according to:
L1 = VOUT ×
f SW
I CIN _ RMS = I OUT ×
V IN − VOUT
× V IN × I OUT × 26%
ΔVIN =
Another important parameter for the inductor is the
current rating. After fixing the inductor value, the
peak inductor current can be expressed as:
Jul. 2012
⎛ VOUT
⎜⎜1 −
V IN
⎝
⎞
⎟⎟
⎠
Where ICIN_RMS is the RMS current of input capacitor.
As indicated by the RMS current equation above,
ICIN_RMS reaches the highest level at the duty cycle of
50%. So the RMS current of input capacitor should
be greater than half of the output current under this
worst case. For reliable operation and best
performance, ceramic capacitors are preferred for
input capacitor because of their low ESR and high
ripple current rating. And X5R or X7R type
dielectric ceramic capacitors are preferred due to
their better temperature and voltage characteristics.
Additionally, when selecting ceramic capacitor,
make sure its capacitance is big enough to provide
sufficient charge to prevent excessive voltage ripple
at input. The input ripple voltage can be
approximately expressed as below:
Where VIN is the input voltage, IOUT is the output
current, and fSW is the oscillator frequency.
I PEAK = I OUT +
VOUT
V IN
I OUT
V
× OUT
f SW × C IN VIN
⎛ VOUT
⎜⎜1 −
VIN
⎝
⎞
⎟⎟
⎠
4.4 Output Capacitor Setting
The output capacitor can be selected based upon the
desired output ripple and transient response. The
output voltage ripple depends directly on the ripple
current and is affected by two parameters of the
(VIN − VOUT ) × VOUT
2 × f SW × VIN × L1
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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output
Application Note 1063
output capacitor: total capacitance and the
Equivalent Series Resistance (ESR). The output
ripple voltage can be expressed as:
power loss and increase conversion efficiency. The
reverse breakdown voltage of the Schottky diode
should be larger than the output voltage and the
average current rating of the Schottky should be
larger than the IPEAK.
⎡
⎤
1
ΔVO = ΔI L × ⎢ RESR +
⎥
8 × COUT × f SW ⎦
⎣
4.6 Feedback Resistor Network Selection
The AP3211 integrates loop compensation inside,
optimal compensation depends on the output
capacitor, inductor, load, compensation network,
feedback resistor ratio and also the device itself. For
a stable system, the values for the feedback resistor
network are shown in Table 1.
Where ∆VO is the output ripple voltage and RESR is
ESR of output capacitor. For lower output ripple
voltage across the entire operating temperature range,
X5R or X7R ceramic dielectric capacitor, or other
low ESR tantalum capacitor or aluminum
electrolytic capacitor are recommended.
VOUT (V)
1.8
2.5
3.3
5
The output capacitor selection will also affect the
output drop voltage during load transient. The output
drop voltage during load transient is dependent on
many factors. However, approximations of the
transient drop ignoring loop bandwidth can be
expressed as:
L × ΔI TRAN
+
COUT × (VIN − VOUT )
4.7 Bootstrap Capacitor
The bootstrap capacitor provided is used to drive the
power switch’s gate above the supply voltage. The
bootstrap capacitor is supplied by an internal 5V
supply and placed between SW pin and BS pin to
form a floating supply across the power switch
driver. So the bootstrap capacitor should be a good
quality and high-frequency ceramic capacitor. For
best performance, the bootstrap capacitor should be
X5R and X7R ceramic capacitor, and is
recommended to be 10nF.
Where ∆ITRAN is the output transient load current
step, and VDROP is the output voltage drop (ignoring
loop bandwidth).
Both the voltage rating and RMS current rating of
the capacitor need to be carefully examined when
designing a specific output ripple or transient drop.
The output capacitor voltage rating should be greater
than 1.5 times of the maximum output voltage. In the
buck converter, output capacitor current is
continuous. The RMS current is decided by the
peak-to-peak inductor ripple current. It can be
expressed as:
I COUT _ RMS =
R2(kΩ)
64.9 (1%)
23.7 (1%)
16.2 (1%)
9.53 (1%)
Table 1. Resistor Selection for Common Output
Voltages
2
VDROP = ΔI TRAN × RESR
R1(kΩ)
80.6 (1%)
49.9 (1%)
49.9 (1%)
49.9 (1%)
4.8 External Bootstrap Diode
A low-cost external diode, such as IN4148, is
recommended for higher efficiency when the system
has a 5V fixed input or a 5V/3.3V output voltage.
The bootstrap capacitor is also recommended to be
used to increase the available voltage for power
switch in the applications that the duty cycle is larger
than 65% or the output voltage (VOUT) is larger than
12V.
ΔI L
12
Where ICOUT_RMS is the RMS current of output
capacitor.
IN4148
4.5 Schottky Diode Selection
There are two principles in selecting a Schottky
diode in buck application with the AP3211. The first
is low forward voltage drop and the second is fast
switching speed, which can decrease the Schottky
Jul. 2012
Input 5V
BS
VIN
AP3211
10nF
SW
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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Application Note 1063
Figure 4. Application of Adding Optional External
Bootstrap Diode
5. PCB Layout Guidance
PCB layout is an important part for DC-DC
converter design. Poor PCB layout may reduce the
converter performance and disrupt its surrounding
circuitry due to EMI. A good PCB layout should
follow guidance below:
Figure 5. Demo Board Top Layer (Silkscreen)
5.1 Power Path Length
The power path of AP3211 includes an input
capacitor, output inductor, Schottky diode and output
capacitor. Place them on the same side of PCB and
connect them with thick traces or copper planes on
the same layer. The power components must be kept
together closely. The longer the paths, the more they
act as antennas, radiating unwanted EMI.
5.2 Coupling Noise
The external control components should be placed as
close to the IC as possible.
5.3 Feedback Net
Special attention should be paid to the route of the
feedback wring. The feedback trace should be routed
far away from the inductor and noisy power traces.
Try to minimize trace length to the FB pin and
connect feedback network behind the output
capacitors.
Figure 6. Demo Board Top Layer
(Component Side)
5.4 Via Hole
Be careful to the via hole. Via hole will result in high
resistance and inductance to the power path. If heavy
switching current must be routed through via holes
and/or internal planes, use multiple parallel via holes
to reduce their resistance and inductance.
Typical examples of AP3211 PCB layer are shown
in Figure 5, 6, 7.
Figure 7. Demo Board Bottom Layer
Jul. 2012
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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