AN1055

Application Note 1055
Design Consideration with AP3407/A
Prepared by Yong Wang
System Engineering Dept.
1. Introduction
2. Function Block Description
The AP3407/A is a 1.4MHz fixed frequency, current
mode, PWM synchronous buck (step-down) DC-DC
converter, capable of driving a 1.2A load with high
efficiency, excellent line and load regulation. The
device integrates synchronous P-channel and
N-channel power MOSFET switches with low
on-resistance. It is ideal for powering portable
equipment that runs from a single Li-ion battery.
The pin configuration and the representative block
diagram of the AP3407/A are respectively shown in
Figure 1 and Figure 2.
A standard series of inductors are available from
several different manufacturers optimized for use
with the AP3407/A. This feature greatly simplifies
the design of switch-mode power supplies.
EN
1
GND
2
SW
3
5
4
(For AP3407)
The AP3407/A is available in SOT-23-5 package.
FB
VIN
VIN
1
GND
2
EN
3
5
SW
4
FB
(For AP3407A)
Figure 1. Pin Configuration of AP3407/A (Top View)
Figure 2. Functional Block Diagram of AP3407/A
Nov. 2010
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1055
3. Operation
No external Schottky diode is required in practical
application. The AP3407/A enters PSM at extremely
light load condition. The equivalent switching
frequency is reduced to increase the efficiency in
PSM.
The AP3407/A is a synchronous step-down converter
operating with typically 1.4MHz fixed frequency
pulse width modulation (PWM) at moderate to heavy
load currents and in power-saving moderation (PSM)
at operating to light load currents. It is capable of
delivering a 1.2A output current over a wide input
voltage range from 2.5 to 5.5V.
As the input supply voltage decreases to a value
approaching the output voltage, the duty cycle
increases to the maximum. Further reduction of the
supply voltage forces the P-channel main switch to
remain on for more than one cycle until it reaches
100% duty cycle. The output voltage will then be
determined by the input voltage minus the voltage
drop across the P-channel MOSFET and the inductor.
This is particularly useful in battery powered
applications to achieve longest operation time by
taking full advantage of the whole battery voltage
range.
At the beginning of each cycle initiated by the clock
signal (from the internal oscillator), the P-channel
MOSFET switch is turned on, and the inductor
current ramps up until the comparator trips and the
control logic turns off the switch. The current limit
comparator also turns off the switch in case the
current limit of the P-channel MOSFET is exceeded.
Then the N-channel synchronous switch is turned on
and the inductor current ramps down. The next cycle
is initiated by the clock signal again, turning off the
N-channel synchronous switch and turning on the
P-channel switch (See Figure 2).
4. Application
A general AP3407/A application circuit is shown in
Figure 3. External component selection is driven by
the load requirement, and begins with the selection of
the inductor L. Once L is chosen, CIN and COUT can
be selected.
Two operational modes are available: PSM and
PWM. Internal synchronous rectifier with low RDS(ON)
dramatically reduces conduction loss at PWM mode.
np
R2
VIN
EN
R1
FB
SW
AP3407/A-ADJ
CIN
VOUT
L
VIN
2.2 H
COUT
F
GND
F
Figure 3. Typical Application of AP3407/A
Nov. 2010
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BCD Semiconductor Manufacturing Limited
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Application Note 1055
driven by the required ESR to minimize voltage
ripple and load step transients. Typically, once the
ESR requirement is satisfied, the capacitance is
adequate for filtering. The output ripple (△VOUT) is
determined by:
5. Components Selection
5.1 Inductor Selection
Although the inductor does not influence the
operating frequency, the inductor value has a direct
effect on ripple current. The inductor ripple current
△IL decreases with higher inductance and increases
with higher VIN or VOUT.
∆I L =
⎛
1
∆VOUT ≈ ∆I L ⎜⎜ ESR +
8 × f OSC × C OUT
⎝
VOUT
V
× (1 − OUT )
f OSC × L
V IN
The output ripple is highest at maximum input
voltage since △IL increases with input voltage.
Accepting larger values of △IL allows the use of low
inductances, but results in higher output voltage
ripple, greater core losses, and lower output current
capability. △IL typical value is 20% to 40% of
output current.
Once the ESR requirements for COUT have been met,
the RMS current rating generally far exceeds the
IRIPPLE (P-P) requirement, except for an all ceramic
solution. In most applications, a 22µF ceramic
capacitor is usually enough for these conditions.
Another important parameter for the inductor is the
current rating. Exceeding an inductor's maximum
current rating may cause the inductor to saturate and
overheat. If inductor value has been selected, the
peak inductor current can be calculated as the
following:
I PEAK = I OUT + VOUT ×
At light load currents, the device operates in PSM
mode, and the output voltage ripple is independent of
the output capacitor value. The output voltage ripple
is set by the internal comparator thresholds. The
typical output voltage ripple is 1% of the output
voltage VOUT.
V IN − VOUT
2 × f OSC × V IN × L
5.4 Feedback Divider Resistors
The AP3407/A develops a 0.6V reference voltage
between the feedback pin, FB, and the signal ground
as shown in Figure 3. The output voltage is set by a
resistive divider according to the following formula:
It should be ensured that the current rating of the
selected inductor is 1.5 times of the IPEAK.
R1 ⎞
⎛
VOUT = 0.6 × ⎜1 +
⎟
⎝ R2 ⎠
5.2 Input Capacitor Selection
Because the buck converter has a pulsating input
current, a low ESR input capacitor is required. This
results in the best input voltage filtering and
minimizing the interference with other circuits
caused by high input voltage spikes. Also the input
capacitor must be sufficiently large to stabilize the
input voltage during heavy load transients. Ceramic
capacitors show a good performance because of the
low ESR value, and they are less sensitive against
voltage transients and spikes. Place the input
capacitor as close as possible to the input pin of the
device for best performance. The typical value is
about 4.7µF. The X5R or X7R ceramic capacitors
have the best temperature and voltage characteristics,
which is good for input capacitor.
Keeping the current small (<40µA) in these resistors
maximizes efficiency, but making them too small
(<20µA) may allow stray capacitance to cause noise
problems and reduce the phase margin of the error
amp loop.
6. Layout Consideration
PCB layout is very important to the performance of
the AP3407/A. The loop which switching current
flows through should be kept as short as possible.
The external components (especially CIN) should be
placed as close to the IC as possible. Therefore use
wide and short traces for the main current paths, as
indicated in bold in Figure 4.
5.3 Output Capacitor Selection
The output capacitor is the most critical component
of a switching regulator, it is used for output filtering
and keeping the loop stable. The selection of COUT is
Nov. 2010
⎞
⎟⎟
⎠
Try to route the feedback trace as far from the
inductor and noisy power traces as possible. You
would also like the feedback trace to be as direct as
possible and somewhat thick. These two sometimes
Rev. 1. 0
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involve
Application Note 1055
involve a trade-off, but keeping it away from
inductor and other noise sources is the more critical
of the two. Locate the feedback divider resistor
network near the feedback pin with short leads.
VOUT
VIN
SW
CIN
4.7 F
GND
EN
Flood all unused areas on all layers with copper.
Flooding with copper will reduce the temperature
rise of power components. These copper areas
should be connected to one of the input supplies:
VIN or GND.
Nov. 2010
L 2.2 H
VIN
R1
COUT
22 F
FB
R2
Figure 4. Layout Diagram
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