AN1126

A Product Line of
Diodes Incorporated
AP3418
APPLICATION NOTE 1126
1.5A, 1.4MHZ HIGH EFFICIENCY SYNCHRONOUS DC-DC BUCK CONVERTER
Introduction
The AP3418 is a 1.4MHz fixed frequency, current mode, PWM synchronous buck (step-down) DC-DC converter, capable of driving a 1.5A 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.
A standard series of inductors are available from several different manufacturers optimized for use with the AP3418. This feature greatly simplifies
the design of switch-mode power supplies.
The AP3418 is available in SOT25 package.
Function Block Diagram
The pin configuration and the representative block diagram of the AP3418 are respectively shown in Figure1 and Figure 2.
(Top View)
EN
1
GND
2
SW
3
5
FB
4
VIN
SOT25
Figure 1. Pin Configuration of AP3418
VIN
4
GND
VIN
2
4
1
EN
VOLTAGE
REFERENCE
OSCILLATOR
CURRENT
SENSE
0.6V
5
FB
ERROR
AMPLIFIER
PWM
COMPARATOR
MAX
CURRENT LIMIT
VOCP
3
SW
DRIVER
0.4V
LOGIC
CLK
SHORT
CIRCUIT
PROTECTION
REVERSE
COMPARATOR
2
GND
Figure 2. Functional Block Diagram of AP3418
AP3418 Rev.1.0
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AP3418
EVB Schematic
A general AP3418 application circuit is shown in Figure 3. External component selection is driven by the load requirement, and begins with the
selection of the inductor L1. Once L1 is chosen, CIN and COUT can be selected.
R4
OPT
R2
R1
EN
VIN
4
5
1
EN
FB
VIN
AP3418-ADJ
CIN
4.7μF
SW
VOUT
L1
3
2.2μH
GND
2
COUT
22μF
Figure 3. Schematic of AP3418
VOUT (V)
R1 (kΩ)
R2 (kΩ)
L1 (µH)
3.3
135
30
2.2
2.5
95
30
2.2
1.8
60
30
2.2
1.2
30
30
2.2
Application Notes
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 V IN or VOUT.
I L 
VOUT
V
 (1  OUT )
f osc  L1
VIN
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.
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 
VIN  VOUT
2  f OSC  VIN  L1
It should be ensured that the current rating of the selected inductor is 1.5 times of the I PEAK.
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.
AP3418 Rev.1.0
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AP3418
Application Notes (Cont.)
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 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:


1

VOUT  I L  ESR 
8

f

C
OSC
OUT


The output ripple is highest at maximum input voltage since ∆IL increases with input voltage.
Once the ESR requirements for COUT have been met, the RMS current rating generally far exceeds the I RIPPLE (P-P) requirement, except for an all
ceramic solution. In most applications, a 22µF ceramic capacitor is usually enough for these conditions.
Feedback Divider Resistors
The AP3418 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:
R1 

VOUT  0.6  1 

R2 

Layout Consideration
PCB layout is very important to the performance of the AP3418. 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.
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 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.
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.
Figure 4. Top Layer
AP3418 Rev.1.0
Figure 5. Bottom Layer
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AP3418
Bill of Materials (BOM)
Location
Quantity
Specification
Size
CIN
1
4.7µF
C1206
COUT
1
22µF
C1206
R1
1
30kΩ (1%)
R1206
R2
1
30kΩ (1%)
R1206
L1
1
2.2µH
4.0(mm) x 4.0(mm)
U1
1
AP3418
SOT25
AP3418 Rev.1.0
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AP3418
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