AN1026

Application Note 1026
Some Application Hints for AP3406
Prepared by Yuan Shan Shan
System Engineering Dept.
1. Introduction
This IC is available in TSOT-23-5 and SOT-23-5
packages.
The AP3406 is a 1.1MHz fixed frequency, current
mode, PWM synchronous buck (step-down) DC-DC
converter, capable of driving a 650mA load with high
efficiency, excellent line and load regulation. The
device integrates a main switch and a synchronous
switch without an external Schottky diode. It is ideal
for powering portable equipment that runs from a
single Li-ion battery.
2. Operation
The AP3406 consists of a reference voltage, slope
compensation circuit, error amplifier, PWM
comparator, P-channel MOSFET (used as a main
switch), N-channel MOSFET (used as a synchronous
switch), current limit circuit, and others (see
functional block diagram in Figure 1 for detailed
information).
A standard series of inductors are available from
several different manufacturers optimized for use
with the AP3406. This feature greatly simplifies the
design of switch-mode power supplies.
EN
VIN
4
1
Shutdown
Control
Slope
Compensation
Current
Sense
Current Limit
Detector
Oscillator
Control
Logic
VREF with
Soft Start
FB
5
3
Driver
SW
0.6V
Error
Amplifier
PWM
Comparator
Over
Temperature
Detector
2
GND
Figure 1. Functional Block Diagram of AP3406
2.1 Main Loop Control
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
Aug. 2008
is initiated by the clock signal again, turning off the
N-channel synchronous switch and turning on the
P-channel switch. (See Figure 2)
2.2 Dropout Operation
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
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Application Note 1026
the initial steady-state output voltage, thereby
reducing start-up stresses and current surges.
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.
2.5 UVLO
If UVLO threshold is not met, all functions of
AP3406 are disabled. The UVLO circuit prevents the
device from misoperation at low input voltage. It
prevents the converter from turning on the main
switch and synchronous switch under undefined
condition.
2.3 Short Circuit Protection
When the output is shorted to ground, the frequency
of the oscillator is reduced to 600 kHz, which ensures
that the inductor current has more time to decay,
thereby preventing damage. The frequency of the
oscillator will gradually increase to 1.1MHz when
feedback voltage gradually increase from 0V to 0.6V.
2.6 Thermal Protection
If the thermal protection circuit senses the junction
temperature exceeding approximately 160oC, the
thermal shutdown circuit will turn off the main
switch and synchronous switch. The thermal
hysteresis is about 30oC, which means that the
converter can return to normal operation when the
junction temperature drops below 130oC. It prevents
the converter from thermal damage under some
unexpected condition.
2.4 Soft Start
The AP3406 has an internal soft start circuit that
limits the inrush current during start-up. The soft start
feature allows the converter output to gradually reach
Figure 2. Typical Application of AP3406
3. Component Select Guide (See
Figure 2)
Care must be taken when ceramic capacitor is used
at the input. When a ceramic capacitor is used at the
input and the power is supplied by a wall adapter
through long wires, a load step at the output can
induce ringing at the input, and this ringing can
couple to the output and be mistaken as loop stability.
The X5R or X7R ceramic capacitors have the best
temperature and voltage characteristics, which is
good for input capacitor.
3.1 Input Capacitor
The input current of the buck converter is
discontinuous, so a bulk capacitor is required to keep
the DC input voltage constant. To ensure a stable
operation, the capacitor should be placed as close to
the VIN pin as possible. The value of the input
capacitor will vary according to different load and
input voltage source impedance characteristics. The
typical value is about 10µF.
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3.2 Output Capacitor
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Application Note 1026
The output capacitor is the most critical component
of a switching regulator, it is used for output filtering
and keeping the loop stable. The typical value is
10µF.
3.4 Feedback Divider Resistors
The output voltage is set by the feedback divider
resistors (see Figure 3) according to the following
formula:
The primary parameters for output capacitor are the
voltage rating and the equivalent series resistance
(ESR). The ESR value has relation to the voltage
rating. For the same product series, the capacitor
with higher voltage rating will have smaller ESR
value. A low ESR capacitor is preferred to keep the
output voltage ripple low. The output ripple is
calculated as the following:
VOUT = 0.6 * (1 +
∆VOUT ≈ ∆IL * (ESR +
R1
)
R2
VOUT
300k
R1
AP3406-ADJ FB
GND
R2
1
)
8 * f * COUT
Figure 3
Where f is the switching frequency, COUT is the
output capacitance and △IL is the ripple current in
the inductor.
The resistor R1 also sets the feedback loop
bandwidth with the internal compensation capacitor,
so choose R1 around 300k for optimal transient
response. Then, solve for R2:
3.3 Inductor
The inductor is used to supply smooth current to
output when it is driven by a switching voltage. The
higher the inductance, the lower the peak-to-peak
ripple current, as the higher inductance usually
means
the larger inductor size, so some trade-offs should be
made when select an inductor. The AP3406 is a
synchronous buck converter. It always works on
continuous current mode (CCM), and the inductor
value can be selected as the following:
L = VOUT *
R2 =
Table 1. Resistor Selection vs. Output Voltage Setting
VOUT
1.2V
1.3V
1.5V
1.8V
2.5V
3.3V
VIN − VOUT
f * VIN * IOUT * k
Where VOUT is the output voltage, VIN is the input
voltage, IOUT is the output current, k is the coefficient
about ripple current, the typical value is 20% to 40%.
R2
300k (1%)
240k (1%)
200k (1%)
150k (1%)
95.3k (1%)
66.5k (1%)
PCB layout is very important to the performance of
AP3406. 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. Special
attention should be paid to the route of the feedback
wiring. 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.
Figure 4 illustrates an example of PCB layout.
VIN − V OUT
2 * f * VIN * L
It should be ensured that the current rating of the
selected inductor is 1.5 times of the IPEAK.
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R1
300k (1%)
280k (1%)
300k (1%)
300k (1%)
300k (1%)
300k (1%)
4. Layout Consideration
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:
IPEAK = IOUT + VOUT *
R1
VOUT
−1
0.6
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Application Note 1026
Via to VIN
Via to VOUT
Via to GND
AP3406
Figure 4
Aug. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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