Application Note 1034
Some Application Hints for AP3406A
Prepared by Zhang Jiu Sheng
System Engineering Dept.
1. Introduction
This IC is available in TSOT-23-5 and MSOP-10
The AP3406A is a 1.1MHz fixed frequency, current
mode, PWM synchronous buck (step-down) DC-DC
converter, capable of driving a 800mA 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 AP3406A 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
A standard series of inductors are available from
several different manufacturers optimized for use
with the AP3406A. This feature greatly simplifies the
design of switch-mode power supplies.
Figure 1. Functional Block Diagram of AP3406A
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
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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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BCD Semiconductor Manufacturing Limited
Application Note 1034
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
2.5 UVLO
If UVLO threshold is not met, all functions of
AP3406A 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
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 AP3406A 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
2.5V to 5.5V
Figure 2. Typical Application of AP3406A
3. Component Select Guide (See
Figure 2)
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.
3.2 Output Capacitor
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
Care must be taken when ceramic capacitor is used
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BCD Semiconductor Manufacturing Limited
Application Note 1034
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:
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
AP3406A is a synchronous buck converter. It always
works on continuous current mode (CCM), and the
inductor value can be selected as the following:
R2 =
PCB layout is very important to the performance of
AP3406A. 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 below 2 items:
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%.
4.1 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.
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
2 * f * VIN * L
4.2 The GND and PGND Connection for
MSOP-10 Package
The AP3406A uses 2 separate ground connections
for MSOP-10 package: PGND for driver, NMOS
power device and GND for sensitive analog control
circuitry. For better performance, GND and PGND
should be tied as closely as possible.
It should be ensured that the current rating of the
selected inductor is 1.5 times of the IPEAK.
3.4 Feedback Divider Resistors
The output voltage is set by the feedback divider
resistors (see Figure 3) according to the following
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4. Layout Consideration
f * VIN * IOUT * k
VOUT = 0.6 * (1 +
Figure 3
Where f is the switching frequency, COUT is the
output capacitance and △IL is the ripple current in
the inductor.
8 * f * COUT
∆VOUT ≈ ∆IL * (ESR +
L = VOUT *
Figure 4 and 5 are examples of PCB layout for
TSOT-23-5 and MSOP-10 packages respectively.
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BCD Semiconductor Manufacturing Limited
Application Note 1034
Figure 4
Figure 5
Mar. 2009
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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