AN1050

Application Note 1050
Design Consideration with AP3615
Prepared by Qingling You
System Engineering Dept.
1. Introduction
1MHz high switching frequency enables the use of
small external flying capacitors.
The AP3615 is a step-up DC/DC converter based on
1x/1.5x charge pump for white LED backlight
applications. The AP3615 integrates 5 channels
current sink and automatic mode selection charge
pump. High efficiency is achieved by utilizing a
1x/1.5x fractional charge pump in combination with
low dropout current sink.
The supply voltage range of AP3615 is from 2.8V to
5.5V which make them ideally suitable for
applications powered by Li-ion battery. Internal
soft-start circuitry prevents excessive inrush current
during start-up and mode transition.
The AP3615 is available in a tiny package of
QFN-3×3-16. Its operating temperature range is from
-40°C to 85°C. The pin configuration and the
representative block diagram of the AP3615 are
respectively shown in Figure 1 and Figure 2:
2
C2+
3
C1+
4
NC
VOUT2
D1
13
E P
5
6
7
8
EN
C2-
14
VIN
1
15
PGND
C1-
16
AGND
Pin 1 Dot by Marking
VOUT1
The AP3615 supports up to 5 white LEDs and
regulates a constant current for brightness control.
The LED current can be changed from 1.25mA to
20mA by applying a pulse signal to the EN pin. There
are totally 16 steps for current control. Additionally,
12
D2
11
D3
10
D4
9
D5
Exposed PAD,
Connected to AGND
:
Note Pin 14 should be connected with Pin 16 on PCB Board.
Figure 1. Pin Configuration of AP3615 (Top View)
May. 2010
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1050
Figure 2. Functional Block Diagram of AP3615
signal
signal is added to the EN pin and remains more than
1ms, the IC will shut down. The shutdown current is
very small. It can minimize the power dissipation and
extend the battery life.
1.1 Soft Start
The AP3615 has an internal soft start circuit to limit
the inrush current during startup and charge pump
mode transition. It prevents possible drop of the input
voltage if a high impedance power source is
connected to the input of the AP3615.
If adding a pulse signal to the EN pin, the AP3615
will operate in dimming mode. The LED current can
be changed from 1.25mA to 20mA, in 16 steps totally.
The detailed operation of brightness dimming please
refer to 4.3.
In mobile phone applications, the AP3615 is powered
by Li-ion battery. The soft start function prevents
excessive output current from leaving the battery,
which can cause excessive voltage drop. It is of
benefit to other circuits powered by Li-ion battery
directly.
1.3 Current Matching
Current matching is most important for WLED
backlights. The AP3615 current sink block
implements the current matching function. It can
achieve less than ±3% current matching between 5
channels.
1.2 Enable and Dimming Control
The AP3615 has an EN pin to enable/disable the IC
or control the brightness of white LEDs. When a
logic high signal is added to the EN pin, the AP3615
starts up and operates normally. When a logic low
May. 2010
As we know, WLED is current control device whose
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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forwa
rd
Application Note 1050
forward voltage has a wide distribution. If the LED
lot number is different, the distribution is much wider.
A highlight feature of AP3615 is that its current
matching precision is independent on LED forward
voltage. It can guarantee ±3% current matching
precision for all types of WLEDs.
1.8 Over Temperature Protection
The AP3615 is also equipped with a thermal
protection and shutdown circuit that continuously
monitors the IC junction temperature. When some
abnormal operations occur, the internal junction
temperature rises. If the thermal protection circuit
senses the junction temperature exceeding
approximately 160°C, it will disable AP3615 and
protect the device from damaged. The thermal
protection and shutdown circuit has 20°C of system
hysteresis
1.4 White LED Connection
The AP3615 supports up to 5 white LEDs that are
connected from VIN to D1, D2, D3, D4 and D5
channels respectively. If a certain channel is not used,
it should be connected to VIN directly. It should not
be left floating.
2. Components Selection
2.1 Capacitor Selection
Careful selection of the four external capacitors CIN,
COUT and CFLY (C1 and C2) is very important because
they will affect charge pump capability, output
voltage ripple, input voltage ripple and power
efficiency. Polarized capacitor such as tantalum or
aluminum is not recommended for using with the
AP3615. For the two flying capacitors C1 and C2,
it is important to use low ESR capacitors to avoid
unnecessary efficiency losses. To reduce the input
and output ripple effectively, low ESR capacitors on
VIN and VOUT are recommended. To get the
optimum performance, low ESR ceramic capacitor
will be the best option for CIN, COUT and CFLY. The
recommended value for each capacitor is 1µF typical.
1.5 Mode Selection
The AP3615 can automatically switch between 1x
mode and 1.5x mode for optimizing the efficiency.
An internal current sensing circuit monitors WLED
current to keep it constant when the battery voltage
fluctuates. The AP3615 switches to 1.5x mode
whenever the dropout condition is detected from any
of the 5 current sinks and returns to 1x mode
whenever the dropout condition releases.
1.6 Under Voltage Lockout
The AP3615 provides an under voltage lockout
(UVLO) circuit to prevent it from undefined status
when startup. The UVLO circuit shuts down the
device when VIN drops below a typical threshold of
2.2V. The UVLO circuit has 250mV hysteresis,
which means the device starts up again when VIN
rises to 2.45V or above. The UVLO hysteresis can
prevent AP3615 from unstable state when startup.
Ceramic Capacitors have features of very low ESR,
low cost, small PCB footprint, and are non-polarized.
All the above are of benefit to the charge pump. The
main materials of ceramic capacitor include NPO
(COG), X7R, X5R, Y5U, Z5U and Y5V. MLCC X7R
and X5R dielectrics are much more desirable because
of their capacitance, precision, operating temperature
range, temperature stability and cost (compare with
NPO ceramic capacitor). Table 1 gives the key
features of X7R and X5R ceramic capacitors.
1.7 Over Voltage Protection
The AP3615 has an internal over voltage protection
circuit. When the white LEDs are disconnected from
the circuit or fail to open, the voltage between input
and output will be clamped at 5.5V.
Table 1. Key Features of X7R and X5R Ceramic Capacitors
Ceramic Capacitor
X7R
X5R
May. 2010
Operating Temperature
Range (℃ )
-55 to 125
-55 to 85
Precision
Capacitance
± 15%
± 15%
150pF to 4.7µF
0.47µF to 10µF
Rev. 1. 0
Temperature
Stability
-5% to 5%
-15% to 5%
BCD Semiconductor Manufacturing Limited
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Application Note 1050
2.2 White LED Selection
The AP3615 is specifically intended for driving white
LEDs with forward voltage from 0V to 4.0V. Since
the D1 to D5 current sinks are independent of the
voltage, the LED brightness will be matched
regardless of the specific LED forward voltage (VF)
levels.
The AP3615 is designed especially for white LED
driver with current matching. Figure 3 and Figure 4
show the typical application circuits. In these
applications, the LED current can be changed from
1.25mA to 20mA by applying a pulse signal to the
EN pin. There are totally 16 steps of current can be
set by users.
In a word, the AP3615 can drive most white LEDs
for backlighting with good current matching.
If the LED is not used, the LED pin should be
connected to VIN directly. Figure 3 and Figure 4
show the connection in 4 WLEDs and 5 WLEDs
applications.
3. Typical application
C2
1µF
C1
1µF
D1
7
Li-ion
Battery
1
4
2
3
C1-
C1+
C2-
C2+
D1
8
EN
5
COUT
D2
AP3615
14
16
D4
VIN
Pulse Input
CIN
1µF
D3
D2
VOUT2
D3
VOUT1
D4
AGND
11
10
9
D5
PGND
1µF
13
12
6
Figure 3. Typical Application of AP3615 (4 WLEDs)
C1
1µF
C2
1µF
1
4
2
3
C1-
C1+
C2-
C2+
D1
7
Li-ion
Battery
CIN
1µF
VIN
D1
Pulse Input
8
14
16
COUT
5
EN
AP3615
D2
VOUT2
D3
VOUT1
D4
AGND
PGND
1µF
D5
D2
D3
D4
D5
13
12
11
10
9
6
Figure 4. Typical Application of AP3615 (5 WLEDs)
May. 2010
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1050
4. Application Hints
be calculated through formula 2 and formula 3.
4.1 Output Ripple
Low ESR ceramic capacitor for COUT and CFLY and
high switching frequency of AP3615 minimize the
output voltage ripple.
η LED =
Formula 1 can be used to calculate the output voltage
ripple.
1 1
ID × ×
f 2 + 2 × I × ESR
VRIPPLE =
D
COUT
C OUT
--------------------------------------(1)
η LED =
VD × I D
VD × I D
VD
(1.5x mode)
≈
=
VIN × I IN VIN × 1.5 I D 1.5VIN
--------------------------------------(2)
VD × I D
V ×I
V
(1x mode)
≈ D D = D
VIN × I IN VIN × I D VIN
--------------------------------------(3)
With a fully charged battery, the charge pump
operates in 1x mode. The efficiency will increase
along with the decrease of battery voltage. When the
battery voltage drops to about 3.5V (VF=3.2V), the
efficiency will reach the top point which is about
91%. As the discharge of battery ceaselessly run out,
leading to the current sinks no longer have enough
voltage overhead to maintain a constant current
regulation, the charge pump will switch into 1.5x
mode. The conversion efficiency is lowest at this
crossover. As the battery discharges further, the
efficiency again increases and reaches a second
maximum at about 2.7V VIN.
ID ---equal to the sum of LED current
f --- operating frequency, 1MHz
COUT --- output capacitor
ESRCOUT --- the equivalent serial resistor of output
capacitor
The current sink is powered by charge pump.
Because of its high bandwidth, the output voltage
ripple is filtered by the current sink. So, there is little
current ripple in the current sink. It is of benefit to the
LED.
4.3 Dimming Control
The AP3615 implements a pulse dimming
mechanism to control the brightness of white LEDs.
4.2 Power Efficiency
The power conversion efficiency of the AP3615 can
be
Figure 5. Dimming Control by Pulse Signal
May. 2010
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1050
The WLED comes to full brightness with about
20mA current on each channel when the AP3615 is
powered on. The dimming control can be achieved by
applying a pulse to EN pin. When the low level
duration time of the pulse is between TLOmin and
TLOmax, and the high-level duration time is larger than
THImin, the ILED will decrease 1/16. After 15 such
pulses
pulses applied, the WLED brightness will be reduced
to 1/16 of its full brightness. And, at that time if one
more such pulse applied to EN pin, the WLED will
return back to its full brightness. And this repeats. If
the low level duration time is larger than TSHDNmax, the
IC will be turned off, apply one pulse to EN pin to
turn on it if necessary.
VEN
ILED
Figure 6. WLED Current vs. Dimming Signal
prevention. These rules are adopted to minimize the
LED current ripple, improve the drive capability and
efficiency, and minimize the input voltage ripple to
prolong the battery life. Figure 7 gives a PCB layout
example of the AP3615. In this example, the package
type of AP3615 is QFN-3×3-16 and the type of CIN,
COUT and CFLY is 0805.
4.4 PCB Layout Consideration
Due to high switching frequency and large transient
current produced by AP3615, careful PCB layout is
necessary. For best performance, all external
capacitors should be located as close as possible to
the device. A true ground plan and short connection is
highly recommended.
In detail, COUT has the first priority to be located
closely to VOUT and GND pin which is
advantageous to reduce noise coupling from charge
pump to LEDs and reduce the output ripple. CFLY has
the second priority and CIN has the third priority.
Furthermore, the traces from pins to flying capacitors
should be short and wide to reduce parasitic
resistance. VIN traces should be wide to minimize
inductance and handle the large current. The exposed
pad, PGND and AGND should be connected to a
strong ground plane for heat sinking and noise
May. 2010
Figure 7. AP3615 PCB Layout Example
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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