AN1029

Application Note 1029
Design Consideration with AP3606/AP3607
Prepared by You Qing Ling
System Engineering Dept.
Additionally, 1MHz high switching frequency
enables the use of small external flying capacitors.
1. Introduction
The AP3606 and AP3607 are step-up DC/DC
converters based on 1x/1.5x charge pump for white
LED backlight application. The AP3606(AP3607)
integrates 4(6) 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 ranges of AP3606 and AP3607
are from 2.7V to 5.5V which make them ideally suit
for applications powered by Li-ion battery. Internal
soft-start circuitry prevents excessive inrush current
during start-up and mode transition.
D1
8
C2-
2
10
D4
C2+
3
9
NC
C1+
4
Exposed PAD,
Connected to AGND
D2
VOUT2
7
D3
D1
NC
6
11
VOUT2
VOUT1
5
1
16
15
14
13
12 D3
11 D4
EP
10 D5
9
5
6
7
8
EN
4
C1-
VIN
3
C1+
EP
12
D2
PGND
C2+
13
AGND
2
14
EN
C2-
15
VIN
1
16
PGND
C1-
Pin 1 Dot by Marking
AGND
Pin 1 Dot by Marking
VOUT1
The AP3606 and AP3607 are 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 diagrams of the AP3606
and AP3607 are respectively shown in Figure 1 and
Figure 2:
The AP3606(AP3607) supports up to 4(6) 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.
D6
Exposed PAD,
Connected to AGND
Note: Pin15 should be connected with Pin 16 on PCB Board.
Note: Pin14 should be connected with Pin 16 on PCB Board.
AP3606
AP3607
Figure 1. Pin Configuration of AP3606/AP3607 (Top View)
Nov. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
1
Application Note 1029
VIN
C1-
C1+
1
4
7
C22
8
3
16
1x /1.5x Charge Pump
VOUT1
5
AGND
6
PGND
14
VOUT2
Current Limitation
1MHz
OSC
EN
C2+
OVP
Soft Start
Shutdown
Delay
OTSD
UVLO
16 Steps Pulse
Dimming Controller
4 Channels Current Sink
13
D1
12
D2
11
D3
10
D4
AP3606
C2- C2+
C1- C1+
1
VIN
7
4
3
2
16
1x /1.5x Charge Pump
5
1MHz
OSC
Current Limitation
6
EN
Shutdown
Delay
UVLO
OTSD
16 Steps Pulse
Dimming Controller
AGND
OVP
Soft Start
8
VOUT1
15
6 Channels Current Sink
14
13
12
11
10
PGND
VOUT2
9
D1 D2 D3 D4 D5 D6
AP3607
Figure 2. Functional Block Diagram of AP3606/AP3607
Nov. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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Application Note 1029
channel is not used, it should be connected to VIN
directly. It should not be floating.
1.1 Soft Start
The AP3606(AP3607) 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 AP3606(AP3607).
In mobile phone applications, the AP3606(AP3607)
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.
1.5 Mode Selection
The AP3606(AP3607) 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 AP3606(AP3607)
switches to 1.5x mode whenever the dropout
condition is detected from any of the 4(6) current
sinks and returns to 1x mode whenever the dropout
condition releases.
1.2 Enable and Dimming Control
The AP3606(AP3607) 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 AP3606(AP3607) starts up and operates
normally. When a logic low 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.
1.6 Under Voltage Lockout
The AP3606(AP3607) 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 AP3606(AP3607) from unstable state when
startup.
If adding a pulse signal to the EN pin, the
AP3606(AP3607) 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.
1.7 Over Voltage Protection
The AP3606(AP3607) 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.
1.3 Current Matching
Current matching is most important for WLED
backlights. The AP3606(AP3607) current sink block
implements the current matching function. It can
achieve less than ±3% current matching between 4
channels for AP3606 and 6 channels for AP3607.
1.8 Over Temperature Protection
The AP3606(AP3607) 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
AP3606(AP3607) and protect the device from
damaged. The thermal protection and shutdown
circuit has 20°C of system hysteresis.
As we know, WLED is current control device whose
forward voltage has a wide distribution. If the LED
lot number is different, the distribution is much wider.
A highlight feature of AP3606(AP3607)) is that it’s
current matching precision is independent on LED
forward voltage. It can guarantee ±3% current
matching precision for all types of WLEDs.
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
1.4 White LED Connection
The AP3606 supports up to 4 white LEDs that are
connected from VIN to D1, D2, D3 and D4 channels
respectively. The AP3607 supports up to 6 white
LEDs which are connected from VIN to D1, D2, D3,
D4, D5 and D6 channels respectively. If a certain
Nov. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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Application Note 1029
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.
aluminum is not recommended for using with the
AP3606(AP3607). 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.
Table 1. Key Features of X7R and X5R Ceramic Capacitors
Ceramic Capacitor
X7R
X5R
Operating Temperature
Range (℃ )
-55 to 125
-55 to 85
Precision
Capacitance
± 15%
± 15%
150pF to 4.7µF
0.47µF to 10µF
Temperature
Stability
-5% to 5%
-15% to 5%
2.2 White LED Selection
The AP3606(AP3607) is specifically intended for
driving white LEDs with forward voltage from 0V to
4.0V. Since the D1 to D6 current sinks are
independent of the voltage, the LED brightness will
be matched regardless of the specific LED forward
voltage (VF) levels.
The AP3606(AP3607) is designed especially for
white LED driver with current matching. Figure 3, 4,
5 and 6 shows 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 AP3606(AP3607) 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 5
show the connection in 3 WLEDs and 5 WLEDs
applications.
3. Typical application
C1
1µF
C2
1µF
D1
U1
7
Li-ion
Battery
CIN
1µF
1
4
2
3
C1-
C1+
C2-
C2+
D2
D3
VIN
Pulse Input
8
14
16
COUT
5
EN
AP3606
D1
VOUT2
D2
VOUT1
D3
AGND
PGND
D4
13
12
11
10
6
1µF
Figure 3. Typical Application of AP3606 (3 WLEDs)
Nov. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
4
Application Note 1029
Figure 4. Typical Application of AP3606 (4 WLEDs)
C1
1µF
C2
1µF
D1
7
Li-ion
Battery
CIN
1
4
2
C1-
C1+
C2-
VIN
D2
8
1µF
15
16
COUT
C2+
D1
Pulse Input
5
EN
AP3607
D3
VOUT2
D4
VOUT1
D5
PGND
AGND
1µF
D2
D3
D4
D5
3
D6
14
13
12
11
10
9
6
Figure 5. Typical Application of AP3607 (5 WLEDs)
Nov. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
5
Application Note 1029
C1
1 F
C2
1 F
D1
7
Li-ion
Battery
CIN
1
4
2
3
C1-
C1+
C2-
C2+
VIN
D1
Pulse Input
1 F
D2
8
15
16
COUT
5
EN
AP3607
D3
VOUT2
D4
VOUT1
D5
AGND
PGND
1 F
D6
D2
D3
D4
D5
D6
14
13
12
11
10
9
6
Figure 6. Typical Application of AP3607 (6 WLEDs)
(AP3607) can be calculated through formula 2 and
formula 3.
4. Application Hints
4.1 Output Ripple
Low ESR ceramic capacitor for COUT and CFLY and
high switching frequency of AP3606(AP3607)
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
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.
4.2 Power Efficiency
4.3 Dimming Control
The AP3606(AP3607) implements a pulse dimming
mechanism to control the brightness of white LEDs.
The power conversion efficiency of the AP3606
Nov. 2008
VD × I D
VD × I D
VD
(1.5x mode)
≈
=
VIN × I IN VIN × 1.5 I D 1.5VIN
--------------------------------------(2)
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
6
Application Note 1029
When AP3606(AP3607) is powered on, the WLED is
in full brightness. Its current is about 20mA. The
dimming control can be achieved by applying a pulse
to EN pin. When the low level duration time of pulse
is between TLOmin and TLOmax, and the high-level
duration time is larger than THImin, the LED current
will decrease 1/16. If the low level duration time is
larger than TSHDNmax, the IC will be turned off. When
AP3606/AP3607 is powered on, the WLED is in full
brightness. And it will keep maximum current until
the pulse is detected. After 15 pulses the WLED
current decreases to 1/16 of full brightness. It will
increase to full brightness if a pulse is added to EN
pin then. Please refer to Figure 7 and Figure 8 for
detailed information and example.
Figure 7. Dimming Control by Pulse Signal
Figure 8. WLED Current vs. Dimming Signal (AP3607)
Nov. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
7
Application Note 1029
4.4 PCB Layout Consideration
Due to high switching frequency and large transient
currents produced by AP3606(AP3607), 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 capacitor
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
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.
AP3607
Figure 9. AP3606/AP3607 PCB Layout Examples
4.5 The Layout Compatibility of AP3606 and
AP3607
There is a good layout compatibility between
AP3606 and AP3607. It is convenient for user to use
AP3606 or AP3607 as backlight drive in the same
PCB board. In other word, the PCB board used for
AP3606 also suits AP3607. Figure 10 illustrates how
to apply AP3606 and AP3607 to the same PCB.
Figure 9 gives an layout example of the
AP3606(AP3607). In this example, the package of
AP3606(AP3607) is QFN-3x3-16 and the package of
CIN, COUT and CFLY is 0805.
VIN
LED1 LED2
R
COUT
16
C1
C2
15
14
13
1
12
2
11
EP
3
LED5
9
6
7
LED4
10
4
5
LED3
LED6
8
VIN
Pulse input
CIN
AP3606
Nov. 2008
Figure 10.
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
8
Application Note 1029
In case AP3607 is used, please remove R as Figure
12 shows.
If AP3606 is applied, please:
1) Remove LED1 & LED6
2) Set R to 0Ω
VIN
The recommended schematics please refer to Figure
11.
12
D2
C2-
2
11
D3
C2+
3
10
D4
C1+
4
9
NC
AP3606
7
LED3
LED4
C2
D2
D1
VOUT2
12
D3
LED3
C2-
2
11
D4
LED4
C2+
3
10
D5
LED5
C1+
4
9
D6
LED6
AP3607
5
6
7
8
Pulse input
CIN
8
EN
6
VIN
AGND
5
C1
LED2
13
EN
1
14
VIN
13
15
1
D1
14
C1-
PGND
C2
15
16
C1-
AGND
16
C1
VOUT2
NC
VOUT1
COUT
LED1
PGND
R=0
COUT
VOUT1
LED1 LED2
VIN
Pulse input
CIN
Figure 12.
Figure 11.
Nov. 2008
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
9
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