AN1031

Application Note 1031
Design Consideration with AP3031
Prepared by Han Lu
System Engineering Dept.
1. Introduction
10’ LCD panels, including digital photo frame, GPS
receiver, EPC, PDVD, etc. As is well known, the
main feature of these handheld devices is
compactness. To satisfy the requirements for small
mounting space constraints, this IC is available in
small SOT-23-6, TSOT-23-6 and SOIC-8 packages
to save space.
The AP3031 is an inductor-based DC/DC boost
converter designed to drive LED arrays. 1.4A
switching current allows AP3031 to be used in
different 7' to 10' LCD panel backlights (3*13 LED
arrays typically). The output current accuracy is
determined by the feedback voltage and resistor
RISET.
2. Functional Block Description
To save space and reduce cost, this AP3031 operates
at 1MHz normally, which means that the tiny
periphery components are acceptable. In fact, 1mm
tall 4.7µH inductor and 0.47µF output capacitors for
typical applications are very appropriate.
The pin configuration and the representative block
diagram of the AP3031 are respectively shown in
Figure 1 and Figure 2.
SOT-23-6/TSOT-23-6
SOIC-8
Pin 1 Dot by Marking
An OVP circuit is integrated in this chip, which
protects the IC under open load condition. The
AP3031 includes UVLO, soft-start, current limit and
OTSD to protect the circuit.
SW
To achieve flexible maneuverability, a control
terminal is designed to turn on or turn off this device.
In practice, adding a PWM signal on this terminal to
control the luminous intensity is a good idea. The
main application field of the AP3031 consists of 7’ to
6
1
VIN
GND
2
5
OV
FB
3
4
CTRL
FB
1
8
CTRL
GND
2
7
OV
GND
3
6
GND
SW
4
5
VIN
Figure 1. Pin Configuration of AP3031 (Top View)
FB
SW
3 (1)
1 (4)
SOFT
START
5 (7)
OV
6 (5)
VIN
V REF
1 .25 V
200 mV
A1
A2
CONTROL
LOGIC
COMPARATOR
OVER
TEMPERATURE
DETECTOR
4 (8)
STANDBY
To Control
Logic
Q1
2
(2, 3, 6)
Σ
CTRL
OV
DRIVER
GND
RAMP
GENERATOR
1 MHz
OSCILLATOR
A (B)
A SOT-23-6/TSOT-23-6
B SOIC-8
Figure 2. Functional Block Diagram of AP3031
Oct. 2009
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
1
Application Note 1031
3. Operation
regulation. This voltage level is the output signal of
error amplifier A1, and is the amplified signal of
difference between feedback voltage and reference
voltage of 200mV. So, constant output current can be
provided by this operation mode.
The AP3031 uses a constant frequency, current mode
control scheme to provide excellent line and load
regulation. Operation can be best understood by
referring to Figure 2 and Figure 3. Every switching
cycle can be divided into two time subintervals.
The device will enter a pulse skipping state at light
load to lower the loss. In fact, as the current through
LEDs reduces, the device will begin skipping pulses.
At this state, the device operates at a low frequency
and discontinuous current mode (DCM). One feature
of the DCM is that the SW pin exhibits ringing due to
the LC tank circuit formed by the inductor in
combination with the switch and diode capacitance
after the current through the inductor goes back to
zero. It is necessary to point out that the skipping
pulse will cause a low frequency ripple on the output.
At the start of each switching cycle, the oscillator will
set the logic control, which turns on the power switch
Q1. Then the inductor current will increase linearly.
The voltage, proportional to the inductor current and
sampled by the sense resistor, is added with oscillator
ramp voltage and the resulting voltage is fed into the
non-inversion input of comparator A2. When this
voltage exceeds the level at the inversion input of A2,
the output of A2 is changed and the logic control is
reset to turn off the switch. In this time interval, the
white LED current is provided by the output
capacitor. The output voltage drops slightly.
4. Typical Application
In Figure 3, the inductor L, the schottky diode and the
built-in switch build a typical boost converter. C1 and
C2 are the input and output capacitor respectively.
The white LEDs act as the load. RISET resistor is the
feedback resistor, which determines the LEDs'
current. In this application, the CTRL pin should be
connected to a high level voltage to enable the circuit
or a low level voltage to disable the circuit. It should
not be floating.
After the switch turns off, the inductor provides
current to the white LEDs and charges the output
capacitor. Due to negative voltage applied to the
inductor, the inductor current decreases linearly. The
switch will not turn on until the oscillator set logic
control again. The oscillator frequency is set at
1MHz.
It is clear that the voltage level at the inversion input
of A2 sets the peak current level to keep the output in
VIN
5V
CIN
10 µF
L 4.7µH
VIN
VOUT
SW
COUT
0.47µF
OV
CTRL
On/Off
D
AP3031
GND
3*13
FB
R ISET
Figure 3. Typical Application of AP3031 ( 3×13 WLEDs)
Oct. 2009
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
2
Application Note 1031
5. Components Selection
VCTRL
Inductor Selection
To obtain stable output current and greater output
current capability, the 4.7µH inductor is
recommended. The inductor should have low core
loss
at
1.0MHz
and
low
DCR.
CDRH5D16NP-4R7NC provided by Sumida is a
good choice.
VSW
5V/div
IL
500mA/div
VOUT
10V/div
Capacitor Selection
The small size of ceramic capacitors makes them
ideal for this application. Compared with other types
of capacitors, X7R and X5R types feature wider
voltage and temperature ranges. So, ceramic
capacitors such as those provided by Murata are
recommended. Considering that the operation
frequency is 1.0MHz, the 10µF input capacitor and
0.47µF output capacitor are acceptable.
Time
Standby and Dimming
To avoid audio noise and achieve high frequency
dimming, AP3031 set up a standby function. If logic
low time on CTRL pin is less than about 0.7ms and
then enable the IC, AP3031 will hold on standby
mode and start directly to achieve high frequency
dimming. Details please refer to Figure 5.
VCTRL
6. Application Hints
5V/div
<0.7ms
VSW
LED Current Control
Refer to Figure 3, the LED current is controlled by
the feedback resistor RISET. LEDs' current accuracy is
determined by the feedback voltage and resistor RISET,
so the precise resistors are preferred. The resistance
of RISET is in inverse proportion to the LED current
since the feedback reference is fixed at 200mV. The
relation for RISET and LED current (ILED) can be
expressed as below:
5V/div
IL
500mA/div
VOUT
10V/div
Time
80µs/div
Figure 5. Standby Waveform
200mV
ILED
Two typical types of dimming control circuit are
present as below. First, controlling CTRL Pin voltage
to change operation state is a good choice. Second,
changing the feedback voltage to get appropriate duty
and luminous intensity is also useful.
Over Voltage Protection
The AP3031 has an internal open load protection
circuit. When the LEDs are disconnected from circuit
or fail open, the output voltage is clamped at about
17V. The AP3031 will switch at a low frequency, and
minimize current to avoid input voltage drop.
(1) Adding a Control Signal to CTRL Pin
Adding a PWM signal to CTRL pin directly, the
AP3031 is turned on and off by this signal. When the
PWM frequency is lower than 1kHz(Typ.), the IC
works in the soft-start mode to dimming the light. On
contrary, when the PWM frequency is higher than
1kHz(Typ.), the IC works in the standby mode: the
converter ceaselessly switches off and directly starts
to achieve light dimming. This standby function
allows AP3031 to support high frequency dimming
(up to 25kHz or higher) to avoid audio noise. Great
notice should be taken to the dimming PWM
Soft Start
The AP3031 has an internal soft start circuit to limit
the inrush current during startup. If logic low time on
CTRL pin is more than about 0.7ms and then enable
the IC, AP3031 will start smoothly to protect the
supplier. The time of startup is controlled by internal
soft start capacitor. Details please refer to Figure 4.
Oct. 2009
80µs/div
Figure 4. Soft Start Waveform
VIN=5V, 3×13 LEDs, ILED=260mA
Resistor Selection
In order to obtain an accurate LED current, a high
precision resistor is needed.
RISET =
>0.7ms
5V/div
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
3
Application Note 1031
divider to FB pin can control the dimming. Changing
the DC voltage or resistor between the FB Pin and the
DC voltage can get appropriate luminous intensity.
Comparing with all kinds of PWM signal control, this
method features a stable output voltage and LEDs
current. Please refer to Figure 8.
frequency when it close to 1kHz, with duty cycle
changes, IC may work both in soft-start mode and
standby mode. This causes output current a
discontinuity change which is not wanted. More
details please refer to Figure 6 and Figure 7.
AP3031
AP3031
CTRL
FB
VDC
Figure 6. Dimming Control
Using a PWM Signal in CTRL Pin
R3
R1
R2
Effective
Feedback Voltage
Figure 8. Dimming Control
Using DC Voltage
VCTRL
2.5V/div
Second, using a filtered PWM signal can do it. The
filtered PWM signal can be considered as a varying
and adjustable DC voltage. Please refer to Figure 9.
VSW
5V/div
IL
500mA/div
Time
AP3031
20µs/div
FB
PWM
Figure 7. High Frequency
(25kHz) Dimming Waveform
R4
(2) Changing the Effective Feedback Voltage
There are two popular methods to change the
effective feedback voltage.
C
R3
Effective
Feedback Voltage
R2
R1
Figure 9. Dimming Control
Using Filtered PWM Voltage
First, adding a constant DC voltage through a resistor
Oct. 2009
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
4
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