1MHz, High-Efficiency, Step-Up Converter for 2 to 6 White LEDs
Features
•Wide Input Voltage from 2.5V to 6V
•104mV Reference Voltage
•Fixed 1MHz Switching Frequency
•High Efficiency up to 88%
•100Hz to 100kHz PWM Brightness Control Fre-
quency
•Open-LED Protection
•Under-Voltage Lockout Protection
•Over-Temperature Protection
•<1µA Quiescent Current During Shutdown
•SOT-23-6 Package
•Lead Free and Green Devices Available
(RoHS Compliant)
Applications
•White LED Display Backlighting
General Description
The APW7208 is a current-mode and fixed frequency
boost converter with an integrated N-FET to drive up to 6
white LEDs in series.
The series connection allows the LED current to be identical for uniform brightness. Its low on-resistance of NFET and feedback voltage reduces power los s and
achieves high efficiency. Fast 1MHz current-mode PWM
operation is available for input and output capacitors and
a small inductor while minimizing ripple on the input
supply. The OVP pin monitors the output voltage and stops
switching if exceeds the over-voltage threshold. An internal soft-start circuit eliminates the inrush current during
start-up.
The APW7208 also integrates under-voltage lockout,
over-temperature protection and current limit circuits.
The APW7208 is available in a SOT-23-6 package.
Pin Configuration
•Cell Phone and Smart Phone
•PDA, PMP, and MP3
LX 1
GND 2
FB 3
6 VIN
5 OVP
4 EN
•Digital Camera
SOT-23-6
(Top View)
Simplified Application Circuit
C2
1µF
V
OUT
Up to 6
WLEDs
R1
5.1Ω
www.anpec.com.tw1
V
IN
C1
2.2µF
OFF ON
ANPEC reserves the right to make changes to improve reliability or manufacturability without notice, and
advise customers to obtain the latest version of relevant information to verify before placing orders.
Note : ANPEC lead-free products contain molding compounds/die attach materials and 100% matte tin plate termination finish; which
are fully compliant with RoHS. ANPEC lead-free products meet or exceed the lead-free requirements of IPC/JEDEC J-STD-020D for
MSL classification at lead-free peak reflow temperature. ANPEC defines “Green” to mean lead-free (RoHS compliant) and halogen
free (Br or Cl does not exceed 900ppm by weight in homogeneous material and total of Br and Cl does not exceed 1500ppm by
weight).
W08X
Package Code
C : SOT-23-6
Operating Ambient Temperature Range
I : -40 to 85 oC
Handling Code
TR : Tape & Reel
Assembly Material
G : Halogen and Lead Free Device
X - Date Code
Absolute Maximum Ratings(Note 1)
Symbol
VIN VIN Supply Voltage (VIN to GND) -0.3 ~ 7 V
FB, EN to GND Voltage -0.3 ~ VIN V
VLX LX to GND Voltage -0.3 ~ 34 V
V
OVP to GND Voltage -0.3 ~ 32 V
OVP
TJ Maximum Junction Temperature 150 °C
T
Storage Temperature -65 ~ 150 °C
STG
T
Maximum Lead Soldering Temperature, 10 Seconds 260 °C
SDR
Note 1: Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only and functional operation of the device at these or any other conditions beyond those indicated in the operational
sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device
reliability.
Parameter Rating Unit
Thermal Characteristics (Note 2)
Symbol
θJA
Note 2: θJA is measured with the component mounted on a high effective thermal conductivity test board in free air. The exposed pad
of package is soldered directly on the PCB.
Junction to Ambient Thermal Resistance
Parameter Typical Value Unit
(Note 2)
SOT-23-6
250
°C/W
Recommended Operating Conditions (Note 3)
Symbol
VIN VIN Input Voltage 2.5 ~ 6 V
V
Converter Output Voltage Up to 24 V
OUT
CIN Input Capacitor 2.2 or higher
C
Output capacitor 0.47 or higher
OUT
L1 Inductor 6.8 ~ 22
TA Ambient Temperature -40 ~ 85 °C
TJ Junction Temperature -40 ~ 125 °C
Note 3: Refer to the application circuit for further information.
1 LX Switch pin. Connect this pin to inductor/diode here.
2 GND Power and signal ground pin.
3 FB
4 EN
5 OVP Over-Voltage Protection pin. OVP is connected to the output capacitor of the converter.
6 VIN
Feedback Pin. Reference voltage is 104mV. Connect this pin to cathode of the lowest LED and
resistor (R1). Calculate resistor value according to R1=104mV/I
Enable Control Input. Forcing this pin above 1.0V enables the device, or forcing this pin below 0.4V
to shut it down. In shutdown, all functions are disabled to decrease the supply current below 1µA. Do not leave this pin floating.
Main Supply Pin. Must be closely decoupled to the GND with a 2.2µF or greater ceramic capacitor.
The APW7208 is a constant frequency current-mode
switching regulator. During normal operation, the internal N-channel power MOSFET is turned on each cyc le
when the oscillator sets an internal RS latch and turned
off when an internal comparator (ICMP) resets the latch.
The peak inductor current at which ICMP resets the RS
latch is controlled by the voltage on the COMP node, which
is the output of the error amplifier (EAMP). An external
resistive divider connected between V
and ground al-
OUT
lows the EAMP to receive an output feedback voltage V
at FB pin. When the load current increases, it causes a
slightly decrease in VFB relative to the 104mV reference,
which in turn causes the COMP voltage to increase until
the average inductor c urrent matches the new load
current.
VIN Under-Voltage Lockout (UVLO)
The Under-Voltage Lockout (UVLO) circuit compares the
input voltage at VIN with the UVLO threshold (2.3V rising,
typical) to ensure the input voltage is high enough for
reliable operation. The 100mV (typ) hysteresis prevents
supply transients from causing a restart. Once the input
voltage exceeds the UVLO rising threshold, start-up
begins. When the input voltage falls below the UVLO falling threshold, the controller turns off the converter.
Soft-Start
The APW7208 has a built-in soft-start to control the Nchannel MOSFET current rise during start-up. During softstart, an internal ramp, connected to one of the inverting
inputs, raise up to replace the output voltage of error amplifier until the ramp voltage reaches the V
COMP
.
Current-Limit Protection
The APW7208 monitors the inductor current, flowing
through the N-c hannel MOSFET, and limits the current
peak at current-limit level to prevent loads and the
APW7208 from damages during overload or short-circuit
conditions.
Over-Temperature Protection (OTP)
The over-temperature circuit limits the junction temperature of the APW7208. When the junction temperature exceeds 150οC, a thermal sensor turns off the power
MOSFET, allowing the devices to cool. The thermal sensor allows the converters to start a soft-start process and
to regulate the output voltage again after the junction temperature cools by 40οC. The OTP is designed with a 40οC
hysteresis to lower the average Junction Temperature
(TJ) during continuous thermal overload conditions, increasing the lifetime of the device.
FB
Enable/Shutdown
Driving EN to the ground places the APW7208 in shutdown mode. When in shutdown, the internal power
MOSFET turns off, all internal circuitry shuts down and
the quiescnet supply current reduces to 1µA maximum.
This pin also could be used as a digital input allowing
brightness control using a PWM signal from 100Hz to
100kHz. The 0% duty cycle of PWM signal corresponds to
zero LEDs current and 100% corresponds to full one.
Open-LED Protection
In driving LED applications, the feedback voltage on the
FB pin falls down if one of the LEDs, in series, is failed.
Meanwhile, the c onverter unceasingly boosts the output
voltage lik e a open-loop operation. Therefore, an overvoltage protection (OVP), monitoring the output voltage
via OVP pin, is integrated into the chip to prevent the LX
and the output voltages from exceeding their maximum
voltage ratings. When the voltage on the OVP pin rises
above the OVP threshold (28V typical), the converter stops
switching and prevents the output voltage from rising.
The converter can work again when the falling OVP voltage falls below the OVP voltage thres hold.
The input capacitor (CIN) reduces the ripple of the input
current drawn from the input supply and reduces noise
injection into the IC. The reflected ripple voltage will be
smaller when an input capacitor with larger c apacitance
is used. For reliable operation, it is recommended to
select the capacitor with maximum voltage rating at least
1.2 times of the maximum input voltage. The capacitors
should be placed close to the VIN and the GND.
Inductor Selection
Selecting an inductor with low dc resistance reduces conduction losses and achieves high efficiency. The efficiency
is moderated while using small chip inductor which operates with higher inductor core losses. Therefore, it is
necessary to take further consideration while choosing
an adequate inductor. Mainly, the inductor value determines the inductor ripple current: larger inductor value
results in smaller inductor ripple current and lower c onduction losses of the converter. However, larger inductor
value generates slower load transient response. A reasonable design rule is to set the ripple current, ∆IL, to be
30% to 50% of the maximum average inductor current,
I
. The inductor value can be obtained as below,
L(AVG)
2
V
IN
≥
L
×
V
OUT
−
VV
INOUT
⋅
IF
η
×
)MAX(OUTSW
∆
I
L
I
()
AVGL
where
VIN = input voltage
V
= output voltage
OUT
FSW = switching frequency in MHz
I
= maximum output current in amp.
OUT
η = Efficiency
∆IL /I
= inductor ripple current/average current
L(AVG)
(0.3 to 0.5 typical)
To avoid the saturation of the inductor, the inductor should
be rated at least for the maximum input current of the
converter plus the inductor ripple current. The maximum
input current is c alculated as below:
⋅
VI
=
I
)MAX(IN
OUT)MAX(OUT
η⋅
V
IN
The peak inductor current is c alculated as the following
equation:
VVV21
−⋅
LX
INOUTIN
FLV
⋅⋅
SWOUT
D1
I
SW
I
PEAK
I
V
OUT
OUT
ESR
C
OUT
I
LIM
∆I
L
I
IN
I
OUT
II
V
I
IN
IN
C
IN
I
L
I
SW
I
D
⋅+=
)MAX(INPEAK
I
L
N-FET
Output Capacitor Selection
The current-mode c ontrol scheme of the APW7208 allows the usage of tiny ceramic capacitors. The higher
capacitor value provides good load transients response.
Ceramic capacitors with low ESR values have the lowest
output voltage ripple and are recommended. If required,
tantalum capacitors may be used as well. The output ripple
is the sum of the voltages across the ESR and the ideal
output capacitor.
ΔV
where I
= ΔV
OUT
V
COUT
is the peak inductor current.
PEAK
+ ΔV
ESR
I
OUT
C
OUT
RIV⋅≈∆
COUT
⋅≈∆
ESRPEAKESR
−
VV
INOUT
⋅
FV
SWOUT
Copyright ANPEC Electronics Corp.
www.anpec.com.tw9
Rev. A.4 - Aug., 2009
APW7208
Recommended
Inductor
Selection
Application Information (Cont.)
Output Capacitor Selection (Cont.)
For ceramic capacitor application, the output voltage ripple
is dominated by the ∆V
. When choosing the input and
COUT
output c eramic capacitors, the X5R or X7R with their
good temperature and voltage charac teristics are
rec ommended.
Diode Selection
Setting the LED Current
In figure 1, the converter regulates the voltage on the FB
pin, connected with the cathod of the lowest LED and the
current-sense resistor R1, at 104mV (typical). Therefore,
the current (I
), flowing via the LEDs and the R1, is cal-
LED
culated by the following equation:
I
= 104mV/R1
LED
To achieve high efficiency, a Schottky diode must be used.
The current rating of the diode must meet the peak current rating of the converter.
For all switching power supplies, the layout is an important step in the design; especially at high peak currents
and switching frequencies. If the layout is not carefully
done, the regulator might show noise problems and duty
cycle jitter.
1. The input c apacitor should be placed close to the VIN
and the GND. Connec ting the capacitor with VIN and
GND pins by short and wide tracks without using any
vias for filtering and minimizing the input voltage ripple.
2. The inductor should be placed as close as possible to
the LX pin to minimize length of the copper tracks as
well as the noise coupling into other circuits .
3. Since the feedback pin and network is a high impedance c ircuit, the feedback network should be routed
away from the inductor. The feedback pin and feedback network should be shielded with a ground plane
or track to minimize noise coupling into this circuit.
4. A star ground connection or ground plane minimizes
ground shifts and noise is recommended.