AP5724-EVM

AP5724
(application)
WHITE LED STEP-UP CONVERTER
Power Management Business Unit
PWM application note
apply to device family:
AP5724/25/26(SOT26) series
AP5724/25/26pcb_V2(SOT26) Rev. 1
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AP5724
(application)
WHITE LED STEP-UP CONVERTER
General Description
Features
•
•
•
•
•
•
•
•
Inherently Matched LED Current
High Efficiency: 84% Typical
Fast 1.2MHz Switching Frequency
Current limit and UVLO protections
Over Voltage Protect function built-in
Soft-Start function built-in
SOT26 and DFN2020C-6: Available in “Green” Molding
Compound (No Br, Sb)
Lead Free Finish/RoHS Compliant (Note 1)
The AP5724 is a step-up DC/DC converter specifically designed
to drive white LEDs with a constant current. The device can drive
2~6 LEDs in series from a Li-Ion cell. Series connection of the
LEDs provides identical LED currents resulting in uniform
brightness and eliminates the need for ballast resistors. The
AP5724 switches at 1.2MHz that allows the use of tiny external
components. A low 0.1V feedback voltage minimizes power loss
in the current setting resistor for better efficiency.
Applications
•
•
•
•
•
Cellular Phones
PDAs, Hand help Computers
Digital Cameras
MP3 Players
GPS Receivers
AP5724/25/26pcb_V2(SOT26) Rev. 1
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AP5724
(application)
WHITE LED STEP-UP CONVERTER
Functional Description
Inductor Selection
A 10µH~22µH inductor is recommended for most AP5724 applications. Although small size and high
efficiency are major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire
resistance).
Capacitor Selection
The small size of ceramic capacitors are ideal for AP5724 applications. X5R and X7R types are
recommended because they retain their capacitance over wider voltage and temperature ranges than other
types such as Y5V or Z5U. A 1µF input capacitor and a 1µF output capacitor are sufficient for most AP5724
applications.
Diode Selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for
AP5724 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the
diode, while the diode capacitance (CT or CD) represents the switching losses. For diode selection, both
forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current
ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant
switching losses at the 1.2MHz switching frequency of the AP5724. A Schottky diode rated at 100mA to
200mA is sufficient for most AP5724 applications.
LED Current Control
The LED current is controlled by the feedback resistor (RSET in Figure 1). The feedback reference is 0.1V.
The LED current is 0.1V/ RSET. In order to have accurate LED current, precision resistors are preferred (1% is
recommended). The formula and table for RSET selection are shown below.
AP5724 RSET = 0.1V/ILED (See Table 1)
AP5725 RSET = 0.25V/ILED
AP5726 RSET = 0.31V/ILED
Table 1. RSET Resistor Value Selection
ILED (mA)
5
10
15
20
30
AP5724 RSET (Ω)
20
10
6.6
5
3.3
AP5724/25/26pcb_V2(SOT26) Rev. 1
AP5725 RSET (Ω)
50
25
16.6
12.5
8.3
AP5726 RSET (Ω)
62
31
20.6
15.5
10.3
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AP5724
(application)
WHITE LED STEP-UP CONVERTER
Open-Circuit Protection
In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the
feedback voltage will be zero. The AP5724 will then switch at a high duty cycle resulting in a high output
voltage, which may cause the SW pin voltage to exceed the level of the over voltage protect function. The
OVP pin can detect the output voltage and monitor if the output voltage reach to the protect voltage level
(Figure 2). Once OVP is activated, SW pin stops switching.
L1
22uH
VIN
D1
COUT
1uF
C IN
1uF
V IN
SW
x
AP5724 OVP
EN
x
FB
GND
RSET
5
Figure 2. LED Driver with Open-Circuit Protection
Dimming Control
There are four different types of dimming control circuits:
1. Using a PWM Signal to EN Pin
With the PWM signal applied to the EN pin, the AP5724 is turned on or off by the PWM signal. The LEDs
operate at either zero or full current. The average LED current increases proportionally with the duty cycle of
the PWM signal. A 0% duty cycle will turn off the AP5724 and corresponds to zero LED current. A 100%
duty cycle corresponds to full current. The typical frequency range of the PWM signal is below 2kHz.
AP5724/25/26pcb_V2(SOT26) Rev. 1
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AP5724
(application)
WHITE LED STEP-UP CONVERTER
2. Using a DC Voltage
For some applications, the preferred method of brightness control is a variable DC voltage to
adjust the LED current. The dimming control using a DC voltage is shown in Figure 3. As the
DC voltage increases, the voltage drop on R2 increases and the voltage drop on RSET
decreases. Thus, the LED current decreases. The selection of R2 and R3 will make the
current from the variable DC source much smaller than the LED current and much larger than
the FB pin bias current. For VDC range from 0V to 2V, the selection of resistors in Figure 3
gives dimming control of LED current from 0mA to 20mA.
3. Using a Filtered PWM Signal
The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to
replace the variable DC voltage source in dimming control.
AP5724
FB
VDC
R3
100k
R2
5k
RSET
5
Figure 3. Dimming Control Using a DC Voltage
4. Using a Logic Signal
For applications that need to adjust the LED current in discrete steps, a logic signal can be
used as shown in Figure 4. RSET sets the minimum LED current (when the NMOS is off). RSET
sets how much the LED current increases when the NMOS is turned on.
AP5724
FB
RINC
Logic
Signal
RSET
Figure 4. Dimming Control Using a Logic Signal
AP5724/25/26pcb_V2(SOT26) Rev. 1
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AP5724
(application)
WHITE LED STEP-UP CONVERTER
Evaluation Board
VIN vs. Efficiency
100
3 LEDs
Efficiency(%)
90
4 LEDs
80
6 LEDs
70
60
50
40
2.5
3
3.5
VIN (V)
4
4.5
5
Specification
Symbol
Parameter
System Supply Input
VIN
Operating Input Voltage
UVLO Under Voltage Lockout
Under Voltage Lockout Hysteretic
IQ
Quiescent Current
ISD
Shutdown Current
Oscillator
FOSC Operation Frequency
Dmax Maximum Duty Cycle
Reference Voltage
VFB
Feedback Voltage
IFB
FB Pin Bias Current
MOSFET
Rds(on) On Resistance of MOSFET
IOCP
Switching Current Limit
Control and Protection
EN
Voltage High
EN
Voltage Low
IEN
EN Pin Pull Low Current
OVP OVP Threshold
θJA
Thermal Resistance Junction-to-Ambient
θJC
Thermal Resistance Junction-to-Case
Notes:
Conditions
Min
Typ.
Max
Unit
FB=0.2V, No Switching
VEN < 0.4V
2.7
-
2.2
85
500
0.1
5.5
2.4
1
V
V
mV
µA
µA
1
86
1.2
90
1.4
-
MHz
%
0.09
10
0.1
45
0.11
100
V
nA
0.5
-
0.75
750
1.0
-
Ω
mA
1.5
26
4
30
162
200
36
30
0.4
6
34
V
V
µA
V
o
C/W
o
C/W
o
C/W
o
C/W
Normal Operation
ON
OFF
SOT26 (Note 3)
DFN2020C-6 (Note 3)
SOT26 (Note 3)
DFN2020C-6 (Note 3)
3. Test condition for SOT26 and DFN2020C-6: Devices mounted on FR-4 substrate PC board, 2oz copper, with minimum recommended pad layout.
AP5724/25/26pcb_V2(SOT26) Rev. 1
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© Diodes Incorporated
AP5724
(application)
WHITE LED STEP-UP CONVERTER
Schematic
Test Circuit
VOUT
VIN
L1
C1
1uF
D2
D1
22uH
B0540WS
C2
1uF
U1
LED
1
2
D3
R1
5ohm
LED
3
SW
GND
Vin
OVP
FB
EN
6
5
4
ON
OFF
AP5724
D4
LED
D5
LED
D6
LED
D7
LED
Bill of Materials
Component
Location
Quantity
Specification
Mark
Maker Part No.
Size
TAIYO YUDEN
TMK212 B7105MG-T
C0805
22uH/0.88A
Wurth Electronics
744042220
4.8 X 4.8 X 1.6
22uH/0.56A
GOTREND
GLP3810PH220N
3.8 X 3.8 X 1.05
22uF/25V
C1,C2
2
(Ceramic
Capacitor)
L1
R1
D1
U1
1
1
1
1
(1%)
YAGEO
40V/0.5A
ZETEX
ZLLS400
SOD323
40V/0.5A
DIODES
B0540WS
SOD323
40V/0.25A
DIODES
SDM20U40
SOD523
AP5724_SOT26
Diodes
5Ω
AP5724/25/26pcb_V2(SOT26) Rev. 1
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R0805
SOT26
JULY 2009
© Diodes Incorporated
AP5724
(application)
WHITE LED STEP-UP CONVERTER
PCB Layout
Top Layer
AP5724/25/26pcb_V2(SOT26) Rev. 1
Bottom Layer
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© Diodes Incorporated
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