680mA Buck for one to three LEDs in series (AL8805EV2)

AL8805EV2 EVALUATION BOARD USER GUIDE
DESCRIPTION
AL8805EV2 is an evaluation board showing the application of the Diodes Incorporated AL8805 LED driver device
on a board suitable for use in an MR16 lamp.
The board has four main connections: two power inputs P1 and P2, and two LED connections Anode (A) and
Cathode (C). The evaluation board is preset to drive 680mA into a single LED.
The operating voltage is nominally 12VAC or DC. The 33uH inductor used in the circuit is based on
this nominal supply, which should be connected across the P1 and P2 pins.
Note: The input bridge rectifier provides the board with reverse battery protection.
The nominal current, 680mA, is set with the 0R15 sense resistor, R1.
The CTRL Terminal is not used in this application.
Warning: At 12V nominal operation with 680mA output, the LED will be hot and very bright
12-20VDC
12VAC
Figure 1: AL8805EV2 evaluation board Power input connection P1 and P2
Figure 2: AL8805EV2 evaluation board LED connection Anode (A) and Cathode (K)
Issue 1 – August 2010
© Diodes Incorporated, 2010
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AL8805EV2
AL8805 DEVICE DESCRIPTION
The AL8805 is a continuous mode inductive driver in a SOT25 package, for driving one or more
series-connected LEDs efficiently from a voltage source higher than the LED voltage. The device
includes the output switch and a current sense circuit, which requires an external sense resistor to set
the nominal current up to 1000mA.
AL8805 DEVICE FEATURES
• Drives one or more series-connected LEDs
• LEDs up to 1000mA.
• Internal 30V switch.
• Wide input voltage: 6V to 30V.
• Inherent open circuit LED protection.
• Brightness control using DC or PWM.
DEVICE APPLICATIONS
• Low-voltage halogen replacement LEDs
• High Power LED driving.
• Illuminated signs.
AL8805 Device Packages, Pin and Definitions
SOT25 pack
AL8805 Device Pin Definition
Name
Pin No
Description
SW
1
Drain of NDMOS switch.
GND
2
Ground (0V).
CTRL
3
Internal voltage ref. pin (2.5V) :
• Leave floating for normal operation.
• Connect to GND to turn off output current.
• Drive with DC voltage (0.4V to 2.5V) or with PWM signal to
Adjust output current
SET
4
Connect a sense resistor, Rs, from the SET pin to VIN to sense the
nominal output current. Nominal Iout = 0.1/ R1
VIN
5
Input voltage: 6V to 30V. Decouple to ground with a 4.7uF or higher
ceramic capacitor.
ORDERING INFORMATION
EVALBOARD ORDER
NUMBER
AL8805EV2
DEVICE ORDER NUMBER
AL8805E5TA
Please note: Evaluation boards are subject to
availability and qualified sales leads.
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AL8805EV2
AL8805EV2 EVALUATION BOARD REFERENCE DESIGN
The AL8805EV2 is configured to the reference design in Figure 3.
The operating voltage is a nominal 12V AC or DC. The nominal current is set at 680mA with a 0R15
sense resistor, R1 and the circuit operates in continuous mode at approximately 300kHz (with Dc
voltage), with a 33uH inductor and a single LED.
An accurate way of determining the current is to measure the voltage on the sense resistor. A 10K
resistor and a 1uF capacitor can be used to form a low pass filter, the voltage across the capacitor
representing a more stable DC reading of current. 100mV represents 1 Amp when using a 0R1 sense
resistor.
For other reference designs or further applications information, please refer to the AL8805 datasheet.
Figure 3: Schematic diagram
AL8805EV2 Circuit description
P1 and P2 are the power input pads, feeding into the bridge rectifier, comprising of D2, D3, D4 and D5. C5 offers
an optional EMI filtering at the input pads.
C1 and C2 form the bulk reservoir capacitance, used to sustain operation of the device during the low part of the
AC wave. In the case of a system driving a single LED, only one capacitor of 150uF is needed.
L2 and C6 are a filter to reduce conducted EMI if necessary. In many cases they may not be required, and where
that is so then L2 pads are shorted out and C6 is not fitted.
C3 provides local decoupling for the AL8805 device U1. It is important that this is as close as possible to U1, as
reflected in the layout.
R1 sets the LED current, in this case to 680mA.
C4 decouples the LED connections, again to minimise EMI, as well as smoothing the current.
L1 smoothes the switching at LX into a DC current for the LED string. D1 operates as the freewheeling diode,
preventing large voltage spikes at LX.
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AL8805EV2
AL8805 Operation
In normal operation, when voltage is applied at +Vin, the AL8805 internal NDMOS switch is turned on.
Current starts to flow through sense resistor R1, inductor L1, and the LED. The current ramps up
linearly, and the ramp rate is determined by the input voltage +Vin and the inductor L1. This rising
current produces a voltage ramp across R1. The internal circuit of the AL8805 senses the voltage
across R1 and applies a proportional voltage to the input of the internal comparator. When this
voltage reaches an internally set upper threshold, the NDMOS switch is turned off. The inductor
current continues to flow through R1, L1, the LED and the schottky diode D1, and back to the supply
rail, but it decays, with the rate of decay determined by the forward voltage drop of the LEDs and the
schottky diode. This decaying current produces a falling voltage at R1, which is sensed by the
AL8805. A voltage proportional to the sense voltage across R1 is applied at the input of the internal
comparator. When this voltage falls to the internally set lower threshold, the NDMOS switch is turned
on again. This switch-on-and-off cycle continues to provide the average LED current set by the sense
resistor R1. Please refer to the datasheets for the threshold limits, AL8805 internal circuits, electrical
characteristics and parameters.
AL8805EV2 Component list
QUANTITY
1
PCB
IDENT
VALUE
SUGGESTED
SOURCE
DESCRIPTION
AL8805
LED Driver IC
Diodes Inc.
1
U1
D1, D2,
D3, D4,
D5
DFLS240L
Diodes Inc.
1
R1
0R15
1
1
C2
C1
150uF 20v
---
1
C3
100nF >=25v
Input bridge & freewheeling diode
Resistor, 0805, +/-1% <+/-300ppm
Generic
KOA
SR732ATTDR150F
SMD tantalum Kemet D case,
T491X157K020AT
Not Fitted
X7R 0805 Generic
Kemet C0805C104K5RAC (50v)
NIC NMC0805X7R104K50TRPF (50v)
1uF >=25v
----33uH
-----
X7R 1206 Generic
Kemet C1206105K5RAC7800 (50v)
NIC NMC1206X7R105K50F (50v)
Not Fitted
Not Fitted
LPS6235 - 333MLB
Not Fitted
1
1
1
1
1
C4
C5
C6
L1
L2
Kemet
Kemet
Kemet
NIC Components
Kemet
NIC Components
Coilcraft
Note: The component part numbers are correct at the time of publication. Diodes Inc reserves the
right to substitute other parts where necessary, without further notification.
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AL8805EV2
AL8805EV2 Basic operation at full voltage
1. Connect P1 and P2 to the power supply
2. Set the PSU to 12VAC or 12VDC
3. Turn on the PSU and the LED will illuminate and the current should be approximately 680mA.
Warning: Do not stare at the LED directly.
Changing the LED current
1. Remove R1.
2. Calculate and fit a new sense resistor, R1, the value of which is based on the required LED
current without dimming. R1 can be calculated using following equation :
R1 = 0.1V/IOUT
where IOUT = the LED current.
R1 = the sense resistor value in ohms.
0.1V is the nominal sense voltage with ‘CTRL’ open circuit or set to 2.5V.
An on-line Calculator is available to speed up the design process at:
http://www.diodes.com/destools/calculators.html
PERFORMANCE
The system efficiency depends on the sense resistor, supply voltage, switching inductor and the
number of LEDs.
With a 12VDC supply, the switching frequency is typically 300kHz, and the efficiency level is >80% .
For further advice, please contact our local FAE or contact our local sales offices as listed
below.
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AL8805EV2
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AL8805EV2
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