AL8808EV2 User Guide Issue 1

AL8808EV2 EVALUATION BOARD USER GUIDE
DESCRIPTION
The AL8808EV2 is an evaluation board showing the application of the Diodes Incorporated AL8808
LED driver device on a board suitable for use in an MR16 LED lamp. The board has four main
connections: two power inputs P1 and P2, and two LED connections Anode (A) and Cathode (K). 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 pin on the device is not used in this application.
Warning: The LED will be hot and very bright
12-20VDC
12VAC
Figure 1. AL8808EV2 evaluation board power input connection P1 and P2
Figure 2. AL8808EV2 evaluation board LED connection - Anode (A) and Cathode (K)
Issue 1 – January 2013
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AL8808EV2
AL8808 DEVICE DESCRIPTION
The AL8808 is a continuous mode inductive driver in a TSOT25 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.
AL8808 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.
• Built in over temperature protection.
• Brightness control using DC or PWM.
DEVICE APPLICATIONS
• LED flashlights.
• High Power LED driving.
• Low-voltage halogen replacement LEDs.
• Automotive lighting.
• Illuminated signs.
AL8808 Device Package, Pin and Definitions
TSOT25 package
Pin Definition
Name
Pin No
SW
1
GND
2
CTRL
3
SET
4
VIN
5
Description
Drain of NDMOS switch.
Ground (0V).
Internal voltage ref. pin (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
Connect a sense resistor, Rs, from the SET pin to VIN to sense the
nominal output current. Nominal Iout = 0.1/ R1
Input voltage: 6V to 30V. Decouple to ground with a 4.7uF or higher
ceramic capacitor.
ORDERING INFORMATION
EVALBOARD ORDER NUMBER
AL8808EV2
DEVICE ORDER NUMBER
AL8808WT-7
NOTE: Evaluation boards are subject to availability and qualified sales leads.
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AL8808EV2
AL8808EV2 EVALUATION BOARD REFERENCE DESIGN
The AL8808EV2 is configured to the reference design shown in Figure 3. For other reference designs
or further applications information, please refer to the AL8808 datasheet.
The operating voltage is a nominal 12V AC or DC. The nominal current is set at 680mA with a 0R15
sense resistor R1. The circuit operates in continuous mode at approximately 300kHz, (with DC
voltage), with a 33uH inductor and one LED.
•
P1 and P2 are the power input pads, feeding into the bridge rectifier, comprising of D2, D3,
D4 and D5. C5 offers 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 C2 is needed.
C1 is not fitted.
•
C3 provides local decoupling for the AL8808 device U1. It is important that this is as close as
possible to U1, as shown in the layout. C4 decouples the LED connections to minimise EMI
and to smooth the current.
•
L1 smooth’s the switching at LX into a DC current for the LED string. D1 operates as the
freewheeling diode, preventing large voltage spikes at LX.
•
L2 is not fitted, and the pads are shorted with a link on this evaluation board. Its purpose is to
reduce EMI. Both L2 and C6 form a filter circuit 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.
•
R1 sets the LED current. The 0R15 resistor shown gives a LED current of 680mA.
•
Optional extra parts are marked by an asterisk and are NOT fitted on the PCB.
Figure 3. Schematic diagram of full PCB including not fitted components options on PCB
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AL8808EV2
AL8808 Operation
In normal operation, when voltage is applied at +Vin, the AL8808 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 AL8808 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
AL8808. 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, AL8808 internal circuits, electrical
characteristics and parameters.
AL8808EV2 Component List
PCB IDENTITY
VALUE
DESCRIPTION
SUGGESTED SOURCE
U1
D1, D2, D3, D4,
D5
AL8808
DFLS240L
Diodes Inc.
R1
0R15
C2
C1
150uF 20v
---
C3
100nF >=25v
C4
1uF >=25v
C5
C6
L1
L2
100nF >=25v
--33uH
0R0
LED Driver IC
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)
X7R 1206 Generic
Kemet C1206105K5RAC7800
(50v)
NIC NMC1206X7R105K50F
(50v)
X7R 0603 Generic
Not Fitted
LPS6235 - 333MLB
Shorting link fitted
Diodes Inc.
Kemet
Kemet
---
Kemet
NIC Components
Kemet
NIC Components
Farnell
--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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AL8808EV2
AL8808EV2 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. 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 5V.
An on-line Calculator is available to speed up the design process at:
http://www.diodes.com/destools/calculators.html
PCB Layout
Figure 4. PCB top copper and component overlay
Figure 5. PCB bottom copper and component overlay
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AL8808EV2
PERFORMANCE
System Efficiency
The system efficiency depends on the sense resistor, supply voltage, switching inductor and the
number of LEDs.
With a 12VDC supply, L1=33uH and two series LEDs with LED current = 680mA, the switching
frequency is typically 310kHz, and the efficiency level is ~ 86%.
Using the same components, but with three series LEDs, the switching frequency is typically 180kHz
and the efficiency rises to ~ 89%.
For further advice, please contact our local FAE or contact our local sales offices
as listed below.
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AL8808EV2
INTENTIONALLY BLANK
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AL8808EV2
IMPORTANT NOTICE
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Diodes Incorporated products are specifically not authorized for use as critical components in life support devices or
systems without the express written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
A. Life support devices or systems are devices or systems which:
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Copyright © 2013, Diodes Incorporated
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