AL8806 High Current MR16 Design Report »

Report number: DIAPR0165
Issue: 1
Project number: AL8806 User guide
Engineer: Silvestro Russo
Applications Report
Date: 26 January 2011
Customer: internal
Title: AL8806 preliminary Evaluation Board for MR16 user guide
AL8806 DESCRIPTION
AL8806EV2 is an evaluation board showing the application of the Diodes Incorporated AL8806 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 1300mA into two LEDs.
The operating voltage is nominally 12VAC or DC. The 10uH 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, 1300mA, is set with the 0R075 sense resistors, R1 and R2 in parallel.
The CTRL Terminal is not used in this application.
Warning: At 12V nominal operation with 1300mA output, the LED will be hot and very bright
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Figure 1: Top Layer
Figure 2: Bottom Layer
AL8806 DEVICE DESCRIPTION
The AL8806 is a continuous mode inductive driver in a MSOP8 EP 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 1500mA.
AL8806 DEVICE FEATURES
DEVICE APPLICATIONS
•
•
•
•
•
•
•
•
•
Drives one or more series-connected LEDs
LEDs up to 1500mA.
Internal 30V switch.
Wide input voltage: 6V to 30V.
Inherent open circuit LED protection.
Brightness control using DC or PWM.
Low-voltage halogen replacement LEDs
High Power LED driving.
Illuminated signs.
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AL8806 Device Packages, Pin and Definitions
SET
GND
GND
CTRL
1
2
3
4
AL8806
8
7
6
5
VIN
N/C
SW
SW
MSOP8 EP
AL8806 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) :
•
•
•
SET
4
VIN
5
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
DEVICE ORDER NUMBER
AL8806E5TA
AL8806EV2
Please note: Evaluation boards are subject to
availability and qualified sales leads.
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AL8806EV2 EVALUATION BOARD REFERENCE DESIGN
The AL8806EV2 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 1300mA with two 0R15 sense
resistor, R1 and R2, in parallel and the circuit operates in continuous mode at approximately 600kHz (with Dc
voltage), with a 10uH inductor and a two 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 AL8806 datasheet.
D1 DFLS240
SW
R2
0R15
VIN
Anode
Anode
C5
100nF
C3
100nF
GND
Figure 3: Schematic diagram
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GND
150uF 150uF
SW
CTRL
NC
D5
SBR2A30
D4
SBR2A30
150uF 150uF
L1
AL8806
4
GND
C7
3
C6
2
C2
TAB
C1
AC2
9
P2
7
P1
C4
1uF
U1
SET
AC1
8
D2
SBR2A30
Vin
D3
SBR2A30
1
R1 0R15
SW
Cathode
5
6
AL8806
10uH
Cathode
AL8806EV2 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 two LED, four capacitors of 150uF is needed.
C3 provides local decoupling for the AL8806 device U1. It is important that this is as close as possible to U1, as reflected in
the layout.
R1 and R2 set the LED current, in this case to 1300mA.
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.
AL8806 Operation
In normal operation, when voltage is applied at +Vin, the AL8806 internal NDMOS switch is turned on. Current
starts to flow through sense resistor, 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
the sense resistor. The internal circuit of the AL8806 senses the voltage across R1||R2 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||R2, 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||R2, which is sensed by the AL8806. A voltage proportional to the sense voltage across R1||R2 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||R2. Please refer to the datasheets for the threshold limits, AL8806 internal circuits, electrical
characteristics and parameters.
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AL8806EV2 Component list
QUANTITY
PCB IDENT
VALUE
DESCRIPTION
1
U1
AL8806
Diodes Zetex LED Driver IC
5
D1, D2, D3, D4, D5
DFLS230HL
Diodes Zetex freewheeling diode and
bridge
2
R1, R2
0R15
Resistor, 0805, +/-1%
Generic
SR732ATTDR150F
4
C1, C2, C6, C7
150uF 20v
SMD tantalum Kemet
T491X157K020AT
<+/-300ppm
KOA
D
case,
1
C3
100nF >=25v
X7R 0805
Generic
Kemet
C0805C104K5RAC
(50v)
NIC NMC0805X7R104K50TRPF (50v)
1
C4
100nF >=25v
X7R 1206 Generic
1
C5
100nF >=25v
X7R 0603 Generic
0
C6
1
L1
NOT FITTED
10uH
MSS7341– 103ML
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.
AL8806EV2 Basic operation at full voltage
1. Connect P1 and P2 to the power supply
2. Set the PSU to 12VAC or 12VDC
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3. Turn on the PSU and the LED will illuminate and the current should be approximately 1300mA.
Warning: Do not stare at the LED directly.
Changing the LED current
1. Remove R1||R2.
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 600kHz, 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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Performances
VIN
Power IN
VLED
ILED
Temp
Temp
Inductor
T Bridge
Efficiency
PFC
AL8806
12 DC
9.84W
6.69
1.27A
82.3
77.8
86C
86.3%
NA
12 Ac
10.2W
6.7V
1.26A
89.5C
78C
91.5C
82%
0.614
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Figure 4: Switching waveform with VIN=12V DC
Figure 5: AL8806 surface temperature with VIN=12VDC
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Figure 6: AL8806 surface temperature with VIN=12VAc
Figure 7: Bridge rectifier and freewheeling diode surface temperature with VIN=12VDAC
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IMPORTANT NOTICE
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