AL8808EV1 User Guide Issue 1

AL8808EV1 EVALUATION BOARD USER GUIDE
DESCRIPTION
The AL8808EV1, Figure 1, is a double sided evaluation board for the AL8808 step-down, or ‘buck’,
LED driver with internal switch. The evaluation board is preset to drive 680mA into a single LED, or
multiple LEDs, the maximum number of which depends on their total forward voltage drop and the
supply voltage. (The maximum drive current of the AL8808 is 1000mA)
The operating voltage is nominally 30 volts, but it can be reduced to a minimum of 6 volts. The 68uH
inductor used in the circuit is based on this nominal supply. The evaluation board should be
connected as in Figure 1 below.
Note: The evaluation board does not have reverse supply protection.
The nominal current, 680mA, is set with the 0R15 sense resistor, R1.
Terminal CTRL provides a connection point for DC or PWM dimming and shutdown.
Warning: At 30V nominal operation with 680mA output, the LED will be hot and very bright
+
V
1-8
LEDs
6-30VDC
Figure 1. AL8808EV1 evaluation board and connection diagram
AL8808EV1 Connection Point Definition
Name
Description
Vin
Positive supply voltage: 6 to 30V
GND
Supply Ground (0V).
CTRL
LED A
LED K
Internal voltage ref. pin (5V). This pin can be used to achieve dimming and for
switching the output current off. Leave floating for normal operation.
LED A connects to the external LED anode.
LED K connects to the external LED cathode.
Issue 1 – January 2013
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AL8808EV1
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 (up to 5V
logic level) signal to adjust output current.
Connect a sense resistor, R1, 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 2.2uF or higher
ceramic capacitor.
ORDERING INFORMATION
EVALBOARD ORDER NUMBER
AL8808EV1
DEVICE ORDER NUMBER
AL8808WT-7
NOTE: Evaluation boards are subject to availability and qualified sales leads.
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AL8808EV1
AL8808EV1 EVALUATION BOARD REFERENCE DESIGN
The AL8808EV1 is configured to the reference design shown in Figure 2.
The operating voltage is a nominal 30V. The nominal current is set at 680mA with a 0R15 sense
resistor R1. The circuit operates in continuous mode at approximately 188kHz, with a 68uH inductor
and one LED.
•
Both DC and PWM dimming can be achieved by driving the CTRL pin. For DC dimming, the
CTRL pin may be driven between 0.5V and 2.5V adjusting the output current from 25% to
100% of ILED.
•
Driving the CTRL pin below 0.4V will shut down the output current.
•
A PWM signal (low level ≤ 0.4V and high level > 2.5) allows the output current to be adjusted
above or below the level set by the resistor connected to SET input pin. The PWM frequency
can be between 100Hz to 1kHz, providing a resolution of 10 bits.
•
For low frequency PWM, C2 should be removed from the evaluation board, to give a more
accurate duty cycle.
•
Shorting R2 will connect the test pin CTRL to device pin CTRL, if required.
For other reference designs or further applications information, please refer to the AL8808 datasheet.
Figure 2. Schematic diagram
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AL8808EV1
AL8808 Operation
In normal operation, when a 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, the ramp rate being 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.
AL8808EV1 Component list
Ref
Value
Package
Part Number
Manufacturer
Notes
U1
AL8808
TSOT25
AL8808WT-7
Diodes Zetex
D1
R1
R2
C1
40V, 2A
0R15
0R
4.7uF 50V
B240A
1206
0805
1210
Diodes Zetex
Generic
Generic
Generic
KEMET
DC-DC
converter
Schottky diode
1%
1%
X7R
C2
Not Fitted
C3
100nF, 100V
0805
NMC0805X7R104K100 PF
GRM21BR71H104KA01L
C4
100nF 100V
1206
NMC1206X7R104K100
L1
68uH
C1210X475K5RAC
MSS1038-683ML
NPIS24H680MTRF
Generic
NIC Comps
MURATA
Generic
NIC Comps
Coilcraft
NIC Comps
Optional soft
start capacitor
X7R
X7R
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.
AL8808EV1 Basic operation at full voltage
1. Connect Vin and GND
Warning: The board does not feature reverse battery/supply protection.
2. Set the PSU to 30V
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.
Switching the output current off
Shorting the CTRL pin to GND will cause the LED current to go to zero.
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AL8808EV1
Soft start
Adding a C2 capacitor will create a soft-start power-up sequence (0.1ms/nF). This delay will reduce
the PWM dimming performance.
Changing the LED current
1. Remove R1.
2. Calculate and replace 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.
The device calculator at the address below can be used to speed up the redesign phase:
http://www.diodes.com/destools/calculators.html
PERFORMANCE
The system efficiency depends on the sense resistor, supply voltage, switching frequency and the
number of LEDs.
With a 30V supply and one LED, the switching frequency is typically 188kHz. The efficiency level is
>85%.
For further advice, please contact your local Diodes Field Applications Engineer, or one of our
sales offices listed on the back page of this document.
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AL8808EV1
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