AL8807AEV3 User Guide

AL8807AEV3 EVALUATION BOARD USER GUIDE
DESCRIPTION
The AL8807AEV3, Figure 1, is a double sided evaluation board for the AL8807A step-down, or ‘buck’,
LED driver with internal switch. The evaluation board is preset to drive 1A into a single LED, or
multiple LEDs, the maximum number of which depends on their total forward voltage drop and the
supply voltage.
The operating voltage is between 6 and 36 volts, but it must be greater than the voltage drop across
the LED chain. 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 output current, 1A, is set with the 0R1 sense resistor, R1.
Terminal CTRL provides a connection point for DC dimming.
Warning: At 36V nominal operation with 1A output, the LED will be hot and very bright
+
V
1-8
LEDs
6-36VDC
Figure 1: AL8807AEV3 evaluation board and connection diagram
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AL8807AEV3
AL8807AEV3 Connection Point Definition
Name
Description
Vin
Positive supply voltage. 6 to 36V
GND
Supply Ground (0V).
CTRL
LED A
LED K
Internal voltage ref. pin (2.5V). This pin can be used for dimming.
Leave floating for full brightness.
LED A connects to the external LED anode
LED K connects to the external LED cathode
AL8807A DEVICE DESCRIPTION
The AL8807A 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 1A.
AL8807A DEVICE FEATURES
• Drives one or more series-connected LEDs
• LEDs up to 1A.
• Internal 40V switch.
• Wide input voltage: 6V to 36V.
• Inherent open circuit LED protection.
• Brightness control using a DC voltage.
DEVICE APPLICATIONS
• LED flashlights.
• High Power LED driving.
• Low-voltage halogen replacement LEDs.
• Automotive lighting.
• Illuminated signs.
AL8807A Device Packages, Pin and Definitions
SET
1
8
GND
2
7
6
5
GND 3
CTRL 4
VIN
N.C.
SW
SW
Top View
SOT25 package
MSOP8EP package
AL8807A 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.
• Drive with DC voltage (0.25V to 2.5V) to adjust output current
SET
4
Connect a sense resistor, R1, from the SET pin to VIN to sense the
nominal output current. Nominal Iout = 0.1/ R1
VIN
5
Input voltage: 6V to 36V. Decouple to ground with a 2.2uF or higher
ceramic capacitor.
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AL8807AEV3
ORDERING INFORMATION
EVALBOARD ORDER
NUMBER
AL8807AEV3
DEVICE ORDER NUMBER
AL8807AMP-13
Please note: Evaluation boards are subject to
availability and qualified sales leads.
AL8807AEV3 EVALUATION BOARD REFERENCE DESIGN
The AL8807AEV3 is configured to the reference design in Figure 2.
The operating voltage is up to 36V. The nominal current is set at 1A with a 0R1 sense resistor R1.
The circuit operates in continuous mode between approximately 40kHz and 130kHz, depending on
the input voltage, with a 68uH inductor and one LED.
For DC dimming, the CTRL pin may be driven between 0.25V and 2.5V adjusting the output current
from approximately 10% to 100% of ILED. Shorting R3 will connect the test pin CTRL directly to device
pin CTRL, if required.
For other reference designs or further applications information, please refer to the AL8807A
datasheet.
Figure 2: Schematic diagram
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AL8807AEV3
AL8807A Operation
In normal operation, when a voltage is applied at +Vin, the AL8807A 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 AL8807A 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
AL8807A. 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, AL8807A internal circuits, electrical
characteristics and parameters.
AL8807AEV3 Component list
Ref
Value
Package
Part Number
U1
AL8807A
MSOP
AL8807AMP-13
D1
60V, 2A
R1
R2
0R1
R3
C1, C5
1k0
4.7uF 50V
0805
1210
C2, C3
100nF, 100V
0805
DFLS260
1206
NOT FITTED
C1210X475K5RAC
NMC0805X7R104K100 PF
GRM21BR71H104KA01L
C4
Manufactu
rer
Diodes
Zetex
Diodes
Zetex
Generic
1uF 100V
1206
NMC1206X7R104K100
C6, C7,
C8, C9
C12
0805
Generic
Generic
KEMET
Generic
NIC Comps
MURATA
Generic
NIC Comps
Generic
NOT FITTED
C10, C11
1n5, 100V
L1
68uH
0805
Generic
MSS1038-683ML
NPIS24H680MTRF
NPIS104F680MTRF
Notes
DC-DC converter
Schottky diode
1% 250mW
Fit with R1 to
adjust LED
current if
required
1% 125mW
X7R
X7R
X7R
Fit to improve
EMC
performance if
required
C0G
Coilcraft
NIC Comps
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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AL8807AEV3
AL8807AEV3 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 36V
3. Turn on the PSU. The LED will illuminate and the current should be approximately 1A.
Warning: Do not stare at the LED directly.
Minimising the output current
Shorting the CTRL pin to GND will cause the LED current to be reduced. On some boards, however,
the LED may not be completely turned off.
Soft start
Adding capacitor C3 will create a soft-start power-up sequence (0.1ms/nF).
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 2.5V.
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 12V supply and two LEDs, the switching frequency is typically 102kHz, and the efficiency level
is 88%.
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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AL8807AEV3
IMPORTANT NOTICE
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