Data Sheet

ManualsBrandsSparkfun Electronics ManualsLightingPicoBuck LED Driver

AL8805

Document number: DS35030 Rev. 4 - 2

1 of 16

www.diodes.com

July 2012

L8805

HIGH EFFICIENCY 36V 1A BUCK LED DRIVER

Description

The AL8805 is a step-down DC/DC converter designed to drive LEDs

with a constant current. The device can drive up to 8 LEDs,

depending on the forward voltage of the LEDs, in series from a

voltage source of 6V to 36V. Series connection of the LEDs provides

identical LED currents resulting in uniform brightness and eliminating

the need for ballast resistors. The AL8805 switches at frequency up to

1MHz. This allows the use of small size external components, hence

minimizing the PCB area needed.

Maximum output current of AL8805 is set via an external resistor

connected between the V

and SET input pins. Dimming is achieved

by applying either a DC voltage or a PWM signal at the CTRL input

pin. An input voltage of 0.4V or lower at CTRL switches off the output

MOSFET simplifying PWM dimming.

Features

• LED Driving Current up to 1A

• Better than 5% Accuracy

• High Efficiency up to 98%

• Operating Input Voltage from 6V to 36V

• High Switching Frequency up to 1MHz

• PWM/DC Input for Dimming Control

• Built-In Output Open-Circuit Protection

• SOT25: Available in “Green” Molding Compound (No Br,Sb) with

lead Free Finish/ RoHS Compliant

 Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2)

 Halogen and Antimony Free. “Green” Device (Note 3)

Pin Assignments

4SET

CTRL

GND

(Top View)

SOT25

Applications

• MR16 Lamps

• General Illumination Lamps

Notes: 1. No purposely added lead. Fully EU Directive 2002/95/EC (RoHS) & 2011/65/EU (RoHS 2) compliant.

2. See http://www.diodes.com for more information about Diodes Incorporated’s definitions of Halogen- and Antimony-free, "Green" and Lead-free.

3. Halogen- and Antimony-free "Green” products are defined as those which contain <900ppm bromine, <900ppm chlorine (<1500ppm total Br + Cl) and

<1000ppm antimony compounds.

Typical Applications Circuit

Summary of content (16 pages)

PAGE 1
AL8805 HIGH EFFICIENCY 36V 1A BUCK LED DRIVER Description Pin Assignments The AL8805 is a step-down DC/DC converter designed to drive LEDs (Top View) with a constant current. The device can drive up to 8 LEDs, depending on the forward voltage of the LEDs, in series from a voltage source of 6V to 36V. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors. The AL8805 switches at frequency up to 1MHz.
PAGE 2
AL8805 Pin Descriptions Pin Number Pin Name 1 SW 2 GND Function Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to reduce EMI. GND Pin Dimming and On/Off Control Input. 3 CTRL 4 SET 5 VIN • Leave floating for normal operation. (VCTRL = VREF = 2.5V giving nominal average output current IOUTnom = 0.1/RS) • Drive to voltage below 0.4V to turn off output current • Drive with DC voltage (0.5V < VCTRL < 2.
PAGE 3
AL8805 Electrical Characteristics (VIN = 12, @TA = +25°C, unless otherwise specified.) Symbol Parameter Conditions VINSU Internal Regulator Start Up Threshold VINSH Internal Regulator Hysteresis Threshold VIN falling Quiescent Current Output not switching (Note 4) IS Input Supply Current CTRL pin floating f = 250kHz Set Current Threshold Voltage VTH-H Max Unit 5.9 V 300 mV 350 µA 1.
PAGE 4
AL8805 Typical Performance Characteristics (@TA = +25°C, unless otherwise specified.) 400 900 VCRTL = 0V VSET = VIN T A = 25°C 350 700 FREQUENCY (kHz) 250 IIN (µA) VIN = 12V 1 LED RSET = 150mΩ TA = 25°C 800 300 200 150 600 L = 68µH 500 400 300 100 200 50 L = 100µH 100 0 0 0 18 30 36 24 VIN (V) Figure 1 Supply Current (not switching) vs. Input Voltage 6 12 1 0.9 0.8 0 1 2 3 4 5 VCTRL (V) Figure 2 Switching Frequency vs.
PAGE 5
AL8805 4.5 0.9 4.0 0.8 3.5 0.7 3.0 0.6 2.5 0.5 2.0 0.4 1.5 0.3 1.0 0.2 0.5 0.1 0.0 0% 300 240 180 120 VCTRL = Open VSET = VIN TA = 25 °C 60 0 40% 60% 80% 100% PWM DUTY CYCLE Figure 7 ILED vs. PWM Duty Cycle 0 20% 400 6 12 24 30 36 VIN (V) Figure 8 SW RDS(ON) vs.
PAGE 6
AL8805 Typical Performance Characteristics (cont.) (@TA = +25°C, unless otherwise specified.) 0.36 100% 6 LEDs 7 LEDs 98% 4 LEDs 96% 8 LEDs 5 LEDs 0.35 3 LEDs 2 LEDs EFFICIENCY 2 LEDs 92% 90% 88% 86% 3 LEDs LED CURRENT (A) 94% 1 LED 0.34 0.33 1 LED 0.30 6 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 13 Efficiency vs. Input Voltage 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 14 330mA LED Current vs. Input Voltage L = 100µH RS = 150mΩ TA = 25° C 300 0.710 0.
PAGE 7
AL8805 Typical Performance Characteristics (cont.) (670mA LED Current) (@TA = +25°C, unless otherwise specified.) 350 5% L = 100µH RS = 150m Ω TA = 25° C 4% 300 3% FREQUENCY (kHz) 2% % ERROR 1% 0% -1% -2% 250 200 150 2 LEDs 100 1 LED -3% 50 -4% -5% 6 5 LEDs 3 LEDs 4 LEDs 0 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 18 LED Current Deviation vs. Input Voltage 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 19 Switching Frequency vs.
PAGE 8
AL8805 Typical Performance Characteristics (cont.) (1A LED Current) (@TA = +25°C, unless otherwise specified.) 250 10% 6% SWITCHING FREQUENCY (kHz) 8% 3 LEDs 5 LEDs 2 LEDs 4 LEDs 6 LEDs 7 LEDs 8 LEDs 4% % ERROR 2% 1 LED 0% -2% -4% -6% L = 100µH RS = 100mΩ TA = 25° C -8% -10% 6 150 100 50 0 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 25 Switching Frequency vs. Input Voltage 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 24 LED Current Deviation vs.
PAGE 9
AL8805 Application Information AL8805 Operation In normal operation, when voltage is applied at +VIN, the AL8805 internal switch is turned on. Current starts to flow through sense resistor R1, inductor L1, and the LEDs. 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.
PAGE 10
AL8805 Application Information (cont.) PWM Dimming LED current can be adjusted digitally, by applying a low frequency Pulse Width Modulated (PWM) logic signal to the CTRL pin to turn the device on and off. This will produce an average output current proportional to the duty cycle of the control signal. In particular, a PWM signal with a max resolution of 10bit can be applied to the CTRL pin to change the output current to a value below the nominal average value set by resistor RSET.
PAGE 11
AL8805 Application Information (cont.) Soft Start The AL8805 does not have in-built soft-start action – this provides very fast turn off of the output the stage improving PWM dimming accuracy; nonetheless, adding an external capacitor from the CTRL pin to ground will provide a soft-start delay. This is achieved by increasing the time taken for the CTRL voltage to rise to the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator.
PAGE 12
AL8805 Application Information (cont.) Diode Selection For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the maximum operating voltage and temperature. The Schottky diode also provides better efficiency than silicon PN diodes, due to a combination of lower forward voltage and reduced recovery time.
PAGE 13
AL8805 Application Information (cont.) Thermal and Layout Considerations Figure 35 gives details about the PCB layout suggestions: 1. the capacitor C1 has to be placed as close as possible to VIN 2. The sense resistor R1 has to be placed as close as possible to VIN and SET 3. The D1 anode, the SW pin and the inductor have to be placed as close as possible to avoid ringing. Figure 35 Recommended PCB Layout Application Example Typical application example for the AL8805 is the MR16 lamp.
PAGE 14
AL8805 Application Information (cont.) In Figures 37 and 38 are displayed the top layer and the bottom layer of a typical PCB design for an MR16 solution.
PAGE 15
AL8805 Ordering Information AL 8805 W5 - 7 Device Package Code AL8805W5-7 W5 Note: Package Packing W5 :SOT25 7 : Tape & Reel 7” Tape and Reel Packaging (Note 6) Quantity Part Number Suffix SOT25 3000/Tape & Reel -7 8. Pad layout as shown on Diodes Inc. suggested pad layout document AP02001, which can be found on our website at: http://www.diodes.com/datasheets/ap02001.pdf .
PAGE 16
AL8805 Suggested Pad Layout C2 Z C2 Dimensions Value (in mm) Z 3.20 G 1.60 X 0.55 Y 0.80 C1 G C1 C2 Y 2.40 0.95 X IMPORTANT NOTICE DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION).