AL8807 HIGH EFFICIENCY LOW 36V 1A BUCK LED DRIVER Description Pin Assignments The AL8807 is a step-down DC/DC converter designed to drive LEDs (Top View) with a constant current. The device can drive up to 9 LEDs, depending on the forward voltage of the LEDs, in series from a SW voltage source of 6V to 36V. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating VIN GND the need for ballast resistors. The AL8807 switches at frequency up to 1MHz with controlled rise and fall times to reduce EMI. This allows CTRL the use of small size external components, hence minimizing the PCB SET area needed. SOT25 (Top View) Maximum output current of AL8807 is set via an external resistor connected between the VIN and SET input pins. Dimming is achieved SET VIN GND N/C GND SW CTRL SW 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 1.3A (MSOP-8EP) Better Than 5% Accuracy High Efficiency up to 96% Optimally Controlled Switching Speeds Applications Operating Input Voltage from 6V to 36V MR16 Lamps PWM/DC Input for Dimming Control General Illumination Lamps Built-In Output Open-Circuit Protection 12V Powered LED Lamps SOT25, MSOP-8EP: Available in "Green" Molding Compound 24V Powered LED Lamps MSOP-8EP (No Br, Sb) Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2) Halogen and Antimony Free. "Green" Device (Note 3) 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/quality/lead_free.html 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 D1 DFLS 2100 R1 ANODE 0 R15 D3 DFLS 2100 D2 U1 100nF SET C5 P1 VIN DFLS 2100 C2 C3 D4 DFLS 2100 D5 150F L1 1F C1 SW CTRL P2 C4 150F 100 nF 33H CATHODE GND AL8807 DFLS 2100 GND AL8807 Document number: DS35281 Rev. 5 - 2 1 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Pin Descriptions Pin Name SW GND CTRL SET VIN Pin Number Function SOT25 MSOP-8EP 1 5, 6 Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to reduce EMI. 2 2, 3 GND Pin Dimming and On/Off Control Input. Leave floating for normal operation. (VCTRL = VREF = 2.5V giving nominal average output current IOUTnom = 0.1/RS) 3 4 Drive to voltage below 0.4V to turn off output current Drive with DC voltage (0.5V < VCTRL < 2.5V) to adjust output current from 20% to 100% of IOUTnom A PWM signal (low level 0.4V and high level > 2.6; transition times less than 1us) allows the output current to be adjusted below the level set by the resistor connected to SET input pin. 4 1 Set Nominal Output Current Pin. Configure the output current of the device. Input Supply Pin. Must be locally decoupled to GND with > 2.2F X7R ceramic capacitor - see applications 5 8 section for more information. EP -- EP N/C -- 7 Exposed pad/TAB connect to GND and thermal mass for enhanced thermal impedance. Should not be used as electrical ground conduction path. No Connection Absolute Maximum Ratings (@TA = +25C, unless otherwise specified.) Symbol ESD HBM ESD MM VIN VSW VCTRL ISW-RMS ISW-PK Parameter Ratings 2.5 200 Unit kV V Continuous VIN Pin Voltage Relative to GND -0.3 to +40 V SW Voltage Relative to GND -0.3 to +40 V CTRL Pin Input Voltage -0.3 to +6 V 1.25 1.6 A 2.5 A Human Body Model ESD Protection Machine Model ESD Protection SOT25 MSOP-8EP DC or RMS Switch Current Peak Switch Current (<10%) Junction Temperature 150 C TLEAD Lead Temperature Soldering 300 C TST Storage Temperature Range -65 to +150 C TJ Caution: Stresses greater than the 'Absolute Maximum Ratings' specified above, may cause permanent damage to the device. These are stress ratings only; functional operation of the device at these or any other conditions exceeding those indicated in this specification is not implied. Device reliability may be affected by exposure to absolute maximum rating conditions for extended periods of time. Semiconductor devices are ESD sensitive and may be damaged by exposure to ESD events. Suitable ESD precautions should be taken when handling and transporting these devices. Recommended Operating Conditions (@TA = +25C, unless otherwise specified.) Symbol Parameter Min Max Unit Operating Input Voltage Relative to GND 6.0 36 V VCTRLH Voltage High for PWM Dimming Relative to GND 2.6 5.5 V VCTRLDC Voltage Range for 20% to 100% DC Dimming Relative to GND 0.5 2.5 V 0 0.4 V VIN VCTRLL fSW Voltage Low for PWM Dimming Relative to GND Maximum Switching Frequency ISW Continuous Switch Current TJ Junction Temperature Range AL8807 Document number: DS35281 Rev. 5 - 2 SOT25 MSOP-8EP -40 2 of 20 www.diodes.com 1 MHz 1 1.3 A +125 C March 2013 (c) Diodes Incorporated AL8807 Electrical Characteristics (@VIN = 12, TA = +25C, unless otherwise specified.) Symbol Parameter Conditions VINSU Internal Regulator Start-Up Threshold VIN rising VINSH Internal Regulator Hysteresis Threshold VIN falling IQ Quiescent Current Output not switching (Note 4) IS Input Supply Current CTRL pin floating f = 250kHz VTH VTH-H Set current Threshold Voltage Set Threshold Hysteresis RCTRL CTRL Pin Input Resistance Referred to internal reference VREF Internal Reference Voltage ISW = 1A tR SW Rise Time tF SW Fall Time VSENSE = 10020mV, fSW = 250kHz VSW = 0.1V to 12V to 0.1V, CL = 15pF mV 350 A 1.8 5 mA 105 mV 16 mV 22 50 0.25 V 0.4 ns 0.5 SOT25 (Note 6) MSOP-8EP (Note 7) 250 69 JL Thermal Resistance Junction-to-Lead (Note 8) SOT25 (Note 6) 50 Thermal Resistance Junction-to-case (Note 9) MSOP-8EP (Note 7) 4.3 ns 20 VIN =30V A k 12 Thermal Resistance Junction-to-Ambient (Note 5) JC V 300 100 JA Notes: Unit 5.9 2.5 On Resistance of SW MOSFET ISW_Leakage Switch Leakage Current Max 20 VSET = VIN-0.1 RDS(on) Typ 100 95 SET Pin Input Current ISET Min A C/W 4. AL8807 does not have a low power standby mode but current consumption is reduced when output switch is inhibited: VSENSE = 0V. Parameter is tested with VCTRL 2.5V 5. Refer to figure 35 for the device derating curve. 6. Test condition for SOT25: Device mounted on FR-4 PCB (25mm x 25mm 1oz copper, minimum recommended pad layout on top layer and thermal vias to bottom layer ground plane. For better thermal performance, larger copper pad for heat-sink is needed. 7. Test condition for MSOP-8EP: Device mounted on FR-4 PCB (51mm x 51mm 2oz copper, minimum recommended pad layout on top layer and thermal vias to bottom layer with maximum area ground plane. For better thermal performance, larger copper pad for heat-sink is needed 8. Dominant conduction path via Gnd pin (pin 2). 9. Dominant conduction path via exposed pad. AL8807 Document number: DS35281 Rev. 5 - 2 3 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Typical Performance Characteristics 400 900 350 800 700 FREQUENCY (kHz) 300 IIN (A) 250 200 150 100 0 3 600 L = 68H 500 400 300 200 VCTRL = 0V VSET = VIN TA = 25C 50 0 0 6 9 12 15 18 21 24 27 30 33 36 VIN (V) Figure 1. Supply Current (not switching) vs. Input Voltage 90 60 70 40 ICTRL (A) 80 60 40 L = 100H 100 100 LED CURRENT (A) VIN = 12V 1 LED RSET = 150m TA = 25C L = 33H 0 1 2 3 4 5 VCTRL Figure 2. Switching Frequency vs. VCTRL VSET = VIN = 12V TA = 25C 20 0 30 -20 20 -40 -60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.0 VCTRL (V) Figure 4. ICTRL vs. VCTRL 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 CTRL PIN VOLTAGE (V) Figure 3. LED Current vs. VCTRL 2.52 3 VCTRL = Open VSET = VIN = 12V 2.5 2.51 VCTRL (V) VCTRL (V) 2 1.5 2.50 1 0.5 0 2.49 VCTRL = Open VSET = VIN T A = 25C 0 3 2.48 -40 6 9 12 15 18 21 24 27 30 33 36 VIN (V) Figure 5. VCTRL vs. Input Voltage (CTRL Pin Open Circuit) AL8807 Document number: DS35281 Rev. 5 - 2 4 of 20 www.diodes.com -15 10 35 60 85 110 AMBIENT TEMPERATURE (C) Figure 6. VCTRL VS. TEMPERATURE March 2013 (c) Diodes Incorporated AL8807 Typical Performance Characteristics (cont.) 7 240 6 0.5 LED Current Error 0.4 LED Current 4 270 0.7 0.6 5 300 0.3 3 210 RDS(ON) (m) 8 LED CURRENT ERROR (%) 0.8 L = 68H, RS = 150m TA = 25C, VIN = 12V CTRL = PWM, fPWM = 500Hz 1 LED LED CURRENT (A) 9 150 120 90 0.2 2 180 VCTRL = Open VSET = VIN TA = 25C 60 0.1 1 0 0 20 40 60 80 PWM DUTY CYCLE Figure 7. ILED vs. PWM Duty Cycle 30 0 100 0 6 9 12 15 18 21 24 27 30 33 36 VIN (V) Figure 8. SW RDS(ON) vs. Input Voltage 100 400 90 350 80 DUTY CYCLE (%) RDS(ON) (m) 3 LEDS 300 250 200 VCTRL = Open VSET = VIN = 12V 150 L = 68H RS = 100m TA = 25C VCTRL = Open 70 60 2 LEDS 50 40 30 20 10 100 -40 -15 10 35 60 85 110 Ambient Temperature (C) Figure 9. SW RDS(ON) vs. Temperature Figure. 11 SW Output Rise Time AL8807 Document number: DS35281 Rev. 5 - 2 0 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 10. Duty Cycle vs. Input Voltage Figure. 12 SW Output Fall Time 5 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Typical Performance Characteristics (cont.) (670LED Current) 350 10 6 4 2 0 -2 -4 -6 -8 -10 300 SWITCHING FREQUENCY (kHz) LED CURRENT ERROR (%) 8 200 150 100 50 0 6 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 13. LED Current Deviation vs. Input Voltage 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 14. Switching Frequency vs. Input Voltage 10 500 8 450 SWITCHING FREQUENCY (kHz) LED CURRENT ERROR (%) 6 250 6 4 2 0 -2 -4 -6 400 350 300 250 200 150 100 50 -8 -10 0 6 6 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 16. Switching Frequency vs. Input Voltage 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 15. LED Current Deviation vs. Input Voltage 9 10 1 LED 2 LEDs 2 0 3 LEDs 4 LEDs 5 LEDs 6 LEDs -2 -4 7 LEDs -6 8 LEDs -8 -10 6 SWITCHING FREQUENCY (kHz) 6 4 9 800 L = 33H RS = 150m TA = 25C VCTRL = Open 8 LED CURRENT ERROR (%) 9 600 500 400 300 1 LED 200 7 LEDs 0 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 17. LED Current Deviation vs. Input Voltage Document number: DS35281 Rev. 5 - 2 6 of 20 www.diodes.com 8 LEDs 5 LEDs 100 9 AL8807 L = 33H RS = 150m TA = 25C VCTRL = Open 700 3 LEDs 4 LEDs 6 LEDs 2 LEDs 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 18. Switching Frequency vs. Input Voltage March 2013 (c) Diodes Incorporated AL8807 Typical Performance Characteristics (cont.) (1A LED Current MSOP-8EP) 10 350 L = 100H RS = 100m TA = 25C VCTRL = Open SWITCHING FREQUENCY (kHz) LED CURRENT ERROR (%) 8 6 4 2 0 -2 -4 -6 300 250 200 150 1 LED 100 50 -8 -10 4 LEDs 5 LEDs 6 LEDs 2 LEDs3 LEDs 0 6 6 7 LEDs 8 LEDs 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 20. Switching Frequency vs. Input Voltage 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 19. LED Current Deviation vs. Input Voltage 10 9 350 SWITCHING FREQUENCY (kHz) LED CURRENT ERROR (%) 8 6 4 2 0 -2 -4 -6 300 250 200 150 100 50 -8 -10 6 0 6 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 22. Switching Frequency vs. Input Voltage 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 21. LED Current Deviation vs. Input Voltage 9 600 10 SWITCHING FREQUENCY (kHz) LED CURRENT ERROR (%) 8 6 4 2 0 -2 -4 -6 500 400 300 200 100 -8 -10 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 23. LED Current Deviation vs. Input Voltage AL8807 Document number: DS35281 Rev. 5 - 2 7 of 20 www.diodes.com 0 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 24. Switching Frequency vs. Input Voltage March 2013 (c) Diodes Incorporated AL8807 Typical Performance Characteristics (cont.) (1.3A LED Current MSOP-8EP) 250 10 L = 100H RS = 77m TA = 25C VCTRL = Open SWITCHING FREQUENCY (kHz) LED CURRENT ERROR (%) 8 6 4 2 0 -2 -4 -6 200 150 100 1 LED 50 -8 -10 2 LEDs 3 LEDs 5 LEDs 6 LEDs7 LEDs 8 LEDs 4 LEDs 0 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 25. LED Current Deviation vs. Input Voltage 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 26. Switching Frequency vs. Input Voltage 300 10 L = 68H RS = 77m T A = 25C VCTRL = Open SWITCHING FREQUENCY (kHz) LED CURRENT ERROR (%) 8 6 4 2 0 -2 -4 -6 -8 -10 250 200 150 100 1 LED 50 6 LEDs 2 LEDs 3 LEDs 4 LEDs 5 LEDs 6 0 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 27. LED Current Deviation vs. Input Voltage 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 28. Switching Frequency vs. Input Voltage L = 33H RS = 77m TA = 25C VCTRL = Open SWITCHING FREQUENCY (kHz) 8 LED CURRENT ERROR (%) 9 600 10 6 4 2 0 -2 -4 -6 500 400 300 200 1 LED 100 5 LEDs -8 -10 7 LEDs 8 LEDs 0 6 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 29. LED Current Deviation vs. Input Voltage AL8807 Document number: DS35281 Rev. 5 - 2 8 of 20 www.diodes.com 4 LEDs 2 LEDs 3 LEDs 6 6 LEDs 7 LEDs 8 LEDs 9 12 15 18 21 24 27 30 33 36 INPUT VOLTAGE (V) Figure 30. Switching Frequency vs. Input Voltage March 2013 (c) Diodes Incorporated AL8807 Application Information The AL8807 is a hysteretic (also known as equal ripple) LED driver with integrated power switch. It is available in two packages that provide a PCB area-power dissipation capability compromise. It is recommended that at higher LED currents/smaller PCBs that the MSOP-8EP version is used to maximize the allowable LED current over a wider ambient temperature range. AL8807 Operation In normal operation, when voltage is applied at +VIN, the AL8807 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. The internal circuit of the AL8807 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 internal switch is turned off. The inductor current continues to flow through R1, L1, the LEDs 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 AL8807. 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 internal switch is turned on again. This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor R1. LED Current Control The LED current is controlled by the resistor R1 in Figure 30. Figure 30 Typical Application Circuit Connected between VIN and SET the nominal average output current in the LED(s) is defined as: ILED VTHD R1 For example for a desired LED current of 660mA and a default voltage VCTRL=2.5V the resulting resistor is: R1 VTHD 0.1 150m ILED 0.66 DC Dimming Further control of the LED current can be achieved by driving the CTRL pin with an external voltage (between 0.4V and 2.5V); the average LED current becomes: ILED VCTRL VTHD VREF R SET With 0.5V VCTRL 2.5V the LED current varies linearly with VCTRL, as in figure 2. If the CTRL pin is brought higher than 2.5V, the LED current will V be clamped to approximately 100% and follows ILED THD . RSET When the CTRL voltage falls below the threshold, 0.4V, the output switch is turned off which allows PWM dimming. AL8807 Document number: DS35281 Rev. 5 - 2 9 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 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. To achieve this resolution the PWM frequency has to be lower than 500Hz, however higher dimming frequencies can be used, at the expense of dimming dynamic range and accuracy. Typically, for a PWM frequency of 500Hz the accuracy is better than 1% for PWM ranging from 1% to 100%. 700 LED current [mA] 600 500 400 300 200 100 0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% PWM dimming [%] Figure 31 PWM Dimming at 500Hz Zooming in at duty cycles below 10% shows: Figure 32 Low Duty Cycle PWM Dimming at 300Hz The accuracy of the low duty cycle dimming is affected by both the PWM frequency and also the switching frequency of the AL8807. For best accuracy/resolution the switching frequency should be increased while the PWM frequency should be reduced. The CTRL pin is designed to be driven by both 3.3V and 5V logic levels directly from a logic output with either an open drain output or push pull output stage. AL8807 Document number: DS35281 Rev. 5 - 2 10 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Application Information (cont.) Soft Start The AL8807 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. Adding a capacitor increases the time taken for the output to reach 90% of its final value, this delay is 0.1ms/nF, but will impact on the PWM dimming accuracy depending on the delay introduced. Figure 33 Soft start with 22nF capacitor on CTRL pin (VIN = 30V, ILED = 667mA, 1 LED) Reducing Output Ripple Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor C2 across the LED(s) as shown already in the circuit schematic. A value of 1F will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of LED voltage. By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version without altering the mean current value. Capacitor Selection The small size of ceramic capacitors makes them ideal for AL8807 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Z5U. A 2.2F input capacitor is sufficient for most intended applications of AL8807; however a 4.7F input capacitor is suggested for input voltages approaching 36V. AL8807 Document number: DS35281 Rev. 5 - 2 11 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 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. It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. In particular, it is recommended to have a diode voltage rating at least 15% higher than the operating voltage to ensure safe operation during the switching and a current rating at least 10% higher than the average diode current. The power rating is verified by calculating the power loss through the diode. Schottky diodes, e.g. B240 or B140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8807 applications. Inductor Selection Recommended inductor values for the AL8807 are in the range 33H to 100H. Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range. (See graphs). Figure 34 Inductor value with input voltage and number of LEDs The inductor should be mounted as close to the device as possible with low resistance/stray inductance connections to the SW pin. The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. Suitable coils for use with the AL8807 are listed in the table below: Part No. MSS1038-333 MSS1038-683 NPIS64D330MTRF L (H) 33 68 33 DCR (V) 0.093 0.213 0.124 ISAT (A) 2.3 1.5 1.1 Manufacturer CoilCraft www.coilcraft.com NIC www.niccomp.com The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times over the supply voltage and load current range. AL8807 Document number: DS35281 Rev. 5 - 2 12 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Application Information (cont.) The following equations can be used as a guide, with reference to Figure 1 - Operating waveforms. Switch `On' time Switch `Off' time L I tON VIN VLED IAVG x RS rL RSW tOFF LI VLED VD IAVG x RS rL Where: L is the coil inductance (H) rL is the coil resistance ()RS is the current sense resistance () Iavg is the required LED current (A) I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg} VIN is the supply voltage (V) VLED is the total LED forward voltage (V) RSW is the switch resistance () {=0.5 nominal} VD is the diode forward voltage at the required load current (V) Thermal Considerations For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power dissipation depends on several factors: the thermal resistance of the IC package JA, PCB layout, airflow surrounding the IC, and difference between junction and ambient temperature. The maximum power dissipation can be calculated using the following formula: PD(MAX) = (TJ(MAX) - TA) / JA where TJ(MAX) is the maximum operating junction temperature, TA is the ambient temperature, and JA is the junction to ambient thermal resistance. The recommended maximum operating junction temperature, TJ, is 125C and so maximum ambient temperature is determined by the AL8807's junction to ambient thermal resistance, JA and device power dissipation. JA, is layout dependent and package dependent; the AL8807W5's JA on a 25x25mm single layer PCB with 1oz copper standing in still air is approximately 250C/W (160C/W on a four-layer PCB). The maximum power dissipation at TA = 25C can be calculated by the following formulas: PD(MAX) = (125C - 25C) / (250C/W) = 0.4W for single-layer PCB PD(MAX) = (125C - 25C) / (160C/W) = 0.625W for standard four-layer PCB Figure 35, shows the power derating of the AL8807W5 on two (one single-layer and four-layer) different 25x25mm PCB with 1oz copper standing in still air and the AL8807MP on an FR4 51x51mm PCB with 2oz copper standing in still air. Figure 35 Derating Curve for Different PCB AL8807 Document number: DS35281 Rev. 5 - 2 13 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Application Information (cont.) EMI and Layout Considerations The AL8807 is a switching regulator with fast edges and measures small differential voltages; as a result of this care has to be taken with decoupling and layout of the PCB.To help with these effects the AL8807 has been developed to minimise radiated emissions by controlling the switching speeds of the internal power MOSFET. The rise and fall times are controlled to get the right compromise between power dissipation due to switching losses and radiated EMI. The turn-on edge (falling edge) dominates the radiated EMI which is due to an interaction between the Schottky diode (D1), Switching MOSFET and PCB tracks. After the Schottky diode reverse recovery time of around 5ns has occurred; the falling edge of the SW pin sees a resonant loop between the Schottky diode capacitance and the track inductance, LTRACK, See figure 36. Figure 36 PCB Loop Resonance The tracks from the SW pin to the Anode of the Schottky diode, D1, and then from D1's cathode to the decoupling capacitors C1 should be as short as possible. There is an inductance internally in the AL8807 this can be assumed to be around 1nH. For PCB tracks a figure of 0.5nH per mm can be used to estimate the primary resonant frequency. If the track is capable of handling 1A increasing the thickness will have a minor effect on the inductance and length will dominate the size of the inductance. The resonant frequency of any oscillation is determined by the combined inductance in the track and the effective capacitance of the Schottky diode. An example of good layout is shown in figure 37 - the stray track inductance should be less than 5nH. Figure 37 Recommended PCB Layout AL8807 Document number: DS35281 Rev. 5 - 2 14 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Application Information (cont.) Recommendations for minimising radiated EMI and other transients and thermal considerations are: 1. The decoupling capacitor (C1) has to be placed as close as possible to the VIN pin and D1 Cathode 2. The freewheeling diode's (D1) anode, the SW pin and the inductor have to be placed as close as possible to each other to avoid ringing. 3. 4. The Ground return path from C1 must be a low impedance path with the ground plane as large as possible The LED current sense resistor (R1) has to be placed as close as possible to the VIN and SET pins. 5. The majority of the conducted heat from the AL8807 is through the GND pin 2. A maximum earth plane with thermal vias into a second earth plane will minimise self-heating 6. To reduce emissions via long leads on the supply input and LEDs low RF impedance capacitors (C2 and C5) should be used at the point the wires are joined to the PCB A typical application for the AL8807 is an LED MR16 lamp (schematic shown in Figure 38). Figure 38 MR16 Circuit Schematic An evaluation board for the AL8807 (named the AL8807EV2) for MR16 is available on request from your local Diodes' sales representative. This board follows Diodes' recommendations for low EMI. Images of the top layer and bottom layers are shown in Figure 39. Figure 39 Recommended MR16 PCB Layout AL8807 Document number: DS35281 Rev. 5 - 2 15 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Application Information (cont.) The associated EMI measurements for this board using the AL8807 is shown in figure 40. Figure 40 AL8807EV2 Radiated EMI Performance The EMI performance was measured at 12VDC driving two white LEDs (VF = 3.1V at 660mA) on the AL8807EV2. The red bold line is for EN55022 class B used for domestic equipment including lighting. The bottom magenta line is the noise floor of the test chamber. The middle purple line is the EMI emitted radiation of the AL8807 over 30MHz to 1000MHz. This shows that the AL8807 passes the standard with at least 16dB margin. MR16 lamps typically operate from 12VDC or 12VAC, using conventional electromagnetic transformers or electronic transformers. In enclosed lamps such MR16 the ability for the device to operate at high ambient temperatures is critical and figure 41 shows the surface temperature of the AL8807 on AL8807EV2 in operation under the same conditions as the EMI tests at an free air temperature of 25C. It is anticipated that the internal junction temperature is approximately 6C hotter than the surface temperature. Figure 41 Thermal picture of AL8807EV2 at 12VDC 2 white LEDS at 660mA The thermal image shows that components increasing the board temperature are the inductor, Schottky diodes and the AL8807. An inductor choice of 33H with saturation current higher than 1.1A, will limit the frequency variation between 180kHz and 400kHz over the whole input voltage variation (8V to 18V), and therefore represent the best choice for an MR16 solution also taking into account the size constraint of the lamp. The AL8807 guarantees high performance levels with both 12VAC and 12VDC power supplies. The efficiency is generally higher than 81% and current regulation is better than 0.1mA/V in for a DC input voltage in the range from 8V to 18V. AL8807 Document number: DS35281 Rev. 5 - 2 16 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Ordering Information AL8807 XX - XX Package Packing W5 : SOT25 MP : MSOP-8EP 7 : 7" Tape & Reel 13 : 13" Tape & Reel Part Number Status Package Code Packaging AL8807W5-7 AL8807MP-13 New Product New Product W5 MP SOT25 MSOP-8EP 7" Tape and Reel Quantity Part Number Suffix 3000/Tape & Reel -7 2500/Tape & Reel -13 Marking Information (1) SOT25 (Top View) 4 7 5 XX Y W X 1 2 Part Number AL8807W5-7 (2) 3 XX : Identification code Y : Year 0~9 W : Week : A~Z : 1~26 week; a~z : 27~52 week; z represents 52 and 53 week X : A~Z : Internal code Package SOT25 Identification Code B6 MSOP-8EP AL8807 Document number: DS35281 Rev. 5 - 2 Part Number Package AL8807MP-13 MSOP-8EP 17 of 20 www.diodes.com March 2013 (c) Diodes Incorporated AL8807 Package Outline Dimensions (All dimensions in mm.) Please see AP02002 at http://www.diodes.com/datasheets/ap02002.pdf for latest version. (1) SOT25 A SOT25 Dim Min Max Typ A 0.35 0.50 0.38 B 1.50 1.70 1.60 C 2.70 3.00 2.80 D 0.95 H 2.90 3.10 3.00 J 0.013 0.10 0.05 K 1.00 1.30 1.10 L 0.35 0.55 0.40 M 0.10 0.20 0.15 N 0.70 0.80 0.75 0 8 All Dimensions in mm B C H K J (2) M N D L MSOP-8EP D 4X 10 0.25 D1 x E E2 Gauge Plane Seating Plane a y 1 4X 10 8Xb e Detail C E3 A1 A3 L c A2 A D E1 See Detail C AL8807 Document number: DS35281 Rev. 5 - 2 18 of 20 www.diodes.com MSOP-8EP Dim Min Max Typ A 1.10 A1 0.05 0.15 0.10 A2 0.75 0.95 0.86 A3 0.29 0.49 0.39 b 0.22 0.38 0.30 c 0.08 0.23 0.15 D 2.90 3.10 3.00 D1 1.60 2.00 1.80 E 4.70 5.10 4.90 E1 2.90 3.10 3.00 E2 1.30 1.70 1.50 E3 2.85 3.05 2.95 e 0.65 L 0.40 0.80 0.60 a 0 8 4 x 0.750 y 0.750 All Dimensions in mm March 2013 (c) Diodes Incorporated AL8807 Suggested Pad Layout Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for the latest version. (1) SOT25 C2 Z C2 Dimensions Value (in mm) Z 3.20 G 1.60 X 0.55 Y 0.80 C1 2.40 C2 0.95 C1 G Y X (2) MSOP-8EP X C Dimensions C G X X1 Y Y1 Y2 Y G Y2 Y1 X1 AL8807 Document number: DS35281 Rev. 5 - 2 19 of 20 www.diodes.com Value (in mm) 0.650 0.450 0.450 2.000 1.350 1.700 5.300 March 2013 (c) Diodes Incorporated AL8807 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). Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes without further notice to this document and any product described herein. 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LIFE SUPPORT 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: 1. are intended to implant into the body, or 2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in significant injury to the user. B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. Customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support devices or systems, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of Diodes Incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systems-related information or support that may be provided by Diodes Incorporated. Further, Customers must fully indemnify Diodes Incorporated and its representatives against any damages arising out of the use of Diodes Incorporated products in such safety-critical, life support devices or systems. Copyright (c) 2013, Diodes Incorporated www.diodes.com AL8807 Document number: DS35281 Rev. 5 - 2 20 of 20 www.diodes.com March 2013 (c) Diodes Incorporated