Driving Mid & High Power LEDs From 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Application Note 105 http://www.infineon.com/lowcostleddriver Rev. 1.1, 2007 -11 -19 Power Management & Multimarket Edition 2007-11-19 Published by Infineon Technologies AG 81726 München, Germany © Infineon Technologies AG 2009. All Rights Reserved. LEGAL DISCLAIMER THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. 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Life support devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered. Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Application Note No. 105 Revision History: 2007-11-19, Rev. 1.0 Previous Version: Page Subjects (major changes since last revision) Application Note 3 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 1 Description The BCR450, realized in a bipolar power technology, is a low cost linear regulator LED controller IC for industrial applications designed to operate as a constant current source. The LED controller is capable of driving high current, high brightness LEDs up to 2.5 A by using additional external output stages as “booster” transistors. This device operates over a 8 V - 27 V input voltage range and high output accuracy is maintained over a broad current range, from 0 to 85 mA. For LED currents up to 85 mA the IC can be used as a stand alone device and requires only one external low side current sense resistor which monitors the output current to guarantee accurate current regulation. The voltage drop across the sense resistor is only in the range of 0.12 V - 0.15 V, which contributes to a very small supply voltage overhead of typically 0.5 V. This low voltage drop minimizes wasted DC power and maximizes the number of LEDs that can be used in a series ’stack’. The IC can be switched on and off by applying an external signal to the EN (Enable) pin of the device, which also linearly varies the LED brightness up to the programmed LED current by PWM (Pulse Width Modulation) dimming. The precise internal bandgap stabilizes the circuit and provides constant current over the full temperature range. In addition, the current supply uses a sense control function with feedback mechanism that regulates the LED current. Finally, an over voltage/current protection and temperature shut down mechanism is provided, which protects the LEDs and an Output Short Circuit protection block avoids to damage the IC in the event of a short- circuit at the output pin of the BCR450. The BCR450 typically draws only 1.5 mA when operating in the no-load condition and draws typically less than 50 nA when the device is shut down. In “boost” mode, where an external transistor is used for LED currents over 85 mA, the BCR450 is designed to work with a PWM frequency up to 1KHz in addition with a typical PWM range from 1% to 100%. The IC provides a wide dimming range of 1100:1 at a PWM frequency of 1 kHz. The BCR450 is supplied in a small 6-pin TSOP6 / SC74 package. Advantage of Linear Regulation of LED current A key benefit to use a constant-current LED lamp driving is the ability to measure the change in LED lamp current. Through series configuration of the LEDs, current matching is guaranteed. Electromagnetic Interference (EMI) is minimized with linear regulation methods. Therefore designing with the BCR450 allows faster time to market, system integration and qualification. Additional filters or shielding required to suppress unwanted electromagnetic radiation are therefore not necessary. Furthermore, the linear- mode BCR450 does not need a switching inductor. By eliminating the inductor required for a switching design, overall cost is reduced. Given the rapidly increasing DC power efficiency / efficacy of modern LEDs, a switch-mode driver is often not required to meet overall system DC energy efficiency requirements. The BCR450 can be used with an external power transistor (boost transistor) for 1/2 W and 1 W LEDs, which helps the lighting designer to realize a low cost, EMI-free solution in a small area, while reducing the power dissipation in the BCR450 itself. Application Note 4 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 This modular approach to driver design - using BCR450 in “stand alone” mode for currents up to 85 mA, and in “boost” mode with an external transistor for currents over 85 mA - lets the designer use a building- block approach to different LED lighting designs, enabling the designer to use a common core LED driver (BCR450) for multiple designs, adding a “boost” transistor where necessary. This approach can simplify logistics and reduce overall costs. Features • • • • • • • • • • • • • Voltage drop across sense resistor only 0.12 V - 0.15 V; low side current sensing Maximum operation voltage: 27 V Over voltage protection Over current protection Temperature shut down mechanism Extremely precise bandgap voltage reference Maximum operating output current: 85 mA Maximum LED current of 2.5 A possible by using external transistors (boost transistors) Digital On/Off switch PWM control for LED brightness possible Minimum external component required (only one current sense resistor) Small 6-pin package TSOP6 / SC74 Low shutdown current: <50 nA typ. at operational voltage range Applications • • • • • • • • • • • • • LED Controller for industrial applications (not qualified for automotive applications) General purpose constant current source General purpose constant current LED driver General illumination, e.g. Halogen Retrofit Residential architectural and industrial commercial lighting for indoor and outdoor Decorative and entertainment lighting Backlighting (illuminated advertising, general lighting) Display backlight where high brightness is required e.g. TFT Reading lamps (aircraft, car, bus) Substitution of micro incandescent lamps Signage, Gasoline Canopies, Beacons, Hotel Lighting Signal and symbol luminaries for orientation Marker lights (e.g. steps, exit ways, etc.) Application Note 5 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 2 Pin description Figure 1 BCR450 block diagram Table 1 Pin description Pin number 1 2 3 4 5 6 Application Note Pin Symbol I out GND EN V sense GND V S Function Controlled output current to drive LEDs IC ground Power On control voltage pin (PWM input) Sense control voltage pin for internal feedback mechanism IC ground Supply voltage 6 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 3 Application Board The application board is designed to test the BCR450 with additional external “booster” transistors for high current, high brightness LEDs. 3 LEDs are “stacked” in series, which guarantees the matching of the LED current. An optional Reverse Polarity Protection (RPP) based on Infineon Schottky Diodes BAS3010A-03W is provided on the application board to avoid inverting the inputs when connecting the DC power plug. The MCPCB (Metal Clad Printed Circuit Board) for the current application contains only one diode. The board incorporates two power transistors (boost transistors) to minimize thermal problems in high current high voltage applications. To distribute the hot spots on PCB, a cost effective solution could be the use of some Infineon BC817SU transistors in parallel. Due to the fact that the EN Line of the application board is directly connected to the supply voltage a 270 kΩ resistor is inserted in series to the EN pin of the IC to protect that pin against higher voltages than the pernitted 5 V. A supply voltage of 8 V - 27 V may be applied and depending on the resulting power dissipation a LED current up to 1 A can be realized. However the booster transistor requires a minimum of ~0.5 V from collector-to-emitter to operate properly. The controller BCR450 has to deliver a very small driver current due to the hFE of the power transistor, which drastically reduces the power dissipation in the BCR450 IC. The temperature of the LED is sensed by the BCR450 via two capacitors operating as thermal bridges, which are connected between the ground plane of the IC and one LED. If the ground plane heats up, the BCR450 will also warm up and if the BCR450’s chip temperature exceeds 170 °C (typically), the internal temperature shut down will become active and reduce the LED current. Based on the enable input, the IC can be switched on or off or a PWM signal can be applied, making PWM dimming possible via controlling the output current Iout. Due to the fact that LED junction temperatures must be kept below their maximum ratings in order to ensure long LED lifetime, the PCB is manufactured as a metal-clad-circuit board (MCPCB). Flex-Circuit material ® ® (DuPont “Kapton” ) is attached with adhesive (DuPont “LF” ) low cost “3003” series aluminium sheet for the circuit board design. The aluminium back-plate of the PCB serves as a heat sink for the LEDs, the LED driver IC BCR450 and booster transistor. Only one side of the dielectric has traces or metallization on it. A cross-section diagram of the circuit is given in Figure 3. Note the thin dielectric layer (flex-circuit) of 0.05 mm thickness minimizes thermal resistance, permitting heat to flow from the high power LEDs and circuit components into the aluminium base plate relatively easily. Application Note 7 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Application Note 8 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 OSRAM Platinum Dragon Table 2 Technical key parameters of OSRAM Platinum Dragon Symbol Min. Typ. Max. Value VF IF R thJS P 2.9 3.6 4.3 V 1000 mA 100 tot 11 K/W 4.6 W 4 The BCR450 in “boost” operation for high brightness LEDs 4.1 Calculation of the base voltage divider Assuming an application with one power transistor BCX68-25 and an ILED current of 350 mA. hFE is typically 250. Vsense ~ 150 mV VS = 12 V Referring to Figure 2 → base current of the transistor: IB = ILED / hFE = 1.4 mA Assuming Ix should be 5 times higher than IBtot VBE of the power transistor is ~ 0.56 V (if transistor is heated up) → Iout = IB + IX = IB + 5 x IB = 6 IB → VR2 ~ 0.56 V + Vsense = 0.56 V + 0.15 V = 0.71 V → R2 = 0.71 V / 5 x IB = 0.71 V / 7 mA = 101.4 Ω; Next value E24: 100 Ω Assuming Vout = 8 V, which results in 4 V VCE at the output stage (between pin 6 and pin 1 of the BCR450). Lower VCE helps to minimize the power dissipation in the IC (VCE x ICE). A VCE up to approx. 1 V is feasible for boost operation. → VR1 = Vout - 0.71 V = 7.29 V → R1 = VR1 / 6 x IB = 7.29 V / 8.4 mA = 867.86 Ω; Next value E24: 820 Ω Providing two power transistors results in the same resistor values for the base voltage divider. Note, that the values of the bias circuit are not critical Application Note 9 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 4.2 How to calculate and choose the booster transistor The external boost transistor is the key component to get a design which has a good efficiency in terms of power consumption, size of the PCB and cost. 1. At first we have to set the supply voltage range Vsupply 2. Determine the desired LED current ILED 3. Set the number of stacked diodes; this is very important, because the residual voltage will be dropped down at the booster transistor. Be sure to allow for at least 0.5 V across the collector- emitter of the booster transistor for proper operation 4. Depending on the total power dissipation it could be necessary to use 2 transistors in parallel, which is supported in the application board 5. Sufficient heat sink area should be provided for the power transistors Maximum Power Dissipation calculation as an example: V = 12 V supply ILED = 350 mA 3 Platinum Dragon LEDs with a VFmin = 2.9 V in series = 150 mV typ. V sense = 3 x 2.9 V = 8.7 V V F total This results in a value of Rsense of Rsense = Vsense / ILED = 0.15 V / 350 mA = 0.43 Ω (could be realized 1.8 Ω and 0.56 Ω in parallel) VCEtransistor = 12 V - 0.15 V - 8.7 V = 3.15 V Ptot = VCE x ILED = 3.15 V x 350 mA = 1103 mW If the Total Power Dissipation will exceed 1500 mW, adequate cooling provided by a properly sized heat sink is necessary. 4.3 Calculation with two transistors The value of the sense resistor of each power transistor is half of that as compared to a design using a single booster transistor. Note:Both resistors should have the same value of the sense resistor to ensure both boost transistors have the same collector currents and share the power dissipation burden equally. Regarding the power dissipation, each transistor will dissipate half of power as well. Rsense = Vsense / ILED / 2 = 0.15 V / 175 mA = 0.86 Ω (could be realized 5.6 Ω and 1 Ω in parallel) Ptot one transistor = VCE x ILED / 2 = 3.15 V x 175 mA = 551.3 mW, which results in enough margin for the design Application Note 10 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Three transistor types are recommended Table 3 Recommended transistors SC74 SOT89 1 2 4 5 3 3 2 6 2 1 2.9 x 1.6 x 1.1 mm 4.5 x 2.5 x 1.5 mm BC817SU Ptot = 1000 mW max. ICmax = 500 mA 45 V breakdown (C-E) 1) Thermal Resistance < 50 K/W BCX55-16 Ptot = 1000 mW max. ICmax = 1000 mA 60 V breakdown (C-E) BCX68-25 Ptot = 1500 mW max. ICmax = 1000 mA 20 V breakdown (C-E) Thermal Resistance < 20 K/W Thermal Resistance < 20 K/W 1) Thermal resistance is device Junction to package Soldering point (RthJS) As mentioned previously, if the power dissipation exceeds the maximum level of all transistor packages, it is necessary to split the power consumption by using two transistors. Without any heat sink two BCX68-25 should be used in order to handle Ptot = VCE x ILED. Of course, power consumption issues in the transistor could be relaxed if the number of LEDs used in the stack is increased. Note:Stacking more LEDs, if possible, reduces the collector-emitter voltage VCE across the boost transistor(s), thereby decreasing the power dissipation in the boost transistor(s). But one must ensure that the boost transistors have at least 0.5 V across their collector- emitter connections under all anticipated operating conditions to ensure they operate properly. It is also possible to reduce the junction temperature by providing large copper areas on the PCB connected to the collector of the transistor. If the junction temperature does not exceed 150 °C at the highest ambient temperature, a smaller booster transistor could also be used (e.g. BC817SU). Three transistors BC817SU with SC74 packages are recommended in order to avoid hot spots on the PCB by splitting up the power dissipation between multiple packages, e.g. this approach “spreads out the heat”. Nevertheless, the power dissipation in the BCR450 is very low due to the fact that the output current of the BCR450 when operated with an external “boost” transistor is calculated as Iout = ILED / hFE. In other words, in the “boost” configuration, the current that the BCR450 needs to provide, is the LED current, divided by the DC current gain of the boost transistor(s). Therefore in this case, the BCR450 acts as a ’controller’ with very low power dissipation, and does not require any additional effort in terms of cooling, as the largest part of the power dissipation burden has been shifted to the external boost transistor(s). Application Note 11 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 4.4 Calculation of the Power Dissipation We use the example from Chapter 4.2. Only one power transistor will be used. P =U xI tot (only one transistor) CE = 3.15 V x 350 mA = 1103 mW LED 1) In most of the case the RthSA is not known. Therefore the only method to determine the junction temperature of the transistor is to measure the temperature of the solder point TS. T =T +P xR J S tot thJS If the customer knows the thermal resistance of the board one can easily calculate the temperature of the solder point TS, too. T =T +P xR S A tot thSA A combination of both formulas results in T = T + P x (R J A 4.5 tot +R thJS ) thSA Using BCX68-25 P tot = 1.5 W (TS = 120 °C) max = 1A = 20 K/W (SOT89) I R thJS P Table 4 = 1103 mW tot (only one transistor) T TJ @ RthSA = 20 K/W TJ @ RthSA = 36 K/W TJ @ RthSA = 85 K/W 25 °C 69.1 °C 86.8 °C 140.8 °C 65 °C 109.1 °C 126.8 °C 85 °C 129.1 °C 146.8 °C 180.8 °C1) 200.8 page12 °C 1) A 1) Values exceed the maximum junction temperature of 150 °C. The transistor requires additional heat sink or a design with two transistors in parallel. Table 5 Ptot (each transistor)= 551.3 mW; two power transistors in parallel TA TJ @ RthSA = 20 K/W TJ @ RthSA = 36 K/W TJ @ RthSA = 85 K/W 25 °C 47.1 °C 55.9 °C 82.9 °C 65 °C 87.1 °C 95.9 °C 122.9 °C 85 °C 107.1 °C 115.9 °C 142.9 °C 1) Thermal resistance between soldering point and ambient Application Note 12 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 4.6 Using two BCX55-16 P tot = 1W max = 1A = 20 K/W (SOT89) I R thJS Table 6 Ptot (each transistor)= 551.3 mW; two power transistors in parallel TA TJ @ RthSA = 20 K/W TJ @ RthSA = 36 K/W TJ @ RthSA = 85 K/W 25 °C 47.1 °C 55.9 °C 82.9 °C 65 °C 87.1 °C 95.9 °C 122.9 °C 85 °C 107.1 °C 115.9 °C 142.9 °C 4.7 Using BC817SU This is the most cost effective solution Note: hFE of the BC817SU degrades at 500 mA by using only one transistor P tot = 1W max = 0.5 A = 50 K/W (SC74) I R thJS Table 7 T Ptot (each transistor)= 551.3 mW; two power transistors in parallel TJ @ RthSA = 20 K/W TJ @ RthSA = 36 K/W TJ @ RthSA = 85 K/W 25 °C 63.6 °C 72.4 °C 99.5 °C 65 °C 103.6 °C 112.4 °C 139.5 °C 85 °C 123.6 °C 132.4 °C 159.5 °C A 5 Calculation of the maximum number N of stacked diodes with identical VF in boost mode 1. Determine the supply voltage 2. Set the minimum VCE of the booster transistor. BCX68-25 power transistor works well down to VCE = 0.3 V if ICE is below 1000 mA 3. Calculate the available voltage over the LEDs VLED = Vsupply - Vsense - VCE = Vsupply - 0.15 V - 0.5 V (VCE = 0.5 V with additional 0.2 V margin) 4. N = VLED / VF; it is recommended to round down the nearest integer value Example: VF max = 4.3 V (OSRAM Platinum Dragon) Vsupply = 15 V →VLED = 14.35 V → N = 14.35 V / 4.3 V ~ 3 Application Note 13 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 6 Using a heat sink to decrease solder point temperature TS of the LEDs and the booster transistor If the MCPCB is connected with a heat sink using SK 76 profile, the solder point temperature TS of the boost transistor would be decreased by 45 °C. %&5 . ON U U 6._ _YV Figure 4 SK 76 Profile (all readings in mm); Rth = 8 K/W VS = 12 V ILED = 350 mA; VF ~ 2.9 V TA = 25 °C R R = 20 K/W thJS Trans = 11 K/W thJS LED SK 76 : 37.5 mm long (37.5 x 32 x 20mm) => Rth= 8 K/W Table 8 Current MCPCB Board TS (°C) Ptot (W) TJ (°C) RthSA (K/W) Table 9 Booster Transistor LED 1 LED 2 LED 3 119.0 100.0 101.0 99.5 1.11 1.00 1.02 0.99 141.1 111.0 112.2 110.4 85.0 75.0 74.9 75.5 Using MCPCB Board mounted on a SK 76 cooling element TS (°C) Ptot (W) TJ (°C) RthSA (K/W) Booster Transistor LED 1 LED 2 LED 3 74 61 58 60.5 1.11 1.05 1.06 1.03 96.1 72.6 69.7 71.8 44.3 34.2 31.0 34.6 → TJ max Trans = 150 °C → TJ max Trans -TJ Trans on SK76 = 150 °C - 96.1 °C = 53.9 °C The ambient temperature TA could be increased by 53.9 °C (25 °C + 53.9 °C = 78.9 °C) until TJ exceeds 150 °C. This results in a TJ of the LEDs of 125 °C. Application Note 14 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 How to use the BCR450 in “stand alone” mode 7 --not supported according to the application board-The application needs only one sense resistor for operation Assuming again the worst case scenario Using e.g. OSRAM - Advanced Power Top LED Table 10 OSRAM Advanced Power Top Key technical data Symbol VF I F R thJS P Min. 2.9 Typ. Max. Unit 3.6 4.1 V 250 mA 30 tot 40 K/W 650 mW V Ftyp I LED VS = 3.6 V = = 70 mA 12 V 2 LEDs stacked in series VLED = 2 x 3.6 V = 7.2 V ~ Vout Figure 5 Application circuit BCR450 in “stand alone” mode Application Note 15 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 The curves below are specified at TA = 25 °C Iout vers Vsense Vout = 7.2 V Iout (mA) 100 90 80 70 Vs=9V 60 50 Vs=10V Vs=12V 40 30 20 10 Vs=16V Vs=27V 15 8 155 152 149 146 143 140 137 128 13 1 13 4 125 122 119 116 113 110 0 Vsense (mV) BCR450_Iout vers Vsense_Vout =7.2V.vsd Figure 6 Iout(Vsense); Vout=7.2 V Iout (Vs-Vout) Vs = 12V 90 Vs- Vout = 2V 80 Iout (mA) Vs- Vout = 3V 70 Vs- Vout = 4V 60 Vs- Vout = 5V Vs- Vout = 6V 50 Vs- Vout = 7V 40 Vs- Vout = 8V Vs- Vout = 9V 30 Vs- Vout = 10V 20 120 125 130 135 140 145 150 Vsense (mV) BCR450_Iout vers Vsense_Vs=12V.vsd Figure 7 Iout(VSense); VS = 12 V Vsense (Vs - Vout) Vs=12V 160 Vsense (mV) 140 120 Iout=15mA 100 Iout=30mA Iout=50mA 80 Iout=70mA 60 Iout=80mA 40 Iout=85mA 20 0 1,0 1,2 1,4 1,6 1,8 2,0 4,0 6,0 8,0 10,0 Vs - Vout (V) BCR450_Vsense vers Vs - Vout_Vs=12V.vsd Figure 8 Vsense(VS-Vout); VS = 12 V Application Note 16 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 It must be pointed out that there are criteria a designer should be aware: 1. VS - Vout should not fall below a certain value e.g. 70 mA: ~ 2.5 V (see Figure 8). If VF tends to maximum specification value, enough overhead regarding supply voltage should be guaranteed 2. The minimum VF of the LEDs results in increasing the power dissipation of the output stage transistor in the IC For a stable linear regulation we use a Vsense which gives enough margin in order to the control range (see Figure 6 and Figure 7) Derived from Figure 6 VS - Vout = 12 V - 7.2 V = 4.8 V Results in Vsense = 141 mV @ 70 mA (yellow curve) → Rsense = Vsense / ILED ~ 2 Ω 7.1 Worst case scenario regarding power dissipation Refer to Figure 9 VTrans = VS - 2 x VFmin - Vsense = 12 V - 5.8 V - 0.141 V = 6.06 V Ptot = 6.06 V x 70 mA = 424 mW RthSA = 20 K/W (assuming the RthSA of an imaginary MCPCB Application Board) RthJS = 75 K/W (BCR450 - Thermal resistance - Junction to Solder Point) TJ = TA + Ptot x (RthSA + RthJS) = TA + 40.28 K Table 11 TJ (TA) TA (°C) TJ (°C) 25 65.3 65 105.3 85 125.3 105 145.3 Application Note 17 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 8 General aspects regarding Overhead Voltage for a given Output current Iout V =V overhead +V Trans =V -V sense S LED Figure 9 Voverhead principle typ. V_overhead max 7 (V) 6 V overh ead 5 4 3 2 1 0 0 10 20 30 40 50 60 70 80 90 Iout (mA) BCR450_V_overhead.vsd V Figure 10 overhead (I ) out For a wanted output current Iout of 70 mA one needs approx. 3 V overhead, while Vsense operates in a range of > 130 mV e.g. 3 diodes with a VF of 3 V and 12 V supply voltage → 3 xVF + Voverhead = 9 V + 3 V = 12 V Application Note 18 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 9 Calculate PWM frequency and duty cycle To determine the maximum PWM frequency or a certain PWM duty cycle the knowledge of the rise and falltimes of the BCR450 is necessary Figure 11 Response Time T on T (10-90%) 4 s Maximum value does not exceed 10 s off (90-10%) 50.5 s Maximum value: 70 s For the calculation the maximum value of Ton of 10 s should be used (Ton / Toff) * 100 = tduty in % T = (Ton + Toff) = (Ton + Ton / tduty) = Ton (1 + 100 / tduty (%)) FPWM = 1 / T Maximum frequency according to 1 % duty cycle FPWMmax = 1 / (10 s (1 + 100/1)) = 990 Hz Maximum duty cycle for a given PWM frequency e.g. FPWM = 2 KHz tdutymax(%) = 100 / ( (1 / (FPWM x Ton) - 1) ) tdutymax(%) = 100 / ( (1 / (2 KHz x 10 s) - 1) ) → tdutymax = 2.04 % Application Note 19 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Example of PWM- Dimming in boost mode VS I LED F PWM = 12 V = 353 mA (100% tduty) = 300 Hz Table 12 Dimming Range of 2300 : 1 Tduty (%) ILED (mA) 1 0.15 5 17 10 34 20 68 30 108 40 144 50 180 60 214 70 250 80 286 90 320 95 340 100 353 10 Measurement setup for the boost mode In order to set up and evaluate the BCR450, the following components and equipment are needed: • • • • • • A sense resistor (typically 0.1 Ω to 0.5 Ω depending on the wanted LED current). – See Table 13 A power transistor (the type depends on the LED current and the maximum power dissipation, see Table 3) LED load 8 V to 27 V supply Enable or PWM- signal Digital voltmeter (DVM) Table 13 Sense Resistor Selection R (Ω) I (mA) sense LED 100 1.5 150 1 350 0.43 500 0.3 700 0.21 Application Note 20 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 11 Schematic and Layout Figure 12 Board Schematic of High Power LED Application with OSRAM Platinum Dragon Application Note 21 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 %&5 3O WLQXP_'U RQ_$SSOLF WLRQ \RXW_YVG Figure 13 Layout %&5 3O WLQXP_'U RQ_$SSOLF WLRQ $VVHP O\_YVG Figure 14 Component Placement Specification Application Note 22 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Table 14 Assembly List Name Value Package J2 0Ω 0603 R1 0Ω 0805 R2 820 Ω 0603 R3 100 Ω 0603 0.56 Ω 0805 Set LED current 1.8 Ω 0603 0603 Set LED current Only necessary by using a second booster transistor 0603 Only necessary by using a second booster transistor R41) R5 R6 1) R7 Function R10 270 kΩ 0603 C1 47 nF 0603 C2 1 nF 0805 For heat sink purposes, optional C3 1 nF 0805 For heat sink purposes, optional D1 BAS3010A-03W SOD323 D2 BAS3010A-03W SOD323 Only used in case of RPP circuit D3 BAS3010A-03W SOD323 Only used in case of RPP circuit D4 BAS3010A-03W SOD323 Only used in case of RPP circuit IC1 BCR450 TSOP6 / SC74 LED controller T1 BCX68-25 SOT89 Booster Transistor T2 BCX68-25 SOT89 Not used in the application board S1 CON5 EDGE_CON_TOP DC plug LED1 LW W5SN Platinum Dragon 1W LED, white LED2 LW W5SN Platinum Dragon 1W LED, white LED3 LW W5SN Platinum Dragon 1W LED, white 1) Value is valid only by using one boost transistor Application Note 23 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Figure 15 Photograph of the Application of BCR450 with OSRAM Platinum Dragon LEDs and additional cooling element SK 76 Application Note 24 Rev. 1.0, 2007-11-19 Application Note No. 105 Driving Mid & High Power LEDs from 65mA to 700mA with Thermal Protection LED Controller IC BCR450 Application Note 25 Rev. 1.0, 2007-11-19 www.infineon.com Published by Infineon Technologies AG AN105