BCR450 Driving mid & high power LEDs from 65mA to 700mA with LED controller IC BCR450 with thermal protection Application Note Revision: 1.0 Date June 2009 Power Management and Multimarket
Edition June 2009 Published by Infineon Technologies AG, 81726 Munich, Germany. 2014 Infineon Technologies AG 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. INFINEON TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND (INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE. Information For further information on technology, delivery terms and conditions and prices, please contact the nearest Infineon Technologies Office (www.infineon.com). Warnings Due to technical requirements, components may contain dangerous substances. For information on the types in question, please contact the nearest Infineon Technologies Office. Infineon Technologies components may be used in life-support devices or systems only with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system or to affect the safety or effectiveness of that device or system. 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.
1 Demonstrator board description This application board is designed to demonstrate the ability of the linear LED driver BCR450 to drive 1W LEDs, typically at 350mA current, for General lighting and industrial applications. The BCX68-25, an external NPN transistor boosts the current to the desired 350mA for each LED. The board comes with footprints for 2 other possible external booster transistors, which can be applied in exchange for the BCX68-25 by the user of this board at a later stage. The combination of the external power stage next to an intelligent and feature-rich LED driver IC like the BCR450 is the most cost efficient solution in the industry besides the advantage of spreading the heat dissipation over two devices instead of one monolithic driver IC. A string of LEDs must be applied to the output pins, which leaves flexibility on the supply voltage and the number of LEDs in the string. The board is designed for supply voltages from 12VDC to 27VDC, sufficient for driving a string of 3 to 8 LEDs with a typical forward voltage of 3.2V. To prevent that the LEDs suffer damage from connecting the input voltage inverted to the designed configuration, the full bridge Schottky rectifier diode BAS3007A-RPP serves as a low cost reverse polarity protection device and makes the circuit work with an in either way connected supply voltage. Additionally, with this diode on board, the circuit works with 18VAC voltage as well. Dimming of the LEDs is possible via applying a PWM signal to the enable pin, which controls the output current. The parts are mounted on a double-sided FR-4 PCB with 70µm copper layer each. As LEDs are in general very sensitive to high temperatures, the PCB has thermal vias to conduct the heat away from the top to the bottom layer. Application Note 3 June 2009
PCB drawing: 2 BCR450 The BCR450, with features like high output current accuracy of +/- 1.5%, overcurrent and overvoltage protection and the ability to protect the LEDs from thermal overstress, is designed for high current general lighting applications. The 85mA current in standalone mode can be extended to up to 2.0A with an external booster transistor. This circuit for high current applications is described in Application Note AN105. The combination of advanced protection features and a price performance ratio that is benchmark in the industry, the BCR450 offers a unique, yet cost effective way to drive high power LEDs. Features of the BCR450: High output current precision of +/- 1.5% at 25 C Current range: o Standalone mode: up to 85mA o Booster circuit: 85mA 2000mA Maximum operating voltage: 27V Overvoltage and overcurrent protection Thermal shutdown Low voltage overhead in boost mode of only 0.5V (0.15V at sense resistor + 0.35V at booster transistor) Direct PWM possible due to logic level enable input Small 6-pin SC74 package Application Note 4 June 2009
Benefits: Thermal shutdown protects the LEDs from permanent damage Linear concept eliminates EMI problems External power stage allows improved heat dissipation in comparison to monolithic drivers Higher count of LEDs possible in a string due to very low voltage overhead Less space needed on PCB, as no coils and inductors are required and no external digital transistor for PWM Excellent price-performance ratio, due to separation of power stage from higher-cost IC technology Suitable applications for the BCR450: Architectural lighting Light bars/ strobe lights for emergency vehicles Infrared security cameras and infrared spot light Illumination for machine vision EMI sensitive applications such as Aircraft cabin lighting and medical lighting High power general lighting applications BCR450 Block diagram and pin configuration Application Note 5 June 2009
3 The booster concept To extend the current range of the BCR450 to current levels beyond 85mA, another approach is needed, to reach the 350mA required current for the used Z1 series LEDs for such higher power applications, the LED driver is used as a controller and an external booster transistor is employed to handle the higher current and heat dissipation. For the correct choice of the transistor, the power dissipation and maximum ratings of the devices must be checked and verified for each individual circuit design. As a general guideline, the BC817SU is recommended for ½ Watt LEDs with currents up to 150mA, the BCX68-25 is recommended for 1W LEDs with current up to 350mA and the BDP947 for higher power, mostly 3W LEDs with current levels above 350mA, up to 700mA. In general, the upper limit on output current for this circuit is only limited by the maximum power dissipation & junction temperature of the boost transistor. It is even possible to parallel multiple boost transistors for extremely high current operation. In this particular case, the BCX68-25 was chosen, as the LED current is commonly 350mA for 1W LEDs. In this approach, the LED driver IC and external boost transistor still operate in a closed-loop system and therefore the LED current is still tightly controlled over temperature and power supply voltage variations. The basic concept is simple: the LED driver takes its output current and feeds it into the base terminal of the external NPN boost transistor, in this case the BCX68-25. The boost transistor then multiplies this base current by the DC current gain (hfe) of the boost transistor, with a much higher output current at the collector. The collector current supplies the 3 LEDs in series. Since the required output current from the standard LED driver is reduced or divided by the DC current gain of the boost transistor, most of the power dissipation burden is now placed upon the boost transistor, instead of on the LED Driver IC. The advantages of the low-current, stand-alone BCR450 LED Driver circuit including the high current precision of +/- 1,5%, the thermal shutdown feature which protects the LEDs from damage and the overcurrent and overvoltage protection- are preserved. Application Note 6 June 2009
4 Application circuit key parameters and Pin configuration Supply Voltage V s : 12VDC 27VDC / 18VAC (Applied to pin S1 and S3, see application schematic for details) LED current I f : 350mA (fixed by Resistor R4 and R5) PWM dimming: A PWM signal (0 3.3V) with a frequency between 50Hz and 300Hz can be applied to Pin S2. Additionally the signal has to be grounded on the PWM Grnd marked area. 5 Operation of the board Before taking the board into operation, the decision on two parameters has to be made: The first step is to take a decision on the number of LEDs, which will be applied in a string to the board. Depending on the number of LEDs, a suitable supply voltage has to be chosen. A calculation example shows best how the supply voltage should be matched around the number of LEDs: Calculation example 1: 3 LEDs in a string is one of the most common configurations in the General and Industrial LED lighting segment. Industry standard 350mA white 1W LEDs come with a typical forward voltage of 3.2V. Summing up the forward voltages of the LEDs and adding the voltage drop of the driver circuit components BCR450, the BCX68-25 and the BAS3007A-RPP Schottky diode gives the minimum required voltage needed to get this circuit running: RPP) V smin = 3 x 3.2V (V fled ) + 0.15V (V drop BCR450) + 0.35V (V drop BCX68-25) + 0.50V (V drop BAS3007A- V smin = 10.6V In this case a supply voltage of 12VDC should be applied to the circuit, to keep the voltage overhead at a low level. In this concept the voltage overhead will always be burned at the external booster transistor. The calculation of the power that will be dissipated at the transistor: P diss = (V supply V smin V drop BCX68-25) x I f P diss = (12V 10.25V 0.35V) x 0.35A 0.613W The BCX68-25 is capable of dissipating a maximum power of 3W, so 0.613W is a very good value that will keep the temperature of the transistor at a good level. Application Note 7 June 2009
Calculation example 2: Another very common configuration in the General and Industrial LED lighting segment is a string of 6 LEDs in series. We will once again use the 3.2V typical forward voltage for the LEDs for the calculation of the minimum required voltage: V smin = 6 x 3.2V (V fled ) + 0.15V (V drop BCR450) + 0.35V (V drop BCX68-25) + 0.50V (V drop BAS3007A-RPP) V smin = 20.2 V As the power supply, that is next to this value is a regular 24VDC power supply, we will use this for further calculations. P diss = (V supply V smin V drop BCX68-25) x I f P diss = (24V 20.2 V - 0.35V) x 0.35A 1.45W Dissipating 1.45W is still no problem for the BCX68-25, with its maximum P tot of 3W. With external N-channel MOSFET: The Board can place by an N-channel MOSFET at location T3 Use BSP318S N-Channel MOSFET, it provides lower RDS = 0.09 Ω. PG-SOT223 package (same as BDP947) The advantages are: 1. Lower RDS, increase efficiency. 2. There is no gate current, more accurate LED current control. 3. Driver power LED current up to 2.6 A 4. Lower supply overhead voltage due to low VDS. 5. Higher drain source voltage up to 60 V, drive more LED in series. V smin = 6 x 3.2V (V fled ) + 0.15V (V drop BCR450) + 0.18 (V drop BSP318S ) + 0.50V (V drop BAS3007A-RPP) V smin = 20.03 V P diss = (V supply V smin (V drop BSP318S) x I f P diss = (24V 20.03V - 0.18V) x 0.35A 1.45W Demo board modification: Change R2 = R3 = 500 Ω and mount BSP318S at location T3. Recommended N-Channel MOS: BSP318S BSR302N BSS306N Application Note 8 June 2009
6 Application schematic On the demo board, only BCX68-25 is used, the other transistors are optional; they can be populated as well. Application Note 9 June 2009
BOM list Name Value Package Function S1 CON3 DC Plug J10 0 Ohm 0603 Jumper J2 0 Ohm 0603 Jumper R2 820 Ohm 0603 R3 100 Ohm 0603 R4 0.56 Ohm 0805 set LED current R5 1.8 Ohm 0603 set LED current R10 270K 0603 C1 47nF 0603 IC1 BCR450 SC74 LED Driver RF1 BAS3007A- RPP SOT143 Schottky Diode Array T1 T2 BCX68-25 SOT89 Booster Transistor T3 Application Note 10 June 2009
7 Application Hit operate with high input voltage (Higher than 27 VDC) When the system power supply voltage higher than 27 VDC, it is possible to use BCR450 with external power stage. Example: ZD1 will clamp at 12 VDC, providing BCR450 operation and enable pin voltage (if there is no PWM control). The input voltage can calculate by: Vs > VfLED * NLED + V DS_Q1 + 0.15V By increasing the input voltage, we can increase the number of LEDs to be driven. Application Note 11 June 2009
w w w. i n f i n e o n. c o m Published by Infineon Technologies AG