AN10739 Discrete LED driver Rev. 2 — 21 June 2010 Application note Document information Info Content Keywords LED, constant current source, buck converter Abstract This application note describes a 300 mA discrete LED driver, based on a buck-converter principle, with a cycle-by-cycle current control. It includes a proposal for a BOM and layout of a low cost, low component count solution. AN10739 NXP Semiconductors Discrete LED driver Revision history Rev Date Description 02 20100621 Corrected version, Figure 4 figure notes corrected 01 20090211 Initial version Contact information For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: [email protected] AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 2 of 14 AN10739 NXP Semiconductors Discrete LED driver 1. Introduction This application note describes a 300 mA discrete LED driver, based on a buck-converter principle with an efficiency of 80 to 90 %. It includes a proposal for a BOM and layout leading to a low cost, low component count solution to drive a single LED or a string of LEDs connected in series. The choice of the discrete parts is discussed with respect to NXP’s bipolar low VCEsat (BISS) and ultra low VF MEGA Schottky technologies, i.e. the PBSSxxx series and the PMEGxxx series. Key applications for the driver are lighting applications, where constant LED brightness, high efficiency and low cost are important features. For example automotive lighting applications require that general illumination and signage should not consume too much power when the motor is not running. The input voltage of +6 V to +18 V supports automotive requirements, too. Besides, battery driven handhelds like flash lights or head lamps will benefit from the topology and efficiency the driver delivers. 2. Operating principle Vin C1 R2 R1 TR2 TR1 L1 Vout TR3 D1 R3 C2 C3 D2 006aab405 Fig 1. LED driver schematic 2.1 Basic operating principle The 300 mA discrete LED driver is based on the buck-converter principle with a cycle-by-cycle current control. The input peak current is set by resistor R1 and by modifying R1, the current can easily be set to lower or higher values, i. e. designing a driver from 20 mA to 1 A. AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 3 of 14 AN10739 NXP Semiconductors Discrete LED driver When applying supply voltage Vin, TR3 is switched on, providing the base current for the PNP transistor TR1 and switching it on. With diode D1 reversed biased, current starts to flow through inductor L1 and LED D2. The coil equation described by Equation 1 shows that a desired rise or fall of the inductor current requires a certain voltage step applied to the inductor, with the factor of proportionality L, called the self-inductance of the coil: Δi L ( t ) v L ( t ) = L × --------------Δt (1) With an LED as load and a constant Vin the result is a linearly increasing input current, as depicted in Figure 2: Imax Vin t ton tperiod 006aab406 Fig 2. Input voltage and current As the collector current of TR1 increases, the voltage drop at the current sense resistor R1 increases, too. When the voltage drop reaches TR2s base-emitter turn-on voltage VBE(on) of about 0.65 V, TR2 switches on and pulls the base of TR1 to the supply voltage, i. e. turns TR1 off. The value of R1, therefore, sets the maximum input current in the application, which flows through R1, TR1 and the inductor L1. When switching TR1 off, its collector current almost immediately drops back to zero. The inductor, however, cannot change its current suddenly, according to ΔI/Δt = V/L. The current will decrease but continues to flow in the same direction, with diode D1 now conducting. As D1 is forward biased, the voltage over L1 reverses when TR1 is switched off. The voltage level at the cathode is −VF of the Schottky diode, as long as there is energy stored in the inductor. Solving the inductor equation for this case and taking iL(0) = Imax as boundary condition leads to: – V out × t v L(t) × t i L(t) = ------------------- + I max = --------------------- + Imax L L (2) The current is decreasing until it reaches zero, depicted in Figure 3. AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 4 of 14 AN10739 NXP Semiconductors Discrete LED driver Imax A Iout B tperiod t 006aab407 Fig 3. Inductor current and optimal (DC) output current When all the energy that was stored in the inductor is delivered to the output, D1 becomes reversed biased again and the procedure is restarted. 2.2 Current, current ripple and switching frequency The slope of the current is set by the voltage step across the inductor, and for a fixed input voltage this voltage step is constant because the voltage drop across the LED is nearly independent from its current. The constant voltage step at the inductor leads to a linearly increasing current (remember: ΔI/Δt = V/L) flowing through the inductor and the LED - neglecting losses and other parasitic effects. When no output capacitors are used, the output current is exactly the coil current and the ripple height would be ±50 % (see Figure 3). To get smaller output ripples, the capacitor C2 is added, acting as a charge storage device and smoothing the sawtooth ripple. The value of the capacitor must be chosen according to LED current and flicker requirements of the specific application, the larger the capacitor, the less the ripple. The most important design value for the LED driver is the average output current, which is half the peak current of the coil set by R1 (Imax = VBE(TR2)/R1). AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 5 of 14 AN10739 NXP Semiconductors Discrete LED driver Looking at Equation 1 the energy absorption at the circuit input for one period can be determined as: 1 W in = --- × I max × V in × t on 2 (3) The energy provided to the LED is: W out = Iout × V F × ( t on + t off ) (4) Iout is the desired DC LED current. ton and toff are the turn-on time and the turn-off time of TR1, and the rise time tr and the fall time tf of the coil current, respectively. Their values can be calculated using the two solutions of the coil equation derived above. During ton the coil current needs to rise from 0 to Imax. Thus, using Equation 2, the turn-on time can be calculated to: L t on = I max × -----------------------V in – V out (5) The time the current needs to drop back to 0 A is: L t off = I max × ---------V out (6) Pasting Equation 5 and Equation 6 into Equation 3 and Equation 4 and applying the power conservation law yields (assuming no losses in the circuit): 1 W out = W in ⇒ I out = --- × I max 2 (7) ton and toff determine the switching frequency of the circuit: V in – V out V out 1 f = -------------------- = ------------------------ × ---------t on + t off V in L × I max AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 (8) © NXP B.V. 2010. All rights reserved. 6 of 14 AN10739 NXP Semiconductors Discrete LED driver 3. Dimensioning and choice of discrete parts The choice of the discrete parts on one hand is dependent on the input requirements like input voltage range, LED current and switching frequencies. On the other hand, the performance of the devices like their on-state losses, switching losses or the power dissipation capabilities of a specific package influence the efficiency and the costs of the circuit. 3.1 Inductor L1, Transistor TR1 and Schottky diode D1 3.1.1 Inductor L1 The switching frequency of the circuit is determined by the input voltage Vin, the LED forward voltage VF, the peak current Imax, and the inductor value L (see Equation 8). With given input conditions one can calculate the resulting switching frequency for different values of L to get a guideline for the choice of the inductor. In general, L shall be as small as possible to reduce costs and package size of the device. Smaller inductors usually have a smaller DC resistance, too, leading to higher efficiency of the whole circuit. The minimum coil saturation current rating should be 1.2 times the peak current. Alternatively, one can specify a maximum switching frequency of the application to derive the required inductor, using Equation 8. For the example below, fmax was set to 100 kHz, which is an appropriate value for a bipolar switch and also noise immunity. Example: For fmax = 100 kHz, Imax = 0.6 A, Vin(max) = 18 V, VF = 3.2 V 18V – ( 3, 2V ) 3, 2V L = -------------------------------------- × ------------- = 43, 85μH 100kHz × 0, 6A 18V Taking 47 μH will result in a maximum switching frequency of < 100 kHz for Vin = 18 V. 3.1.2 Transistor TR1 A bipolar transistor in a small SMD package shall be used for the switch as it offers an excellent performance-cost ratio for this application. The final choice of the device is dependent on the required performance. IC and VCEO are given by the input conditions but also the losses of the device, i. e. Ptot, during operation are important. The main parameters contributing to the losses are the saturation voltage VCEsat and the power loss during the fall time tf during turn-off. The best choice to keep the on-state losses low, is using a low VCEsat (BISS) transistor whereas BVCEO shall be at least 1.2 × Vin(max), and IC(max,DC) shall be at least 1.2 × Imax. Besides the on-state losses, switching times are an important factor for the efficiency whereas the main contributor is the fall time. Losses during the rise time are nearly zero as with an inductive load the collector current rises slowly. AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 7 of 14 AN10739 NXP Semiconductors Discrete LED driver PBSS5220T is a good choice for a 300 mA driver with an input voltage from +6 V to +18 V. The device is a 2 A, 20 V bipolar low VCEsat (BISS) transistor, with a typical VCEsat of 70 mV at IC = 600 mA and reasonable switching times. It comes in the very cost-efficient SOT23 package, with a Ptot of 250 mW on standard footprint. To assure saturation for TR1 - in order to benefit from the low VCEsat technology - R3 must be chosen in a way that with IC,TR3 (which equals IB,TR1) an IC/IB ratio of about 30 is adjusted. For a maximum TR1 collector current of 600 mA, IB shall be tuned to 20 mA, with R3 = 510 Ω. For the resulting IC/IB = 30, there is no VCEsat curve in the set of curves shown below for PBSS5220T. To get an idea of the power dissipation during ton, the value for an IC/IB = 50 is taken, which will be at least equal or worse. 006aab408 1 VCEsat (V) 10−1 IC/IB = 100 10−2 IC/IB = 50 IC/IB = 10 10−3 10−1 1 10 102 103 104 IC (mA) VCEsat,typ (at IC = 600 mA and IB = 20 mA) = 70 mV Fig 4. AN10739 Application note PBSS5220T: Collector-emitter saturation voltage as a function of collector current; typical values All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 8 of 14 AN10739 NXP Semiconductors Discrete LED driver 3.1.3 Schottky diode D1 A Schottky diode is chosen for the ’catch diode’, to provide a current path for the LED current during toff. NXP’s MEGA Schottky PMEG series offers an ultra low forward voltage VF, resulting in reduced heat generation during operation and an increased efficiency. PMEG2010EJ is proposed for a 300 mA LED driver, which is a 20 V, 1 A MEGA Schottky diode in the SOD323F (SC-90) package. It offers a Ptot of 360 mW on standard footprint with a VF of typically 340 mV at 0.6 A DC current, whereas the SOD323F (SC-90) package is a cost-efficient solution, which can not only serve for the 300 mA LED driver but also for modifications up to higher output currents. 4. Demo-Board and measurements To demonstrate the performance of the application discussed above, a demonstrator was realized on a 16.5 mm × 49.5 mm PCB, with the BOM proposed (see Section 4.1). Input requirements were an input voltage range from +6 V to +18 V, low LED current ripple and a maximum switching frequency < 100 kHz. 006aab409 Fig 5. AN10739 Application note NXP Discrete LED Driver and OSRAM Golden Dragon All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 9 of 14 AN10739 NXP Semiconductors Discrete LED driver 4.1 BOM proposal: 300 mA driver Table 1. BOM proposal 350 mA LED current BOM part Proposal R1 1.2 Ω (2010), 1 W resistor R2 10 kΩ (0603) R3 510 Ω (0603) C1 1 μF C2 220 μF C3 not connected L1 47 μH, LQH55D series from Murata D1 PMEG2010EJ; 20 V, 1 A Schottky diode (SOD323F/SC-90), NXP D2 1 A LED; OSRAM Golden Dragon LW W5SM TR1 PBSS5220T; 20 V, 2 A PNP low VCEsat (BISS) transistor (SOT23), NXP TR2, TR3 BC847BPN; NPN/PNP general-purpose double transistor (SOT363), NXP 4.1.1 Measurements Measurements have been performed on the final layout regarding efficiency and switching frequencies. 006aab410 100 η (%) (1) 80 (2) 60 40 20 0 6 8 10 12 14 16 18 Vin (V) (1) L1 = 33 μH (2) L1 = 47 μH Fig 6. Efficiency = f(Vin) The efficiency Pin/Pout of the board as shown with L1 = 47 μH is about 80 % for a supply voltage range from 9 V to 12 V. Choosing lower inductor values would result in a higher efficiency, as usually a smaller inductor comes with a lower DC resistance as well as lower inductor core losses. But, the smaller the inductor, the higher the maximum switching frequency of the application. AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 10 of 14 AN10739 NXP Semiconductors Discrete LED driver Using 33 μH instead of 47 μH would increase the efficiency to > 85 %. With 18 V input voltage, the switching frequency would be about 120 kHz (see Equation 8), and the resulting effects on increased switching losses and noise immunity may become an issue for certain application areas. However, the layout should be able to handle a minimum value of L1 of 22 μH. Using an LED string instead of a single LED would result in an increased efficiency, too, as the ratio between input and output voltage in that case would be beneficial for a buck converter. As with increasing input voltage also the switching frequency increases, the efficiency drops because of higher switching losses in the discrete devices. The increase of frequency is shown below as a comparison between the theoretical values using Equation 8 and a real measurement. 006aab411 100 f (kHz) (1) 80 (2) 60 40 20 0 6 8 10 12 14 16 18 Vin (V) (1) measured data (2) calculated data Fig 7. AN10739 Application note Frequency = f(Vin), L1 = 47 μH All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 11 of 14 AN10739 NXP Semiconductors Discrete LED driver 5. Conclusion • Highly efficient constant current LED driver using a switching power conversion solution based on a buck-converter principle, supported by NXP’s low VCEsat BISS and MEGA Schottky technologies • Applicable for a wide input voltage range from +6 V to +18 V • Applicable for a wide range of ambient temperatures due to low power dissipation / low heat generation of the driver • Low cost, low component count solution • Modifiable for a wide range of output currents from 300 mA up to 1 A AN10739 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 21 June 2010 © NXP B.V. 2010. All rights reserved. 12 of 14 AN10739 NXP Semiconductors Discrete LED driver 6. Legal information 6.1 Definitions Draft — The document is a draft version only. The content is still under internal review and subject to formal approval, which may result in modifications or additions. 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Contents 1 2 2.1 2.2 3 3.1 3.1.1 3.1.2 3.1.3 4 4.1 4.1.1 5 6 6.1 6.2 6.3 7 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Operating principle . . . . . . . . . . . . . . . . . . . . . . 3 Basic operating principle . . . . . . . . . . . . . . . . . 3 Current, current ripple and switching frequency 5 Dimensioning and choice of discrete parts. . . 7 Inductor L1, Transistor TR1 and Schottky diode D1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Inductor L1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Transistor TR1 . . . . . . . . . . . . . . . . . . . . . . . . . 7 Schottky diode D1 . . . . . . . . . . . . . . . . . . . . . . 9 Demo-Board and measurements . . . . . . . . . . . 9 BOM proposal: 300 mA driver . . . . . . . . . . . . 10 Measurements . . . . . . . . . . . . . . . . . . . . . . . . 10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Legal information. . . . . . . . . . . . . . . . . . . . . . . 13 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Please be aware that important notices concerning this document and the product(s) described herein, have been included in section ‘Legal information’. © NXP B.V. 2010. All rights reserved. For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: [email protected] Date of release: 21 June 2010 Document identifier: AN10739