LM3430_07

LED Backlighting Solution with LM3430 and LM3432 LED Backlighting Solution with LM3430 and LM3432 National Semiconductor Application Note 1679 TK Man September 2007 Introduction Since the release of high brightness White LED (HB-WLED) in the middle of 1990's, tremendous research efforts have been undergoing to improve the emission efficiency, reliability and thermal packaging technologies in order to expand its range of applications. In the last few years, improvements in HB-WLED performance and price structure attracts more potential applications considering this new lighting source technology as an alternative to conventional one. One area of applications, LCD display backlighting, consider this new backlighting method as the ultimate solution to replace the existing Cold Cathode Fluorescent Lamps (CCFL). Backlight LED-driver solutions need to exhibit following characteristics: • Instead of voltage control, LED-driver need direct LED current control to ensure consistent color and brightness; • High conversion efficiency under various conditions; • Support PWM dimming; • Limit LED's rail voltage while LEDs open circuited; • Ability to shutdown individual channel(s) when short or open circuited LED(s) detected; • Device over temperature detect and shutdown; • Low profile, small size and ease to use. The LM3432 is a 6-channel high voltage current regulator which provides a simple solution for LED backlighting applications and the LM3430 is a companion device to supply high voltage required to drive serially connected LED strings. The LM3430 and the LM3432 provide a complete solution to most HB-WLED backlighting applications for notebook and PC monitor. In this application note, a typical example for a solution to drive six strings of twelve LEDs in series running at 20mA per string is described in details. The Demonstration Board The LM3432 is used to drive 6-channel LED strings with 12 LEDs in series per string running at 20mA. This example provides a simple solution for HB-WLED backlight applications in notebooks and monitors. The LM3432 is powered with its companion device, LM3430, the Dynamic Headroom Control (DHC) feature helps to provide the optimal system efficiency. The design specification for this demonstration board is shown in below: Design Specifications: • Supply Voltage, VIN 6V to 21V • Boost Converter Switching Frequency, FSW 1 MHz • Boost Converter Output Voltage, VLED 50V (With load disconnected) • Number of LED String 6 • Number of LED per String 12 • LED Current per String, ILED 20 mA PWM Dimming Function: • Analog Dimming Mode Control Voltage Input, VPWM 1V to 3V (0% to 100%) Dimming Frequency, FPWM ≊23 kHz • Digital Dimming Mode External PWM Signal 20 kHz to 25 kHz, 2.5Vpeak AN-1679 © 2007 National Semiconductor Corporation 300289 www.national.com AN-1679 Demonstration Board Schematic 30028901 www.national.com 2 AN-1679 Connecting the Demonstration Board to LED Panel 30028902 LM3430+LM3432 Demonstration Board 30028903 LED Panel (6 x 12 LEDs) Connecting the demonstration board outputs to the LED panel is simple; just use a flat cable connecting the output header, CN1 to the respective input points of the LED panel as shown in above diagram. The LED panel is not a part with this demonstration kit; the user needs to build the LED panel for evaluation. 3 www.national.com AN-1679 Configuration of the Demonstration Board for Evaluation 30028904 Analog PWM Dimming Mode • Leave jumper JP3 open to enable the Analog Dimming Mode; • Short pins 1-2 of jumper JP5 to select the Analog control input; • The VR1 is used to derive the analog control voltage from LM3432's VCC output (5V nominal) and pass to LM3432's DIM input pin to control the dimming of the LED; • Adjust VR5 to change the LED brightness. The ON current of the LED is still maintained, only the duty ratio of ON time is adjusted to achieve the effect of dimming. This ensures the correct color is preserved even in dimmed output conditions. Digital PWM Dimming Mode • Short jumper JP3 to enable the Digital Dimming Mode; • Short pins 2-3 of jumper JP5 to select the Digital control input; • The external PWM dimming control signal applied to CN2 is then routed to LM3432's DIM input pin; • The control of LED current ON-OFF is passed to the externally applied PWM dimming control signal. The ON duty of the PWM control signal governs the LED brightness. www.national.com 4 AN-1679 Connect Power Supply to the Demonstration Board Input supply voltage, VIN is connected to the VIN and GND solder pads of the demonstration board. VIN ranges from 8V to 21V. Once the power is applied, all LEDs on the LED panel will be lighted up. The default setting of the demonstration board is in analog dimming mode, by adjusting the potentiometer, VR1; the LED brightness can be adjusted accordingly. For some cases, due to the incorrect power-up sequence timing of the LM3430 and the LM3432, some strings may have problem at cold start-up and the LM3432's fault detect may be falsely triggered. The normal operation can be recovered by shorting JP2 for few mini-seconds to reset the LM3432's fault detect circuit. This condition only happens when the board is powered up with LEDs in maximum brightness. This condition is not common in most backlighting applications. Typical applications normally have some sort of power-up sequence control to make sure the high voltage supply from the LM3430 is available before the LM3432 is being enabled. Dynamic Headroom Control With the LM3432's VDHC output pin connected to LM3430's VDHC input pin through a gain setting resistor (R19), the Dynamic Headroom Control (DHC) function will be in operation. The LM3432's DHC function will interact with the LM3430 to adjust the boost converter output voltage, VLED to LED strings just enough to keep all LED strings current in regulation. This minimum headroom voltage across the LED strings guarantees the best achievable system efficiency. LED OPEN and SHORT Fault Indication The LM3432 has the ability to detect both OPEN and SHORT faults of LEDs string by string. Whenever a fault is detected, the respective LED string will be latched OFF to protect the device from any possible damage and a signal will be issued to communicate with system micro-controller for appropriate actions. The SHORT fault detect threshold is 7.9V (typical) across IOUT pin to ground and OPEN fault check for zero sink current in any channel for more than 50 µs during ON duty. The fault output, FAULTb pin is connected to an indicator LED in the demonstration board. Synchronization of the Multiple Boost Converters In certain applications, more than one LED rail voltages are required in a single system. For example, a RGB backlighting system requires at least two different LED rail voltages, one for RED and one for BLUE and GREEN. Multiple switching converters running at slightly different switching frequency can introduce complex EMI situation. In order to accommodate this problem, the LM3430 can be synchronized to an external master clock that drives all boost converters switching at one single frequency. The external clock must running at a higher frequency than the preset free running oscillator frequency, i.e. > 1 MHz with this demonstration board. This feature can be enabled by applying external synchronization signal to CN3; the device will pass the control to the external synchronization signal automatically. Over-Temperature Monitor and Shutdown If the on-die temperature is over 125°C, the OTMb pin will be pulled to ground to inform the system micro-controller for immediate attention. Corrective action is expected to lower the die temperature, for example, the system can reduce the PWM duty factor to lower the average current into the IOUTs. If the on-die temperature rises further to 165°C, the LM3432 will shutdown all channels to prevent any potential damage to the device. When the device is cooled down to about 145°C, normal operation will resume. Again, in the demonstration board, the over-temperature signal output, OTMb pin is connected to an indicator LED. 5 www.national.com AN-1679 30028905 Demonstration Board in Operation (Powered by a Notebook Adapter) www.national.com 6 AN-1679 Typical Operating Waveforms In below some operating waveforms are presented for reference. The demonstration board is powered by a note-book adaptor with nominal output voltage of 16VDC. Power-Up Digital Dimming Operation (Channel 1 Waveform) 30028906 30028907 PWM Dimming Characteristic Rising Edge (Channel 1 Waveform) PWM Dimming Characteristic Falling Edge (Channel 1 Waveform) 30028908 30028909 Analog PWM Dimming Operation Efficiency 30028920 30028921 7 www.national.com AN-1679 PCB Layout 30028910 LM3430+LM3432 Demonstration PC Board Top Overlay www.national.com 8 AN-1679 30028911 LM3430+LM3432 Demonstration PC Board Top Layout 9 www.national.com AN-1679 30028912 LM3430+LM3432 Demonstration PC Board Bottom Layout www.national.com 10 AN-1679 Bill of Materials for the Demonstration Board Designation U1 M1 D1 L1 C20 "C22 - C25 C28 - C31" C0, C1, C21 C2 C3, C8 C4 C9 R0 R2 R3 R4 R5 R9 R12, R13 R14 R19 R20, R22 R21 Type Controller IC N-MOSFET Schottky Diode Power Inductor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Part Number LM3430 SI2308DS RB160M-60 CDRH8D28NP-220NB ECJ3YB1E106M GRM55DR61H106KA88L "GRM188R71E104KA01B (ECJ1VB1E104K)" "GRM1885C1H102JA01B (ECJ1VB1H102K)" GRM1885C1H101JA01B "GRM188R71E473KA01B (ECJ1VB1E473K)" GRM1885C1H120JA01B WSL2512R20000FEA CRCW06036192F CRCW06031823F CRCW06033010F CRCW06031652F CRCW06033011F CRCW06031183F CRCW06034021F CRCW06039091F CRCW06032004F CRCW06030R00F 60V, 125 mΩ, 4.8nC 60V, 1A 22 µH, 1.6A 10 µF, 25V 10 µF, 50V 0.1 µF, 25V, X7R "1 nF, 50V, COG (1nF, 50V, X7R)" 100 pF, 50V, COG 47 nF, 25V, X7R 12 pF, 50V, COG 0.2Ω, 1W 61.9 kΩ, ±1% 182 kΩ, ±1% 301Ω, ±1% 16.5 kΩ, ±1% 3.01 kΩ, ±1% 118 kΩ, ±1% 4.02 kΩ, ±1% 9.09 kΩ, ±1% 2.0 MΩ, ±1% 0Ω, ±1% Value Package LLP-12 SOT-23 SOD-123 8.3x8.3x3mm 1206 2220 0603 0603 0603 0603 0603 2512 0603 0603 0603 0603 0603 0603 0603 0603 0603 0603 Qty 1 1 1 1 1 8 3 1 2 1 1 1 1 1 1 1 1 2 1 1 2 1 Manufacturer NSC Vishay Rohm Sumida Panasonic MuRata "MuRata (Panasonic)" "MuRata (Panasonic)" Murata "MuRata (Panasonic)" Murata Vishay Vishay Vishay Vishay Vishay Vishay Vishay Vishay Vishay Vishay Vishay Boost Converter Section (LM3430) : 11 www.national.com AN-1679 Designation U2 C34 C35 C36, C38 C37 R7 R25 R27, R28 R38 R39 VR1 FAULTb OTMb JP2, JP3 JP5 CN1 CN2, CN3 Type LED Current Regulator IC Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Resistor Resistor Resistor Resistor Resistor Variable Resistor SMD LED SMD LED Jumper Header Jumper Header Connector Header Connector Header Part Number LM3432 GRM1885C1H681JA01B "GRM188R71E104KA01B (ECJ1VB1E104K)" "GRM188R71H103KA01B (ECJ1VB1H103K)" "GRM188R61A105KA61B (ECJ1VB1A105K)" CRCW06032212F CRCW06035472F CRCW06031001F CRCW06033572F CRCW06031802F PVM4A5003C01B00 Value Package LLP-24 Qty 1 1 1 2 1 1 1 2 1 1 1 1 1 2 1 1 2 Manufacturer NSC MuRata "MuRata (Panasonic)" "MuRata (Panasonic)" "MuRata (Panasonic)" Vishay Vishay Vishay Vishay Vishay MuRata Current Control Section (LM3432) : 680 pF, 50V, COG 0.1 µF, 25V, X7R 10 nF, 50V, X7R 1 µF, 10V, X5R 22.1 kΩ, ±1% 54.7 kΩ, ±1% 1.0 kΩ, ±1% 35.7 kΩ, ±1% 18.0 kΩ, ±1% 0 - 50 kΩ, 4mm GREEN RED 2-pin, 2.54mm pitch 3-pin, 2.54mm pitch 7-pin, 2mm pitch 2-pin, 2mm pitch 0603 0603 0603 0603 0603 0603 0603 0603 0603 0805 0805 www.national.com 12 AN-1679 Design Hints The schematic of the demonstration board is shown in page 2. Some hints on selection of the key parameters and components will be described in below. For full details of the design equations and theories, please refer to the LM3430 and LM3432 datasheets. BOOST CONVERTER DESIGN WITH LM3430 The function of the boost converter is to step up the input voltage to a higher LED rail voltage to drive strings of twelve LEDs connected in series. Determination of the Worst Case LED Rail Voltage, VLED The diode forward voltage, VF of HB-WLED ranges from 3.5V to 4.0V that varies from vendor to vendor. Considering only the worst case situation, i.e. VF = 4.0V, for twelve LEDs connected in series, a minimum of 48V is required to put the LED strings in proper operation. On the top of this voltage, additional control headroom for the constant current regulators is needed. The suggested start up LED rail voltage is 50V and this is also the ceiling of the boost converter output voltage, i.e. in case the load is disconnected, the boost converter output will stay at this voltage, no risk of over-voltage need to be considered. Once the demonstration board is in operation, the LM3432 will communicate with the LM3430 to lower the LED rail voltage to an appropriate level. This feature makes the LM3430 with LM3432 a robust and efficient LED driver solution. Selecting the Switching Frequency, FSW The selection of switching frequency is a trade off between size, cost and efficiency. In general, a lower switching frequency requires larger and more expansive external components. For some of the applications, space is one of the key considerations. A higher switching frequency can fulfill the space requirement, however the switching losses will go up and the overall system efficiency will be lowered. In this demonstration board, a switching frequency of 1MHz was selected. Selection of the Power MOSFET The power MOSFET plays a key role in system efficiency. An ideal power MOSFET should be low gate capacitance, Ciss, low gate charge, Qg, low ON resistance, RDS(ON) and sufficient Drain-Source breakdown voltage, VDS(BD). In this application, the required VDS(BD) is 50V plus the Schottky diode voltage drop, typically its about 0.5~0.7V. The average load current with all LED strings fully ON is 120 mA, by quick estimation; the worst case peak switch current is about 0.9A. The tiny MOSFET selected can operate up to 60V and 2A with RDS (ON) = 125 mΩ, Qg = 4.8 nC and Ciss = 240 pF. Selecting the Boost Schottky Diode The boost Schottky diode current equals to the average load current. The forward voltage drop and reverse recovery time determines the power loss with this component. The lower the forward voltage drop and faster reverse recovery time always results in better performance. The Schottky diode must also be rated to handle the maximum output voltage plus any ringing at the switching node caused by the diode parasitic capacitance and lead inductance. Selection of Power Inductor In fixed switching frequency boost converter applications, the inductance is determined by the allowable peak-to-peak inductor ripple current, ΔiL(p-p) of the maximum load current and the switching frequency. The duty cycle, D is evaluated first at both VIN(MIN) and VIN(MAX). Then the full load average inductor current is calculated at both voltages respectively. With the maximum average inductor current, the allowable inductor ripple current is determined. Finally, the inductance value can be calculated and an off-the-shelf inductance value closest to the calculated value must be selected. The calculation only gives the inductance required but not the size of the inductor, the inductor selected must be capable to handle the maximum peak inductor current without saturating the inductor even at high temperature. The maximum peak inductor current is equal to the maximum average inductor current plus one half of the maximum allowable inductor ripple current, ΔiL(p-p). Selection of Output Capacitor The output capacitor in a boost converter supplies current to the load during the MOSFET on-time and filters the AC components of the load current during the off-time. The selection of this capacitor determines the steady state output voltage ripple, ΔVOUT. The magnitude of this voltage ripple is comprised of three components. The first part of the ripple voltage is the surge current created during the boost diode turns on. The second part is due to the charging and discharging of the output capacitor through the boost diode and the final part is caused by the flow of inductor current through the output capacitor's Equivalent Series Resistance, ESR. Both part 1 and part 3 are related to ESR, in case low ESR ceramic capacitors are used, the contribution of these two parts becomes insignificant. The output voltage ripple can be estimated by the equation in below. Where IOUT is the load current, D is the duty ratio and FSW is the converter switching frequency. From the equation in above, the relationships between different parameters are obvious. A higher output capacitance can reduce the output voltage ripple; however this can slow down the power-up time and the system transient respond. The choice of the output capacitor depends mainly on the application specifications. For most of the cases, multiple iterations are required to come up with an appropriate value. 13 www.national.com AN-1679 Selecting the Current Sensing Resistor The current sensing resistor, RSNS is used for steady state regulation of the inductor current and to provide cycle by cycle current limit function. The resistance selected must be low enough to keep the power dissipation to a minimum and still can maintain good signal-to-noise ratio for the current sensing circuitry. The current limit comparator's threshold is 0.5V. The resistance should be selected so that the switching cycle can be terminated before the inductor current exceeds the saturation rating of the inductor. The required resistor calculation must take into account of both the switch current through the sensing resistor and the compensation ramp current flowing through the internal 2 kΩ resistor and external current sensing network resistors. The worst case average power dissipation in the current sensing resistor, PSNS can be estimated by the equation in below. Where the DMAX is the On Duty Ratio with the input voltage is a minimum. The Control Loop Compensation The control loop is comprised of two parts. The first part is the power stage, which consists of the pulse width modulator, the output filter and the load. The second part is the error amplifier which is realized by an op-amp configured as an inverting amplifier. To close the control loop, compensation is required to ensure stability and optimize system performance. Many techniques exist for selecting the compensation network components. The most popular method is to create the Bode plots of gain and phase for the power stage and error amplifier individually. By combining both stages, the open loop system Bode plots resulted. By using the plots, overall bandwidth, gain margin and phase margin of the regulator can be easily determined. Software tools such as MathCAD, Matlab and Excel can be used to observe how the changes in compensation network and power stage affecting the system gain and phase. One approach to select the compensation network is introduced in the LM3430 datasheet in details. The theoretically calculated compensation network can only be used as the starting point and bench testing and fine tuning is required to come up with the final values. With the demonstration board, a type II compensation network is suggested and the respective component values are listed in below: Power Stage: L = 22 µH ROUT = 417Ω COUT = 22 µF RESR = 350mΩ RSNS = 0.2Ω FSW = 1 MHz Output Voltage Feedback Divider: R13 = 118 kΩ R9 = 3.01 kΩ Compensation Network: R12 = 118 kΩ C4 = 47 nF C9 = 12 pF Where CMODE is in Farads and FPWM is in Hz. In this demonstration board, the capacitor to determine the PWM frequency is C34 and the capacitance used is 680 pF. The PWM frequency is: 30028915 Power Stage Amplifier CURRENT REGULATOR DESIGN WITH LM3432 The LM3432 provides a simple and handy solution to drive strings of serially connected LEDs with precisely controlled constant current. Only couple of external passive components, up to about 120 LEDs in six strings can be lighted up. To control the brightness of the LED strings, both analog and digital dimming method can be used. Programming the LED Current The string current can be programmed by an external resistor, RIREF connected across IREF pin and GND. The equation to calculate the resistance is: With this demonstration board, the string current is 20 mA and the resistor is R25. Applying to the equation: Determination of the Analog Dimming Frequency In analog dimming mode, the internally generated PWM frequency is controlled by the external capacitor, CMODE connected across MODE pin and GND. The equation that governs the relationship is: www.national.com 14 AN-1679 Selection of Gain Setting Resistor for Dynamic Headroom Control By connecting the VDHC pin of LM3432 through a gain setting resistor to the VDHC pin of the LM3430, the LM3432's DHC function will regulate the LED rail voltage from the boost converter to a level just enough to keep all LED strings current in regulation. This gain setting resistor, R19 determines the maximum depth of the rail voltage can be lowered. The rule of thumb on the selection of this resistor is about three times of the resistance of the bottom resistor in the LM3430 output voltage feedback resistor divider. That approximately allows REFERENCE DOCUMENTS Document Number/Type Datasheet Datasheet AN-1529 the rail voltage lowered by one-quarter of the preset boost converter output voltage. In this example, the bottom feedback resistor, R9 is 3.01 kΩ, the gain setting resistor, R19 used is 9.09 kΩ. The small capacitor, C8 connected to the VDHC output pin is for noise filtering, a 100pF capacitor is good enough. The capacitor C35 connected to VDHC pin controls the DHC function respond time, the value needed depends on the PWM dimming frequency. In our demonstration board, the PWM frequency is about 20 kHz, a 100 nF capacitor is used. The DHC function must be kept slow enough to avoid false triggering of the protection logics and fast enough to respond to high PWM dimming frequency with narrow ON pulse width. Bench side testing is required to determine the optimum value for specific requirements Title LM3430 Boost Controller for LED Backlighting LM3432 6-Channel Current Regulator for LED Backlight Application LM3430 Evaluation Board 15 www.national.com LED Backlighting Solution with LM3430 and LM3432 Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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Copyright© 2007 National Semiconductor Corporation For the most current product information visit us at www.national.com AN-1679 National Semiconductor Americas Customer Support Center Email: new.feedback@nsc.com Tel: 1-800-272-9959 National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530-85-86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +49 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: ap.support@nsc.com National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: jpn.feedback@nsc.com Tel: 81-3-5639-7560 www.national.com