National Semiconductor Application Note 2071 SH Wong August 20, 2010 Introduction Standard Settings of the Evaluation Board The LM3464 is a linear LED driver with Dynamic Headroom Control (DHC) technology designed to drive four serial high power / high brightness LED strings. Four individual linear current regulators with low-side N-channel MOSFETs realize regulations of LED currents. The DHC optimizes system efficiency automatically while maintaining stable and accurate LED currents. The LM3464 includes a voltage regulator for powering the internal circuits that operates over wide input range of 12V to 80V, eases the design of driver stage for different source voltage. The Thermal foldback feature is designed to prolong the lifetime of the LEDs by reducing the average LED current for applications with high ambient temperature. The LM3464 features a fault handling circuit that avoids system failure due to accidentally short or open circuit of LED strings. Two or more LM3464 can be cascaded to expand the number of output channels, enabling flexible system architectural design. This evaluation board demonstrates the system efficiency and LED current regulation accuracy of the LM3464 LED driver typical application circuit as a 50W LED lighting system. The schematic, bill of material and PCB layout for the evaluation board are provided in this document. The evaluation board can adapt to different types of primary power supplies with changes of value of a few components only. • Vin range 12V to 80V • 48V LED turn ON voltage • 350mA LED current per channel • 2kHz thermal foldback dimming frequency As the LED turn-on voltage of the LM3464 evaluation board is designed to 48V, it is recommended to set the supply voltage to the evaluation board no more than 60V to avoid high power dissipation on the MOSFETs of the low-side current regulators upon system startup. For driving different number of LEDs, the startup voltage will need to be adjusted by changing the value of the resistors RFB1 and RFB2. Highlight Features • Dynamic Headroom Control (DHC) • Thermal foldback control • High speed PWM dimming • Minimum brightness limit for thermal foldback control • Cascade operation for output channel expansion • Vin Under-Voltage-Lockout • Fault protection and indication • Programmable startup voltage • Thermal Shutdown LM3464 4 Channel LED Driver Evaluation Board LM3464 4 Channel LED Driver Evaluation Board AN-2071 © 2010 National Semiconductor Corporation 301271 www.national.com AN-2071 Evaluation Board Schematic 30127101 FIGURE 1. LM3464 Evaluation Board Schematic www.national.com 2 Designation Description Package Manufacturer Part # U1 LED Driver IC, LM3464 eTSSOP-28 eTSSOP-28 LM3464MH Vendor NSC D1 Schottky Diode 40V 1.1A DO219AB DO219AB SL04-GS08 Vishay MOSFET N-CH 150V 29A D-PAK D-PAK FDD2572 Fairchild MOSFET N-CH 150V 50A TO252–3 TO252–3 IPD200N15N3 Infineon CIN Cap MLCC 100V 2.2uF X7R 1210 1210 GRM32ER72A225KA35L Murata CVCC Cap MLCC 10V 1uF X5R 0603 0603 GRM185R61A105KE36D Murata CDHC Cap MLCC 50V 0.22uF X5R 0603 0603 GCM188R71H224KA64D Murata CFLT Cap MLCC 50V 2.2nF X7R 0603 0603 GRM188R71H222KA01D Murata CTHM Cap MLCC 50V 68nF X7R 0603 0603 GRM188R71H683KA01D Murata Q1,Q2,Q3,Q4 R1 Chip Resistor 8.06Kohm 1% 0603 0603 CRCW06038K06FKEA Vishay RTHM1 Chip Resistor 4.87Kohm 1% 0603 0603 CRCW06034K87FKEA Vishay RTHM2 Chip Resistor 232ohm 1% 0603 0603 CRCW0603232RFKEA Vishay RDMIN1 Chip Resistor 15.4Kohm 1% 0603 0603 CRCW060315K4FKEA Vishay RDMIN2 Chip Resistor 1.05Kohm 1% 0603 0603 CRCW06031K05FKEA Vishay RDHC Chip Resistor 2.67Kohm 1% 0603 0603 CRCW06032K67FKEA Vishay RFB1 Chip Resistor 48.7Kohm 1% 0603 0603 CRCW060348K7FKEA Vishay RFB2 Chip Resistor 2.67Kohm 1% 0603 0603 CRCW06032K67FKEA Vishay RISNS1, RISNS2, RISNS3, RISNS4 Chip Resistor 1.13ohm 1% 0603 0603 CRCW06031R13FKEA Vishay JP1 Chip Resistor 0ohm 1% 0603 0603 CRCW06030000Z0EA Vishay VIN,PGND Banana Jack 5.3(mm) Dia 5.3 (mm) Dia. 575-8 Keystone FAULTb,DIM, SYNC, THM+, THM-, AGND, VFB, VIN, PGND, EN, LED1, LED2, LED3, LED4 Turret 2.35(mm) Dia 2.35 (mm) Dia. 1502-2 Keystone PCB LM3464EVAL PCB 82.5 X 60 (mm) 82.5 x 60 (mm) D2,D3,D4,D5 NA SMA RG1,RG2,RG3,RG4 NA 0603 3 NSC www.national.com AN-2071 Bill of Materials AN-2071 Connectors and Test Pins 30127102 FIGURE 2. Typical Connection Diagram Evaluation Board Quick Setup Procedures Terminal Designation www.national.com Description VIN Power supply positive (+ve) connection PGND Power supply negative (-ve) connection AGND LM3464 analog signal ground LED1 Output Channel 1 (Connect to cathode of LED string 1) LED2 Output Channel 2 (Connect to cathode of LED string 2) LED3 Output Channel 3 (Connect to cathode of LED string 3) LED4 Output Channel 4 (Connect to cathode of LED string 4) EN LM3464 enable pin (pull down to disable) VFB Connect to voltage feedback node of primary power supply for DHC FAULTb Acknowledgement signal for arising of ‘FAULT’ DIM PWM dimming signal input (TTL signal compatible) SYNC Synchronization signal for cascade operation THM+ Connect to NTC thermal sensor for thermal foldback control THM- Connect to NTC thermal sensor for thermal foldback control VLEDFB Connected to LM3464 VLedFB pin OUTP Connected to LM3464 OutP pin VCC LM3464 internal voltage regulator output VDHC Connected to LM3464 VDHC pin 4 The LM3464 evaluation can be powered by an AC/DC power supply through the banana-plug type connectors on the board as shown in figure 2. The component values of this evaluation board is designed to drive 4 LED strings of 12 High Brightness LEDs (HBLEDs) (Vf = 3.5V(typ)) per strings. As this evaluation board is set to drive 48 pieces of HBLEDs at 350mA, which is around 50W power consumption, the primary AC/DC power supply must be able to deliver no less than 50W continuous output power. On the other hand, as every LED strings includes 12 serial LEDs with total forward voltage around 40V, the AC/DC power supply should be able to supply no less than 41V (with around 1V current regulator dropout) to power up the LEDs. In order to cover variations of LED forward voltage under different ambient temperatures, a 50W AC/DC power supply with 48V nominal output voltage is recommended. In order to facilitate Dynamic Headroom Control (DHC), the output voltage of the AC/DC power supply is pushed up to 48V under control of the LM3464 without violating the rated output voltage of the AC/DC converter. The required voltage headroom is obtained by reducing the nominal 48V output voltage of a AC/DC converter. For example: adjusting the nominal output voltage of a AC/DC converter from 48V(nom.) to 36V(nom.). Prior to connect the AC/DC power supply to the LM3464 evaluation board, the nominal output voltage of the AC/DC power supply has to be adjusted down to allow DHC to take place. To reduce the nominal output voltage of the AC/DC converter, the output voltage feedback circuit of the selected AC/DC (1) For VREF(AC/DC) = 2.5V And VRAIL(nom) = 36V: (2) After the modification, the AC/DC power supply will have a 36V nominal output voltage (VRAIL(nom)). As the output of the AC/DC converter is connected to the power rail to supply power to the LEDs, the rail voltage (VRAIL) is pushed up by the LM3464 up to certain level (VDHC_READY) that is adequate to drive 12 serial LEDs with current regulation before turning on the LEDs. In this evaluation board, VDHC_READY is set at 48V which is adequate for most situations with 12 serial LEDs driving at 350mA per channel. Figure 3 shows the changes of VRAIL upon the AC/DC power supply powers up until the system enters steady state operation. 30127103 FIGURE 3. Changes of Rail Voltage Upon Power Up 5 www.national.com AN-2071 converter has to be located and the reference voltage for output voltage sensing should be identified. Figure 2 shows the voltage feedback circuit with LM431 that has been widely used in typical AC/DC power supplies as an example. Assume the output voltage of the AC/DC power supply is 48V and is going to be adjusted down to 36V, the resistance of R2 has to be increased without changing the value of R1. The output voltage and value of R2 are related by the following equation: Selection of AC/DC Power Supply AN-2071 VRAIL(peak) is the maximum voltage that VRAIL can reach if the OutP is accidentally shorted to GND. The VRAIL(peak) must not exceed the rated output voltage of the AC/DC converter even if the OutP pin is shorted to GND (VOutP = 0V). The level of VRAIL(peak) is depends on the value of RCDHC and can be calculated following this equation: creased to reduce power dissipation on the MOSFETs. By default, the VDHC pin is biased internally to 0.9V as shown in figure 4. (3) where (4) In this equation, VREF(AC/DC) is the reference voltage for output voltage feedback of the AC/DC power supply. R1 and R2 are the resistors of the output voltage feedback resistor divider of the AC/DC power supply. When designing the values of the RDHC, it is essential to ensure that the VRAIL(peak) does not exceed the rated output voltage of the AC/DC power supply, otherwise the AC/DC power supply could be damaged. Setting of VDHC_READY As VRAIL reaches VDHC_READY which defines by RFB1 and RFB2, the voltage at the VLedFB pin of the LM3464 should equal to 2.5V. As the VLedFB pin voltage reaches 2.5V, the LM3464 performs a test no long than 400uS to identify the output channels that are not in use (No LED connected). If the voltage of any current sensing input pins of the LM3464 (SE1 — SE4) is below 30mV because of open circuit of LED string (s), that particular output channel will be disabled (latched off) and taken out from DHC control loop by the LM3464 to allow the functional LED strings to operate. Similarly, if any drain voltage of the MOSFETS (DR1 — DRx) is 8.4V exceeding the drain voltage of any other channel due to short circuit of LEDs, that particular channel will be disabled to avoid permanent damage. After the detection is completed, LED strings turn on and current regulation starts. The level of VDHC_READY is defined by the values of RFB1 and RFB2 on the evaluation board and can be adjusted to any level below 80V as desired. By default, the VDHC_READY is set at 48V. The VDHC_READY must set no more than 20V higher than the forward voltages of any LED string connected to the system under possible temperatures, otherwise a ‘short fault’ may arise and results in immediate output channel latch-off to protect the MOSFETs from over-heat. The VDHC_READY is can be adjusted according to the equation (5): 30127104 FIGURE 4. Adjusting the VDHC Pin Voltage To adjust VVDHC on the evaluation board, an additional resistor divider (RA and RB) can be added across the VDHC test pad and VCC or AGND terminals on the board. It is recommended that the resistances of RA and R B are no more than 100kΩ and 16kΩ respectively to ensure the accuracy of the headroom voltage under steady state. The VVDHC is governed by the following equation: (6) where (7) Connecting the LED Strings The LM3464 evaluation board is designed to drive 4 common anode LEDs strings of 12 serial LEDs per string. The board includes four turret connectors, LED1, LED2, LED3 and LED4 for cathode connections of the LED strings. The anode of the LED strings should connect to the positive power output terminal. By default, the output current for every output channel is set at 350mA. The driving currents can be adjusted by changing the value of the resistors RISNS1, RISNS2, RISNS3 and RISNS4 respectively. The LED driving current is governed by the following equation: (5) Adjusting Voltage Headroom The voltage headroom of the LM3464 evaluation board can be increased or decreased by adjusting the voltage at the VDHC pin (VVDHC) in the range of 0.8V to 2V. In applications with low rail voltage ripple, the voltage headroom can be de- www.national.com (8) 6 The frequency response of the LM3464 evaluation board can be adjusted by changing the value of the capacitor, CDHC. Higher capacitance of CDHC results in slower frequency response of the LM3464 driver stage. In order to ensure stable system operation, it is recommended to set the dominant pole of the LM3464 one decade lower than the dominant pole of the AC/DC converter. The default value of the CDHC on the evaluation board is 0.22uF. For applications with slow response AC/DC power supply (e.g. converters with active PFC), the CDHC value should be increased to make the frequency response of the board slower than that of the AC/DC power supply. The cut-off frequency of the LM3464 driver stage is governed by the following equation: (11) (9) Thermal Foldback Control The LM3464 evaluation board features an interface that enables thermal foldback control by connecting a NTC thermal sensor to the THM+ and THM- terminals. With the NTC sensor attached to the chassis of the LED arrays, the thermal foldback control feature reduces the average LED current and effectively reduces the LED temperature to prevent thermal breakdown of the LEDs. The thermal foldback control reduces the LED currents by means of PWM dimming which the dimming frequency is set by the capacitor CTHM following the equation shows below: 30127113 FIGURE 5. Changes of Average LED Current with Thermal Foldback Control Design Example (10) A NTC thermistor is connected to the THM+ and THM- terminals of the LM3464 evaluation board as shown in figure 6 to activate the thermal foldback control function. The default value of the CTHM on the LM3464 evaluation board is 68nF, which set the thermal foldback dimming frequency at 258Hz. 7 www.national.com AN-2071 Thermal foldback control is activated when the voltage at the Thermal pin, VThermalis between 3.25V and 0.4V as shown in figure 5. Thermal foldback control begins when VThermal is below 3.25V. The LED current will be reduced to zero as VThermal falls below 0.4V. The average LED current varies according to the Thermal pin voltage following the equation: Adjusting Frequency Response of the LM3464 Circuit AN-2071 30127114 FIGURE 6. Attaching NTC Thermistor to the LEDs Assuming that thermal foldback control is required to begin at 70°C LED chassis temperature and reduce 55% average LED current (45% dimming duty cycle) when the chassis temperature reaches 125°C. Using the NTC thermister NXFT15WB473FA1B from MURATA, which has 4.704kΩ resistance at 70°C (RNTC(70°C)) and 1.436kΩ at 125°C (RNTC(125°C)). VThermal at 125°C (45% dimming duty cycle): By combining the equations (17) and (18), the values of RTHM1 and RTHM2 can be obtained: (12) (13) (19) (14) The default values of RTHM1 and RTHM2 on the LM3464 evaluation board are 4.87kΩ and 232Ω respectively. In the above equation, VThermal is the voltage at the Thermal pin and DTHMFB is the dimming duty cycle under thermal foldback control. When thermal foldback begins: Minimum Dimming Duty Cycle for Thermal Foldback Control The minimum dimming duty cycle for thermal foldback control (DTHMFB_MIN) can be limited by setting the voltage at the DMIN pin of the LM3464. This limit will override the minimum dimming level defined by RTHM1, RTHM2 and the NTC thermistor. This function is especially useful for the applications that require to maintain certain brightness level under high operation temperature. The minimum duty cycle limit is governed by the following equation: (15) (16) (17) When the temperature goes up to 125°C (20) (18) www.national.com 8 PWM dimming control can be realized by applying PWM dimming signal to the DIM terminal of the board directly. When the DIM pin is pulled to logic high, all output channles are enabled. When the DIM pin is pulled to logic low (GND), all output channels are turned OFF. In cascade operation, the DIM signal should apply to the MASTER unit only. The LM3464 on the MASTER unit propagates the PWM dimming 30127115 FIGURE 7. Thermal Foldback + PWM Dimming Control 9 www.national.com AN-2071 signal on its DIM pin to the slave units one by one through the SYNC pin. PWM dimming control is allowed when thermal foldback control is activated. When PWM dimming and thermal foldback controls are required simultaneously, the PWM dimming frequency should be set at least ten times below the thermal foldback dimming frequency. The thermal foldback dimming signal reduced the LED currents according to the voltage level of the Thermal pin when the signal at the DIM pin is ‘high’ as shown in figure 7. PWM Dimming AN-2071 Cascade Operation 30127162 FIGURE 8. Cascading LM3464 Evaluation Boards for Output Channel Expansion www.national.com 10 • Connect VLEDFB to VCC When connecting the master and slaves, the following connections are required: • Connect the VIN terminal of master and slave units together and then to the POSITIVE output of the AC/DC power supply 1. Detect rail voltage upon system startup • Connect the PGND terminal of master and slave units together and then to the NEGATIVE output of the AC/DC power supply 2. Command slave units to turn on LEDs as its VLedFB voltage reaches 2.5V • Connect the SYNC terminal of the master unit to DIM terminal of the next slave unit in the chain. 3. Provide dimming signal to slave units according to the PWM dimming signal received at its DIM pin • Connect the VFB terminal of master and slave units together and then to the voltage feedback node of the AC/DC power supply. 4. Provide dimming signal to slave units according to the voltage of its Thermal pin If more than one slave unit is required, the SYNC pin of the first slave unit should connect to the DIM pin of the next slave unit to allow propagation of the control signal along the system chain. By default, the LM3464 evaluation board is set to master mode. To set the board to slave mode, the following changes to the board are required: • Remove resistors RFB1 and RFB2 11 www.national.com AN-2071 The total output power of the LED lighting system can be expanded by cascading the LM3464 evaluation boards. In cascade operation, the system involves one master unit and multiple slave units. Both master and slave units are LM3464 evaluation boards with minor modifications to program the LM3464 into master or slave modes. The connection diagram for cascade operation is shown in figure 8. The master unit is responsible to provide functions as listed in below: AN-2071 Typical Performance and Waveforms All curves taken at VIN = 48V with configuration in typical application for driving twelve power LEDs with four output channels active and output current per channel = 350mA. TA = 25°C, unless otherwise specified. Output Current Variation Efficiency (%) 30127120 30127121 VCC Variation (%) DHC in Cascade Operation 30127123 30127122 VSYNC at System Startup VSYNC of Master Unit 30127124 www.national.com 30127125 12 AN-2071 Evaluation Board Layout 30127126 FIGURE 9. Top Layer and Top Overlay 30127127 FIGURE 10. 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