LM3508TLX

LM3508 Synchronous Magnetic Constant Current White LED Driver March 6, 2008 LM3508 Synchronous Magnetic Constant Current White LED Driver General Description The LM3508 is a synchronous boost converter (no external Schottky diode required) that provides a constant current output. It is designed to drive up to 4 series white LEDs at 30mA from a single-cell Li-Ion battery. A single low power external resistor is used to set the maximum LED current. The LED current can be adjusted by applying a PWM signal of up to 100kHz to the DIM pin. Internal soft-start circuitry is designed to eliminate high in-rush current at start-up. For maximum safety, the device features an advanced short-circuit protection when the output is shorted to ground. Additionally, overvoltage protection and an 850kHz switching frequency allow for the use of small, low-cost output capacitors with lower voltage ratings. During shutdown, the output is disconnected from the input preventing a leakage current path through the LEDs to ground. The LM3508 is available in a tiny 9-bump chip-scale micro-SMD package. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Drives 4 Series White LEDs with up to 30mA >80% Peak Efficiency Up to 100kHz PWM Brightness Control Accurate ±5% LED Current Regulation across VIN range Internal Synchronous PFET (No Schottky Diode Required) True Shutdown Isolation Output Short-Circuit Protection 17.5V Over-Voltage Protection Internal Soft-Start Eliminates Inrush Current Wide Input Voltage Range: 2.7V to 5.5V 850kHz Fixed Frequency Operation Low Profile 9-Bump Micro-SMD Package (1.514mm x 1.514mm x 0.6mm) Applications ■ ■ ■ ■ White LED Backlighting Handheld Devices Digital Cameras Portable Applications Typical Application Circuit 30004201 © 2008 National Semiconductor Corporation 300042 www.national.com LM3508 Connection Diagram Top View 30004202 9-Bump (Large) µ-SMD (1.514mm x 1.514mm x 0.6mm) NS Package Number TLA09 Ordering Information Part Number LM3508TL LM3508TLX Package Type 9-Bump micro SMD 9-Bump micro SMD NSC Package Drawing Top Mark TL09SDA TL09SDA D31 D31 Supplied As 250 Units, Tape and Reel 3000 Units, Tape and Reel Pin Descriptions/Functions Pin A1 A2 A3 B1 B2 B3 C1 C2 C3 Name PGND SW OUT ILED DIM IN SET EN AGND Power Ground Connection. Inductor connection and drain connection for both NMOS and PMOS power devices. Output capacitor connection, PMOS source connection for synchronous rectifier, and OVP sensing node. Regulated current source input. Current source modulation input. A logic low at DIM turns off the internal current source. A logic high turns the LEDs fully on (VSET=200mV). Apply a PWM signal at DIM for LED brightness control. Input voltage connection. Current sense connection and current source output. Connect a 1% resistor (RSET) from SET to PGND to set the maximum LED current (ILED = 200mV/RSET) . Enable input. A logic low at EN turns off the LM3508. A logic high turns the device on. Analog ground. Connect AGND to PGND through a low impedance connection. Function www.national.com 2 LM3508 Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN VOUT VSW VILED, VSET, VDIM, VEN Continuous Power Dissipation (Note ) Junction Temperature Lead Temperature (Note 4) Storage Temperature Range ESD Rating(Note 9) Human Body Model −0.3V to 6V −0.3V to 22V −0.3V to 22V −0.3V to 6V Internally Limited +150°C +300°C -65°C to +150°C 2kV Operating Conditions Input Voltage Range Ambient Temperature Range (Note 5) Junction Temperature Range (Notes 1, 2) 2.7V to 5.5V −30°C to +85°C −30°C to +105°C Thermal Properties Junction to Ambient Thermal Resistance (θJA)(Note 6) 64.7°C/W ESD Caution Notice National Semiconductor recommends that all integrated circuits be handled with appropriate ESD precautions. Failure to observe proper ESD handling techniques can result in damage to the device. Electrical Characteristics Specifications in standard type face are for TA = 25°C and those in boldface type apply over the Operating Temperature Range of TA = −30°C to +85°C. Unless otherwise specified VIN =3.6V.(Note 7) Symbol ID VSET VILED VHR RDSON Parameter LED Current Regulation Voltage at SET Pin Voltage at ILED Pin Current Sink Headroom Voltage NMOS Switch On Resistance PMOS Switch On Resistance ICL ILSW IOUT_SHUTDOWN NMOS Switch Current Limit SW Leakage Current VSW = VIN = 5.5V, OUT Floating, VEN = PGND Outout Pull-Down Resistance in Shutdown VEN = 0V 630 17.5 19.8 18.6 715 850 91 VOUT Falling VOUT Rising On Threshold Off Threshold On Threshold Off Threshold VDIM = 1.8V 4.7 1.1 0.5 1.1 0.5 V V µA 0.93×VIN 0.95×VIN V 1150 21.8 V kHz % Where ILED = 95% of nominal, RSET = 20Ω ISW = 100mA VOUT = 10V, ISW = 65mA 370 Conditions RSET = 10Ω RSET = 6.67Ω 3.0V < VIN < 5.5V 190 Min Typ 20 30 200 500 400 0.5 Ω 2.2 500 0.01 620 mA µA 210 Max Units mA mV mV mV Ω VOVP Output Over-Voltage ON Threshold (VOUT rising) Protection OFF Threshold (VOUT falling) Switching Frequency 3.0V < VIN < 5.5V Maximum Duty Cycle Output Voltage Threshold for Short Circuit Detection EN Threshold Voltage DIM Threshold Voltage DIM Bias Current (Note 8) fSW DMAX VSC VEN_TH VDIM_TH IDIM 3 www.national.com LM3508 Symbol IEN IOUT ROUT_SHUTDOWN Parameter EN Bias Current (Note 8) OUT Bias Current Output Pull-Down Resistance in Shutdown Conditions VEN = 1.8V VOUT = 16V, device not switching VEN = 0V, VOUT < VIN Min Typ 4.7 420 Max Units µA µA 630 Ω IQ Quiescent Current VILED > 0.5V, 3.0V < VIN < Device Not Switching 5.5V, SW Floating VEN = 0V, 3.0V < VIN < 5.5V Switching Supply Current From EN Low to High VOUT = 17V, ILED = 20mA to Inductor Current Steady State 0.18 0.01 825 0.3 0.5 mA IQ_SW tSTART_UP µA 470 µs Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: All voltages are with respect to PGND. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown enagaes at TJ = +150°C (typ.) and disengages at TJ = +140°C (typ.). Note 4: For more detailed soldering information and specifications, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip Scale Package (AN-1112), available at www.national.com. Note 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = +125ºC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX). Note 6: Junction-to-ambient thermal resistance (θJA) is taken from thermal modeling performed under the conditions and guidelines set forth in the JEDEC standard JESD51-7. The test board is a 4-layer FR-4 board mesuring (102mm × 76mm × 1.6mm) with a 2 × 1 array of thermal vias. The ground plane on the board is (50mm × 50mm). Thickness of copper layers are (36µm/18µm/18µm/36µm) (1.5oz/1oz/1oz/1.5oz copper). Ambient temperature in simulation is +22°C, still air. Power dissipation is 1W. Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Unless otherwise specified, conditions for typical specifications are VIN = 3.6V, TA = +25°C. Note 8: There is a typical 383kΩ pull-down on this pin. Note 9: The human body model is a 100pF capacitor discharged through 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7). Typical Performance Characteristics VIN = 3.6V, RSET = 10Ω, L = TDK VLF3012AT-220MR33 (22µH), LEDs are OSRAM (LW M67C), COUT = CIN = 1µF, TA = +25°C, unless otherwise noted. 4 LED Efficiency vs ILED (L = TDK VLF3012AT-220MR33, RL = 0.66Ω) 3 LED Efficiency vs ILED (L = TDK VLF3012AT-220MR33, RL = 0.66Ω) 30004242 30004241 www.national.com 4 LM3508 2 LED Efficiency vs ILED (L = TDK VLF3012AT-220MR33, RL = 0.66Ω) Converter Output Voltage vs LED Current 30004243 30004258 Efficiency vs VIN (ILED = 20mA) Efficiency vs VIN (ILED = 30mA) 30004260 30004261 Peak Current Limit vs VIN Switching Frequency vs VIN 30004213 30004214 5 www.national.com LM3508 Maximum Duty Cycle vs VIN Quiescent Current vs VIN (EN = GND) 30004215 30004217 Quiescent Current vs VIN (Device Not Switching, VIN = VSW) Quiescent Current vs VIN (Device Switching) 30004216 30004218 SET Voltage vs DIM Frequency (50% Duty Cycle at DIM) SET Voltage vs VIN 30004259 30004219 www.national.com 6 LM3508 SET Voltage vs DIM Duty Cycle NFET On-Resistance vs VIN (ISW = 250mA) 30004262 30004224 PFET On-Resistance vs Temperature (VSW = 10.4V, VOUT = 10V) Over Voltage Limit vs VIN (VOUT Rising) 30004225 30004227 Over Voltage Limit vs VIN (VOUT Falling) Start-Up Waveform 30004257 4 LEDs, ILED = 30mA, VIN = 3.6V Channel 1: VOUT (10V/div) Channel 2: EN (2V/div) Channel 4: IIN (200mA/div) Time Base: 100µs/div 30004228 7 www.national.com LM3508 Over-Voltage Protection Function Line-Step Response 30004237 30004255 VIN = 3.6V, VOUT = 18.86V Channel 1: VOUT (1V/div) Channel 4: IIN (500mA/div) Time Base: 400µs/div VIN = 3.6V, 4 LEDs Channel 1: VOUT (AC Copupled, 1V/div) Channel 3: VIN (AC Coupled, 500mV/div) Channel 4: ILED (DC Coupled, 5mA/div) Time Base: 200µs/div Output Short-Circuit Response Typical Operating Waveforms (DIM High) 30004256 30004229 VIN = 3.6V, ILED = 30mA Channel 1: VOUT (10V/div) Channel 2: IIN (100mA/div) Time Base: 200µs/div VIN = 3.6V, 4 LEDs, ILED = 30mA, VOUT = 15.8V Channel 1: VOUT (AC Coupled, 100mV/div) Channel 2: VSW (DC Coupled, 10V/div) Channel 4: IL (DC Coupled, 100mA/div) Time Base: 400ns/div Typical Operating Waveforms (DIM With 20kHz Square Wave) DIM Operation (ILED changing from 30mA to 15mA) 30004238 30004230 VIN = 3.6V, 4 LEDs, ILED = 15mA Channel 1: VOUT (AC Coupled, 200mV/div) Channel 3: VIN (AC Coupled, 100mV/div) Channel 2: IL (DC Coupled, 100mA/div) Channel 4: DIM (DC Coupled, 2V/div) Time Base: 10µs/div VIN = 3.6V Channel 4: ILED (DC Coupled, 10mA/div) Channel 2: VOUT (AC Coupled, 2V/div) Channel 1: DIM (DC Coupled, 2V/div, 20kHz, 50% duty cycle) Channel 3: IIN (DC Coupled, 200mA/div) Time Base: 400µs/div www.national.com 8 LM3508 Operation 30004203 FIGURE 1. LM3508 Block Diagram The LM3508 utilizes a synchronous step-up current mode PWM controller and a regulated current sink to provide a highly efficient and accurate LED current for white LED bias. The internal synchronous rectifier increases efficiency and eliminates the need for an external diode. Additionally, internal compensation eliminates the need for external compensation components resulting in a compact overall solution. Figure 1 shows the detailed block diagram of the LM3508. The output of the boost converter (OUT) provides power to the series string of white LED’s connected between OUT and ILED. The boost converter regulates the voltage at ILED to 500mV. This voltage is then used to power the internal current source whose output is at SET. The first stage of the LM3508 consists of the synchronous boost converter. Operation is as follows: At the start of each switching cycle the oscillator sets the PWM controller. The controller turns the low side (NMOS) switch on and the synchronous rectifier (PMOS) switch off. During this time current ramps up in the inductor while the output capacitor supplies the current to the LED’s. The error signal at the output of the error amplifier is compared against the sensed inductor current. When the sensed inductor current equals the error signal, or when the maximum duty cycle is reached, the NMOS switch turns off and the PMOS switch turns on. When the PMOS turns on, the inductor current ramps down, restoring energy to the output capacitor and supplying current to the 9 LED’s. At the end of the clock period the PWM controller is again set and the process repeats itself. This action regulates ILED to 500mV. The second stage of the LM3508 consists of an internal current source powered by the ILED voltage and providing a regulated current at SET. The regulated LED current is set by connecting an external resistor from SET to PGND. VSET is adjusted from 0 to 200mV by applying a PWM signal of up to typically 100kHz at DIM (see Typical Performance Characteristic of SET voltage vs DIM frequency). The PWM signal at DIM modulates the internal 200mV reference and applies it to an internal RC filter resulting in an adjustable SET voltage and thus an adjustable LED current. Start-Up The LM3508 features a soft-start to prevent large inrush currents during start-up that can cause excessive voltage ripple on VIN. During start-up the average input current is ramped up at a controlled rate. For the typical application circuit, driving 4LED’s from a 3.6V lithium battery at 30mA, when EN is driven high the average input current ramps from zero to 160mA in 470µs. See plot of Soft Start functionality in the Typical Performance Characteristics. www.national.com LM3508 DIM Operation DIM is the input to the gate of an internal switch that accepts a logic level PWM waveform and modulates the internal 200mV reference through an internal RC filter. This forces the current source regulation point (VSET) to vary by the duty cycle (D) of the DIM waveform making ILED = D × 200mV / RSET. The cutoff frequency for the filter is approximately 500Hz. DIM frequencies higher than 100kHz cause the LED current to drastically deviate from their nominal set points. The graphs of SET voltage vs DIM frequency, SET voltage vs VIN and SET voltage vs DIM duty cycle (see Typical Performance Characteristics) show the typical variation of the current source set point voltage. figure 1) will be fully on, appearing as a 5Ω resistor between ILED and SET. Output Short Circuit Protection The LM3508 provides a short circuit protection that limits the output current if OUT is shorted to PGND. During a short at OUT when VOUT falls to below VIN × 0.93, switching will stop. The PMOS will turn into a current source and limit the output current to 35mA. The LM3508 can survive with a continuous short at the output. The threshold for OUT recovering from a short circuit condition is typically VIN × 0.95. Output Over-Voltage Protection When the load at the output of the LM3508 goes high impedance the boost converter will raise VOUT to try and maintain the programmed LED current. To prevent over-voltage conditions that can damage output capacitors and/or the device, the LM3508 will clamp the output at a maximum of 21.8V. This allows for the use of 25V output capacitors available in a tiny 1.6mm × 0.8mm case size. During output open circuit conditions when the output voltage rises to the over voltage protection threshold (VOVP = 19.8V typical) the OVP circuitry will shut off both the NMOS and PMOS switches. When the output voltage drops below 18.6V (typically) the converter will begin switching again. If the device remains in an over voltage condition the cycle will be repeated resulting in a pulsed condition at the output. See waveform for OVP condition in the Typical Performance Characteristics. Enable Input and Output Isolation Driving EN high turns the device on while driving EN low places the LM3508 in shutdown. In shutdown the supply current reduces to less than 1µA, the internal synchronous PFET turns off as well as the current source (N2 in figure 1). This completely isolates the output from the input and prevents leakage current from flowing through the LED’s. In shutdown the leakage current into SW and IN is typically 400nA. EN has an internal 383kΩ pull-down to PGND. Peak Current Limit/Maximum Output Current The LM3508 boost converter provides a peak current limit. When the peak inductor current reaches the peak current limit the duty cycle is terminated. This results in a limit on the maximum output power and thus the maximum output current the LM3508 can deliver. Calculate the maximum LED current as a function of VIN, VOUT, L and IPEAK as: Light Load Operation During light load conditions when the inductor current reaches zero before the end of the switching period, the PFET will turn off, disconnecting OUT from SW and forcing the converter into discontinuous conduction. At the beginning of the next switching cycle, switching will resume. (see plot of discontinuous conduction mode in the Typical Performance Characteristics graphs). Boost converters that operate in the discontinuous conduction mode with fixed input to output conversion ratios (VOUT/ VIN) have load dependent duty cycles, resulting in shorter switch on-times as the load decreases. As the load is decreased the duty cycle will fall until the converter hits its minimum duty cycle (typically 15%). To prevent further decreases in the load current altering the VOUT/VIN ratio, the LM3508 will enter a pulsed skip mode. In pulse skip mode the device will only switch as necessary to keep the LED current in regulation. and fSW = 850kHz. Efficiency and IPEAK can be found in the efficiency and IPEAK curves in the Typical Performance Characteristics. Output Current Accuracy The LM3508 provides highly accurate output current regulation of ±5% over the 3V to 5.5V input voltage range. Accuracy depends on various key factors. Among these are; the tolerance of RSET, the frequency at DIM (ƒDIM), and the errors internal to the LM3508 controller and current sink. For best accuracy, use a 1% resistor for RSET and keep ƒDIM between 1kHz and 100kHz. Refer to the Typical Performance Characteristics for VSET vs VIN, VSET vs ƒDIM, and VSET vs DIM duty cycle. Thermal Shutdown The LM3508 provides a thermal shutdown feature. When the die temperature exceeds +150°C the part will shutdown, turning off both the NMOS and PMOS FET’s. The part will startup again with a soft-start sequence when the die temperature falls below +115°C. Voltage Head Room at ILED If the LED current is increased to a point where the peak inductor current is reached, the boost converter's on-time is terminated until the next switching cycle. If the LED current is further increased the 500mV regulated voltage at ILED begins to drop. When VILED drops below the current sink headroom voltage (VHR = 400mV typ.) the current sink FET (see N2 in www.national.com 10 Applications Information Brightness Adjustment A logic high at DIM forces SET to regulate to 200mV. Adjust the maximum LED current by picking RSET (the resistor from SET to GND) such that: LM3508 Once ILED_MAX is set, the LED current can be adjusted from ILED_MAX down to ILED_MIN by applying a logic level PWM signal to DIM. This results in: input capacitor, the output voltage ripple is composed of two parts, the ripple due to capacitor discharge (delta VQ) and the ripple due to the capacitors ESR (delta VESR). Most of the time the LM3508 will operate in continuous conduction mode. In this mode the ripple due to capacitor discharge is given by: where D is the duty cycle of the PWM pulse applied to DIM. The LM3508 can be brought out of shutdown while a signal is applied to DIM, allowing the device to turn on into a low LED current mode. A logic low at DIM will shut off the current source making ILED high impedance however, the boost converter continues to operate. Due to an offset voltage at SET (approximately +/-2mV) the LED’s can faintly illuminate even with DIM pulled to GND. If zero LED current is required then pulling EN low will shutdown the current source causing the LED current to drop to zero. DIM has an internal 383kΩ pull down to PGND. Input Capacitor Selection Choosing the correct size and type of input capacitor helps minimize the input voltage ripple caused by the switching action of the LM3508’s boost converter. For continuous inductor current operation the input voltage ripple is composed of 2 primary components, the capacitor discharge (delta VQ) and the capacitor’s equivalent series resistance (delta VESR). The ripple due to strictly to the capacitor discharge is: The output voltage ripple component due to the output capacitors ESR is found by: Table 1. Recommended Output Capacitor Manufacturers Manufact urer Murata TDK Part Number GRM39X5 R105K25 D539 C1608X5 R1E105M Value 1µF Case Size 0603 Voltage Rating 25V 1µF 0603 25V Inductor Selection The LM3508 is designed to operate with 10µH to 22µH inductor’s. When choosing the inductor ensure that the inductors saturation current rating is greater than The ripple due to strictly to the capacitors ESR is: In the typical application circuit, a 1µF ceramic input capacitor works well. Since the ESR in ceramic capacitors is typically less than 5mΩ and the capacitance value is usually small, the input voltage ripple is primarily due to the capacitive discharge. With larger value capacitors such as tantalum or aluminum electrolytic the ESR can be greater than 0.5Ω. In this case the input ripple will primarily be due to the ESR. Output Capacitor Selection In a boost converter such as the LM3508, during the on time, the inductor is disconnected from OUT forcing the output capacitor to supply the LED current. When the PMOS switch (synchronous rectifier) turns on the inductor energy supplies the LED current and restores charge to the output capacitor. This action causes a sag in the output voltage during the on time and a rise in the output voltage during the off time. The LM3508’s output capacitor is chosen to limit the output ripple to an acceptable level and to ensure the boost converter is stable. For proper operation use a 1µF ceramic output capacitor. Values of 2.2µF or 4.7µF can be used although startup current and start-up time will be increased. As with the 11 Additionally, the inductor’s value should be large enough such that at the maximum LED current, the peak inductor current is less than the LM3508’s peak switch current limit. This is done by choosing L such that Values for IPEAK and efficiency can be found in the plot of peak current limit vs. VIN in the Typical Performance Characteristics graphs. www.national.com LM3508 Table 2. Recommended Inductor Manufacturers Manufacture r TDK L Part Number Size 2.6mm×2. 8mm×1m m 2.6mm×2. 8mm×1.2 mm Saturation Current 330mA 22µ VLF3010 H AT-220M R33 22µ VLF3012 H AT-220M R33 TDK 330mA Toko 22u D3313FB 3.3mm×3. H (1036FB- 3mm×1.3 220M) mm 350mA Layout Considerations Proper layout is essential for stable, jitter free operation, and good efficiency. Follow these steps to ensure a good layout. 1, Use a separate ground plane for power ground (PGND) and analog ground (AGND). 2, Keep high current paths such as SW and PGND connections short. 3, Connect the return terminals for the input capacitor and the output capacitor together at a single point as close as possible to PGND. 4, Connect PGND and AGND together as close as possible to the IC. Do not connect them together anywhere else. 5, Connect the input capacitor (CIN) as close as possible to IN. 6, Connect the output capacitor (COUT) as close as possible to OUT. 7, Connect the positive terminal of RSET as close as possible to ILED and the negative terminal as close as possible to PGND. This ensures accurate current programming. www.national.com 12 LM3508 Physical Dimensions inches (millimeters) unless otherwise noted 9-Bump Micro SMD Package (TL09AAA) For Ordering, Refer to Ordering Information Table NS Package Number TLA09AAA X1 = 1.514mm (±0.03mm), X2 = 1.514mm (±0.03mm), X3 = 0.6mm (±0.075mm) 13 www.national.com LM3508 Synchronous Magnetic Constant Current White LED Driver Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock Conditioners Data Converters Displays Ethernet Interface LVDS Power Management Switching Regulators LDOs LED Lighting PowerWise Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/displays www.national.com/ethernet www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH Analog University App Notes Distributors Green Compliance Packaging Design Support www.national.com/webench www.national.com/AU www.national.com/appnotes www.national.com/contacts www.national.com/quality/green www.national.com/packaging www.national.com/quality www.national.com/refdesigns www.national.com/feedback Quality and Reliability Reference Designs Feedback THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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