LM3528

LM3528 High Efficiency, Multi Display LED Driver with 128 Exponential Dimming Steps and Integrated OLED Power Supply in micro SMD August 3, 2008 LM3528 High Efficiency, Multi Display LED Driver with 128 Exponential Dimming Steps and Integrated OLED Power Supply in a 1.2mm × 1.6mm µSMD Package General Description The LM3528 current mode boost converter offers two separate outputs. The first output (MAIN) is a constant current sink for driving series white LED’s. The second output (SUB/FB) is configurable as a constant current sink for series white LED bias, or as a feedback pin to set a constant output voltage for powering OLED panels. As a dual output white LED bias supply, the LM3528 adaptively regulates the supply voltage of the LED strings to maximize efficiency and insure the current sinks remain in regulation. The maximum current per output is set via a single external low power resistor. An I2C compatible interface allows for independent adjustment of the LED current in either output from 0 to max current in 128 exponential steps. When configured as a white LED + OLED bias supply the LM3528 can independently and simultaneously drive a string of up to 6 white LED’s and deliver a constant output voltage of up to 21V for OLED panels. Output over-voltage protection shuts down the device if VOUT rises above 22V allowing for the use of small sized low voltage output capacitors. Other features include a dedicated general purpose I/O (GPIO) and a multi-function pin (HWEN/ PGEN/GPIO) which can be configured as a 32 bit pattern generator, a hardware enable input, or as a GPIO. When configured as a pattern generator, an arbitrary pattern is programmed via the I2C compatible interface and output at HWEN/PGEN/GPIO for indicator LED flashing or for external logic control. The LM3528 is offered in a tiny 12-bump µSMD package and operates over the -40°C to +85°C temperature range. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 128 Exponential Dimming Steps Programmable Auto-Dimming Function Up to 90% Efficient Low Profile 12 Bump µ-SMD Package (1.2mm x 1.6mm x 0.6mm) Integrated OLED Display Power Supply and LED Driver Programmable Pattern Generator Output for LED Indicator Function Drives up to 12 LED’s at 20mA Drives up to 5 LED’s at 20mA and delivers 18V at 40mA 1% Accurate Current Matching Between Strings Internal Soft-Start Limits Inrush Current True Shutdown Isolation for LED’s Wide 2.5V to 5.5V Input Voltage Range 22V Over-Voltage Protection 1.25MHz Fixed Frequency Operation Dedicated Programmable General Purpose I/O Active Low Hardware Reset Applications ■ ■ ■ ■ Dual Display LCD Backlighting for Portable Applications Large Format LCD Backlighting OLED Panel Power Supply Display Backlighting with Indicator Light 30020563 Typical PCB Layout 30020501 Typical Application Circuit © 2008 National Semiconductor Corporation 300205 www.national.com LM3528 Connection Diagram Top View 30020502 12-Bump (1.215mm × 1.615mm × 0.6mm) TMD12AAA Ordering Information Order Number LM3528TME LM3528TMX Package Type 12-Bump µSMD 12-Bump µSMD NSC Package Drawing TMD12AAA TMD12AAA Top Mark SE SE Supplied As 250 units, Tape-and-Reel, No Lead 3000 units, Tape-and-Reel, No Lead Pin Descriptions/Functions Pin A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 Name OVP MAIN SUB/FB GPIO1 SCL SET HWEN/PGEN/ GPIO SDA IN VIO SW GND Function Over-Voltage Protection Sense Connection. Connect OVP to the positive terminal of the output capacitor. Main Current Sink Input. Secondary Current Sink Input or 1.21V Feedback Connection for Constant Voltage Output. Programmable General Purpose I/O. Serial Clock Input LED Current Setting Connection. Connect a resistor from SET to GND to set the maximum LED current into MAIN or SUB/FB (when in LED mode), where ILED_MAX = 192×1.244V/RSET. Active High Hardware Enable Input. Programmable Pattern Generator Output, and Programmable General Purpose I/O. Serial Data Input/Output Input Voltage Connection. Connect IN to the input supply, and bypass to GND with a 1µF ceramic capacitor. Logic Voltage Level Input Drain Connection for Internal NMOS Switch Ground www.national.com 2 LM3528 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 VSW, VOVP, VSUB/FB, VMAIN VSCL, VSDA, VRESET\GPIO, VIO , VSET Continuous Power Dissipation Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temperature (Soldering, 10s)(Note 3) ESD Rating(Note 10) Human Body Model −0.3V to 6V −0.3V to 25V −0.3V to 23V −0.3V to 6V Internally Limited +150°C -65°C to +150°C +300°C 2.5kV Operating Ratings VIN VSW, VOVP, VSUB/FB, VMAIN Junction Temperature Range (TJ)(Note 4) Ambient Temperature Range (TA)(Note 5) (Notes 1, 2) 2.5V to 5.5V 0V to 23V 0V to 21V -40°C to +110°C -40°C to +85°C Thermal Properties Junction to Ambient Thermal Resistance (θJA)(Note 6) 68°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 = −40°C to +85°C. Unless otherwise specified VIN = 3.6V, VIO = 1.8V, VRESET/GPIO = VIN, VSUB/FB = VMAIN = 0.5V, R = 12.0kΩ, OLED = ‘0’, ENM = ENS = ‘1’, BSUB = BMAIN = Full Scale.(Notes 2, 7) SET Symbol ILED Parameter Output Current Regulation MAIN or SUB/FB Enabled Maximum Current Per Current Sink Conditions UNI = ‘0’, or ‘1’, 2.5V < VIN < 5.5V RSET = 8.0kΩ UNI = ‘1’, 2.5V < VIN < 5.5V (Note 11) 3.0V < VIN < 5V Min 18.5 Typ 20 30 0.15 1.244 192 500 Max 22 Units mA 1 % V ILED-MATCH VSET ILED/ISET VREG_CS VREG_OLED VHR RDSON ICL VOVP IMAIN to ISUB/FB Current Matching SET Pin Voltage ILED Current to ISET Current Ratio Regulated Current Sink Headroom Voltage mV 1.237 V mV Ω 900 23 V 21.5 1.4 MHz % % mA VSUB/FB Regulation Voltage 2.5V < VIN < 5.5V, OLED = ‘1’ in OLED Mode Current Sink Minimum Headroom Voltage NMOS Switch On Resistance Output Over-Voltage Protection ILED = 95% of nominal ISW = 100mA 1.170 1.21 300 0.43 NMOS Switch Current Limit 2.5V < VIN < 5.5V ON Threshold, 2.5V < VIN < 5.5V OFF Threshold, 2.7V < VIN < 5.5V 645 20.6 19.25 1.0 770 22 20.6 1.27 90 10 fSW DMAX DMIN Switching Frequency Maximum Duty Cycle Minimum Duty Cycle 3 www.national.com LM3528 Symbol IQ Parameter Quiescent Current, Device Not Switching Conditions VMAIN and VSUB/FB > VREG_CS, BSUB = BMAIN = 0x00, 2.5V < VIN < 5.5V VSUB/FB > VREG_OLED, OLED=’1’, ENM=ENS=’0’, RSET Open, 2.5V < VIN < 5.5V Min Typ Max Units 350 390 µA 250 260 ISHDN Shutdown Current ENM = ENS = OLED = '0', 2.5V < VIN < 5.5V 2.5V < VIN 0.3V. OUTPUT CAPACITOR SELECTION The LM3528’s output capacitor supplies the LED current during the boost converters on time. When the switch turns off the inductor energy is discharged through the diode supplying power to the LED’s and restoring charge to the output capacitor. This causes a sag in the output voltage during the on time and a rise in the output voltage during the off time. The output capacitor is therefore chosen to limit the output ripple to an acceptable level depending on LED or OLED panel current requirements and input/output voltage differentials. For proper operation ceramic output capacitors ranging from 1µF to 2.2µF are required. As with the 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). For continuous conduction mode, the ripple components are found by: 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. Table 7 lists different manufacturers for various capacitors and their case sizes that are suitable for use with the LM3528. When configured as a dual output LED driver a 1µF output capacitor is adequate. In OLED mode for output voltages above 12V a 2.2µF output capacitor is required (see Low Output Voltage Operation (OLED) Section). www.national.com 26 LM3528 TABLE 7. Recommended Output Capacitors Manufacturer TDK Murata TDK Murata Part Number C1608X5R1E105M GRM39X5R105K25D539 C2012X5R1E225M GRM219R61E225KA12 Value 1µF 1µF 2.2µF 2.2µF Case Size 0603 0603 0805 0805 Voltage Rating 25V 25V 25V 25V INDUCTOR SELECTION The LM3528 is designed for use with a 10µH inductor, however 22µH are suitable providing the output capacitor is increased 2×. When selecting the inductor ensure that the saturation current rating (ISAT) for the chosen inductor is high enough and the inductor is large enough such that at the maximum LED current the peak inductor current is less than the LM3528’s peak switch current limit. This is done by choosing: Values for IPEAK can be found in the plot of peak current limit vs. VIN in the Typical Performance Characteristics graphs. Table 8 shows possible inductors, as well as their corresponding case size and their saturation current ratings. TABLE 8. Recommended Inductors Manufactur er TDK Coilcraft TDK Coilcraft TOKO Part Number VLF3012AT-100MR49 LPS3008-103ML VLF4012AT-100MR79 LPS4012-103ML A997AS-100M Value 10µH 10µH 10µH 10µH 10µH Dimensions 2.6mm×2.8mm×1mm 2.95mm×2.95mm×0.8mm 3.5mm×3.7mm×1.2mm 3.9mm×3.9mm×1.1mm 3.8mm×3.8mm×1.8mm ISAT 490mA 490mA 800mA 700mA 580mA DC Resistance 0.36Ω 0.65Ω 0.3Ω 0.35Ω 0.18Ω DIODE SELECTION The output diode must have a reverse breakdown voltage greater than the maximum output voltage. The diodes average current rating should be high enough to handle the LM3528’s output current. Additionally, the diodes peak current rating must be high enough to handle the peak inductor current. Schottky diodes are recommended due to their lower forward voltage drop (0.3V to 0.5V) compared to (0.6V to 0.8V) for PN junction diodes. If a PN junction diode is used, ensure it is the ultra-fast type (trr < 50ns) to prevent excessive loss in the rectifier. For Schottky diodes the B05030WS (or equivalent) work well for most designs. See Table 9 for a list of other Schottky Diodes with similar performance. 27 www.national.com LM3528 TABLE 9. Recommended Schottky Diodes Manufacturer Part Number Package Reverse Breakdown Voltage 30V 30V 30V 30V 30V 20V Average Current Rating 200mA 200mA 200mA 200mA 0.5A 1A On Semiconductor On Semicondcuctor On Semiconductor Diodes Inc. Diodes Inc. Philips NSR0230P2T5G NSR0230M2T5G RB521S30T1 SDM20U30 B05030WS BAT760 SOD-923 (0.8mm×0.6mm×0.4mm) SOD-723 (1mm×0.6mm×0.52mm) SOD-523 (1.2mm×0.8mm×0.6mm) SOD-523 (1.2mm×0.8mm×0.6mm) SOD-323 (1.6mm×1.2mm×1mm) SOD-323 (1.6mm×1.2mm×1mm) OUTPUT CURRENT RANGE (OLED MODE) The maximum output current the LM3528 can deliver in OLED mode is limited by 4 factors (assuming continuous conduction); the peak current limit of 770mA (typical), the inductor value, the input voltage, and the output voltage. Calculate the maximum output current (IOUT_MAX) using the following equation: OUTPUT VOLTAGE RANGE (OLED MODE) The LM3528's output voltage is constrained by 2 factors. On the low end it is limited by the minimum duty cycle of 10% (assuming continuous conduction) and on the high end it is limited by the over voltage protection threshold (VOVP) of 22V (typical). In order to maintain stability when operating at different output voltages the output capacitor and inductor must be changed. Refer to Table 10 for different VOUT, C OUT, and L combinations. TABLE 10. Component Values for Output Voltage Selection VOUT 18V 15V 12V COUT 2.2µF 2.2µF 4.7µF 10µF 10µF 22µF L 10µH 10µH 10µH 10µH 4.7µH 4.7µH VIN Range 2.7V to 5.5V 2.7V to 5.5V 2.7V to 5.5V 2.7V to 5.5V 2.7V to 5.5V 2.7V to 4.5V For the typical application circuit with VOUT = 18V and assuming 70% efficiency, the maximum output current at VIN = 2.7V will be approximately 70mA. At 4.2V due to the shorter on times and lower average input currents the maximum output current (at 70% efficiency) jumps to approximately 105mA. Figure 11 shows a plot of IOUT_MAX vs. VIN using the above equation, assuming 80% efficiency. In reality, factors such as current limit and efficiency will vary over VIN, temperature, and component selection. This can cause the actual IOUT_MAX to be higher or lower. 9V 7V 5V 30020562 FIGURE 18. Typical Maximum Output Current in OLED Mode (assumed 80% efficiency) www.national.com 28 LM3528 APPLICATION CIRCUITS 30020561 FIGURE 19. LED Backlight + OLED Power Supply LAYOUT CONSIDERATIONS Refer to AN-1112 for µSMD package soldering The high switching frequencies and large peak currents in the LM3528 make the PCB layout a critical part of the design. The proceeding steps should be followed to ensure stable operation and proper current source regulation. 1, CIN should be located on the top layer and as close to the device as possible. The input capacitor supplies the driver currents during MOSFET switching and can have relatively large spikes. Connecting the capacitor close to the device will reduce the inductance between CIN and the LM3528 and eliminate much of the noise that can disturb the internal analog circuitry. 2, Connect the anode of the Schottky diode as close to the SW pin as possible. This reduces the inductance between the internal MOSFET and the diode and minimizes the noise generated from the discontinuous diode current and the PCB trace inductance that will add ringing at the SW node and filter through to VOUT. This is especially important in VOUT mode when designing for a stable output voltage. 3, COUT should be located on the top layer to minimize the trace lengths between the diode and PGND. Connect the positive terminal of the output capacitor (COUT+) as close as possible to the cathode of the diode. Connect the negative terminal of the output capacitor (COUT-) as close as possible to the PGND pin on the LM3528. This minimizes the inductance in series with the output capacitor and reduces the noise present at VOUT and at the PGND connection. This is important due to the large di/dt into and out of COUT. The returns for both CIN and COUT should terminate directly to the PGND pin. 4, Connect the inductor on the top layer close to the SW pin. There should be a low impedance connection from the inductor to SW due to the large DC inductor current, and at the same time the area occupied by the SW node should be small so as to reduce the capacitive coupling of the high dV/dt present at SW that can couple into nearby traces. 5, Route the traces for RSET and the feedback divider away from the SW node to minimize the capacitance between these nodes that can couple the high dV/dt present at SW into them. Furthermore, the feedback divider and RSETshould have dedicated returns that terminate directly to the PGND pin of the device. This will minimize any shared current with COUT or CIN that can lead to instability. Avoide routing the SUB/FB node close to other traces that can see high dV/dt such as the I2C pins. The capacitive coupling on the PCB between FB and these nodes can disturb the output voltage and cause large voltage spikes at VOUT. 6, Do not connect any external capacitance to the SET pin. 7, Refer to the LM3528 Evaluation Board as a guide for proper layout. 29 www.national.com LM3528 Physical Dimensions inches (millimeters) unless otherwise noted 12 Bump µSMD For Ordering, Refer to Ordering Information Table NS Package Number TMD12 X1 = 1.215mm (±0.1mm), X2 = 1.615mm (±0.1mm), X3 = 0.6mm www.national.com 30 LM3528 Notes 31 www.national.com LM3528 High Efficiency, Multi Display LED Driver with 128 Exponential Dimming Steps and Integrated OLED Power Supply in micro SMD 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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