CAT37 CMOS White LED Driver Boost Converter Description The CAT37 is a DC/DC step up converter that delivers a regulated output current. Operation at a constant switching frequency of 1.2 MHz allows the device to be used with very small value external inductor and ceramic capacitors. The CAT37 is targeted to drive multiple white light−emitting diodes (LEDs) connected in series and provides the necessary regulated current to control the brightness and the color purity. An external resistor R1 controls the output current level. LED currents of up to 40 mA can be supported over a wide range of input supply voltages from 2.5 V to 7 V, making the device ideal for battery−powered applications. A high voltage output stage allows up to 4 White LEDs to be driven in series. Series drive provides inherent current matching. LED dimming can be done by using a DC voltage, a logic signal, or a pulse width modulation (PWM) signal. The shutdown input pin allows the device to be placed in power−down mode with “near zero” quiescent current. In addition to overcurrent limiting protection, the device also includes detection circuitry to ensure protection against open−circuit load fault conditions. The device is available in a low profile (1 mm max height) 5−lead TSOT−23 package. Low Quiescent Ground Current (0.5 mA Typical) Power Efficiency Over 80% Compatible Pinout with LT1937 Adjustable Output Current (up to 40 mA) High Frequency 1.2 MHz Operation Input Voltage Operation down to 2.5 V Low Resistance (0.5 W) Power Switch Drives up to 4 White LEDs in Series Shutdown Current Less than 1 mA Load Fault Protection Against Open−circuits Low Value External Components Low Profile (1 mm) TSOT−23 5−lead Package These Devices are Pb−Free, Halogen Free/BFR Free and are RoHS Compliant Applications • • • • • • February, 2010 − Rev. 21 1 TSOT−23 TD SUFFIX CASE 419AE PIN CONNECTIONS 1 VIN SW GND SHDN FB (Top View) 1 mm Maximum Height qJA = 250°C/W (free air) LMYM ULYM LM = CAT37TDI−T3 UL = CAT37TDI−GT3 Y = Production Year (Last Digit) M = Production Month (1−9, A, B, C) ORDERING INFORMATION Device Package Shipping CAT37TDI−T3 (Note 1) TSOT−23 (Pb−Free) 3,000/ Tape & Reel CAT37TDI−GT3 (Note 2) TSOT−23 (Pb−Free) 3,000/ Tape & Reel 1. Matte−Tin Plated Finish (RoHS−compliant). 2. NiPdAu Plated Finish (RoHS−compliant). Color LCD and Keypad Backlighting Cellular Phones Handheld Terminals Digital Cameras PDAs/Games Portable MP3 Players © Semiconductor Components Industries, LLC, 2010 5 MARKING DIAGRAM Features • • • • • • • • • • • • • http://onsemi.com 1 Publication Order Number: CAT37/D CAT37 Typical Application Circuit L1 6.8 mH VIN 3 V to 5 V C1 1 mF D1 C2 1 mF 1 SW 5 VIN CAT37 ON OFF 4 FB SHDN 3 GND 2 15 mA C1: Taiyo Yuden JMK212BJ475 C2: Taiyo Yuden EMK212BJ105 L1: Panasonic ELJEA6R8 or equivalent R1 6.34 W Figure 1. Driver for Four High−Brightness White LEDs Table 1. PIN DESCRIPTION Pin Number Name Function 1 SW Switch pin. This is the drain of the internal power switch. For minimum EMI, minimize the trace area connected to this pin. 2 GND Ground pin. Connect pin 2 to ground. 3 FB 4 SHDN 5 VIN LED (cathode) connection pin. Shutdown pin. Input supply pin. This pin should be bypassed with a capacitor to ground. A 1 mF capacitor mounted close to the pin is recommended. Table 2. ABSOLUTE MAXIMUM RATINGS Parameter Rating Unit VIN, FB, SHDN voltage 8 V SW voltage 20 V −65 to +160 °C Junction Temperature 125 °C Lead Soldering Temperature (10 secs) 300 °C ESD Rating – Human Body Model 2000 V Storage Temperature Range Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect device reliability. Table 3. RECOMMENDED OPERATING CONDITIONS Parameter Range Unit 2.5 to 7 V −40 to +85 °C Inductor L1 6.8 ± 20% typical mH Input Capacitor C1 1.0 ± 20% typical mF Output Capacitor C2 1.0 ± 20% typical mF 0 to 20 mA VIN Ambient Temperature Range ILED with 1 to 4 LEDs in series NOTE: Typical application circuit with external components is shown above. http://onsemi.com 2 CAT37 Table 4. ELECTRICAL OPERATING CHARACTERISTICS (Over recommended operating conditions unless otherwise specified. TA = 25°C, VIN = 3 V and VSHDN = 3 V.) Symbol Parameter Conditions Input Voltage Range IQ Quiescent Current ISD Shutdown Current VFB FB Pin Voltage IFB FB Pin Leakage Current Min Typ 2.5 VFB = 0.2 V 0.5 VSHDN = 0 V 4 LEDs at 15 mA Shutdown High Threshold 85 Units 7 V 0.7 mA 0.05 1 mA 95 105 mV 1 2 mA 0.85 V Shutdown Low Threshold ISHDN Max Shutdown Pin Current 0.25 V 1 nA 1.6 MHz fSW Boost Converter Frequency 0.8 1.2 DC Maximum Switch Duty Cycle 90 95 Switch Current Limit 400 550 780 mA ISW = 300 mA 150 200 mV Switch Off, VSW = 5 V 0.01 5 mA Figure 1 with specified components 83 ISWL Switch Saturation Voltage Switch Leakage Current Efficiency http://onsemi.com 3 % % CAT37 TYPICAL CHARACTERISTICS (VIN = 3.6 V, TAMB = 25°C, CIN = 4.7 mF, COUT = 1 mF, L = 6.8 mH, unless otherwise specified.) 30 16.0 20 mA LED CURRENT (mA) LED CURRENT (mA) 3 LEDs 20 10 mA 10 0 2 3 4 5 15.5 4 LEDs 15.0 14.5 14.0 −50 6 −25 0 25 50 75 INPUT VOLTAGE (V) TEMPERATURE (°C) Figure 2. LED Current vs. Input Voltage Figure 3. LED Current vs. Temperature 110 100 100 100 FB PIN VOLTAGE (mV) FB PIN VOLTAGE (mV) 105 3 LEDs 95 90 95 90 85 85 80 4 LEDs at 15 mA VIN = 3.6 V 0 5 10 15 80 −50 20 25 50 75 TEMPERATURE (°C) Figure 4. FB Pin Voltage vs. LED Current Figure 5. FB Pin Voltage vs. Temperature 100 1.3 SWITCHING FREQUENCY (MHz) SWITCH FREQUENCY (MHz) 0 LED CURRENT (mA) 1.3 1.2 1.1 1.0 0.9 4 LEDs 0.8 −25 1 2 3 4 5 1.2 1.1 LOAD = 20 mA 1.0 0.9 0.8 −50 6 LOAD = 3 LEDs VIN = 3.6 V −25 0 25 50 75 INPUT VOLTAGE (V) TEMPERATURE (°C) Figure 6. Switching Frequency vs. Input Voltage Figure 7. Switching Frequency vs. Temperature http://onsemi.com 4 100 CAT37 TYPICAL CHARACTERISTICS 90 86 85 84 EFFICIENCY (%) EFFICIENCY (%) (VIN = 3.6 V, TAMB = 25°C, CIN = 4.7 mF, COUT = 1 mF, L = 6.8 mH, unless otherwise specified.) 80 75 3 LEDs at 15 mA VOUT = 10 V 70 65 60 3 4 5 LOAD = 10 mA 80 78 74 −50 6 LOAD = 15 mA 82 76 2 LOAD = 20 mA LOAD = 3 LEDs VIN = 3.6 V −25 0 25 50 75 INPUT VOLTAGE (V) TEMPERATURE (°C) Figure 8. Efficiency vs. Input Voltage Figure 9. Efficiency vs. Temperature Figure 10. VSW, IL, & VOUT Waveforms Figure 11. PWM on SHDN Pin at 1 kHz Figure 12. Enable Power−Up Waveforms http://onsemi.com 5 100 CAT37 Operation The CAT37 device is a high efficiency, constant frequency, current regulating boost driver for white LEDs. The device includes a switch and an internally compensated loop for the regulation of the current in the LEDs. Operation can be best understood by examining the block diagram. The FB pin is regulated at 95 mV and the current through the external resistor will set the regulated current in the LEDs at: While maintaining LED current regulation, the CAT37 boost converter automatically adjusts the FB pin voltage to be as low as possible. A low FB pin voltage ensures high efficiency. Current through the internal power switch is continuously monitored cycle−by−cycle. If the current limit is exceeded, the switch is immediately turned off, protecting the device, for the remainder of the cycle. PWM dimming operation can be achieved by switching the SHDN pin or by pulling the FB pin higher than 95 mV. I LED + 0.095 R1 Block Diagram D1 L1 VIN 5 C1 1 4 C2 SW VIN SHDN 1.2 MHz Oscillator Over Voltage Protection (22 V) PWM & Logic + 90 kW − Error Amp − CAT37 GND Current Sense Amp 2 Figure 13. CAT37 Block Diagram http://onsemi.com 6 6 kW ILED FB 3 + 0.095 V REF R1 CAT37 Application Information Capacitor Selection Low ESR (equivalent series resistance) capacitors should be used at the output to minimize the output ripple voltage. The low ESR and small package options available with multilayer ceramic capacitors make them excellent choices. The X5R and X7R capacitor types are preferred because they retain their capacitance over wider voltage and temperature ranges than the Y5V or Z5U types. A 1.0 mF output capacitor is recommended for most applications. The voltage rating of the output capacitor C2 depends on the number of LEDs driven in the series. A 16 V ceramic capacitor is recommended when driving 3 or 4 LEDs. Low profile ceramic capacitors with a 1 mm maximum height/thickness are available for designs height requirements. Ceramic capacitors also make a good choice for the input capacitor, which should be mounted as close as possible to the CAT37. A 1 mF or 4.7 mF input capacitor is recommended. Table 5 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for detailed information as new products and package options are introduced regularly. Table 6. SCHOTTKY DIODE SUPPLIERS Part MBR0520 MBR0530 MBR0540 The LED current is programmed with a single resistor connected to the FB pin. The FB pin is internally regulated to 95 mV, which sets the current flowing through R1 and the LEDs as equal to 0.095/R1. For the best accuracy, a 1% or better resistor is recommended. Table 7 shows several typical 1% R1 values. Table 7. R1 RESISTOR VALUES ILED (mA) R1 (W) 40 2.37 30 3.16 20 4.75 15 6.34 12 7.87 10 9.53 5 19.1 Web Taiyo Yuden www.t−yuden.com Murata www.murata.com Kemet www.kemet.com AVX www.avxcorp.com ON Semiconductor www.onsemi.com 800.282.9855 LED Current Programming Table 5. CERAMIC CAPACITOR MANUFACTURERS Supplier Supplier For other LED current values, use the following equation to choose R1. R1 + 0.095 ILED Diode Selection Schottky diodes, with their low forward voltage drop and fast switching speed, are the ideal choice for high efficiency applications. Table 6 shows several different Schottky diodes that work well with the CAT37. Make sure that the diode has a voltage rating greater than the output voltage. The diode conducts current only when the power switch is turned off (typically less than one−third the time), so a 0.4 A or 0.5 A diode will be sufficient for most designs. Most white LEDs are driven at maximum currents of 15 mA to 20 mA. Some higher power designs will use two parallel strings of LEDs for greater light output, resulting in 30 mA to 40 mA (two strings of 15 mA to 20 mA) flowing through the R1 resistor. http://onsemi.com 7 CAT37 LED Dimming with DC Signal LED Dimming with a Logic Signal Dimming the LEDs can be done by applying a variable DC voltage as shown on Figure 14. As the VDC increase the voltage across R1 decreases and therefore lower the LED current. The resistors R2 and R3 must be large enough so that their current (tens of mA) is much smaller than the LED current but much larger than the FB leakage current (IFB). When adjusting VDC between 0 V and 2 V, the resistors shown on Figure 14 will set the LED current between 0 mA and 15 mA. For applications that need to adjust the LED brightness in discrete steps, a logic signal can be used as shown in Figure 16. R1 sets the minimum LED current value (when the NMOS switch is OFF): R1 + RINCR determines how much LED current increases when the external NMOS switch is turned ON. LED Dimming with PWM Signal R INCR + PWM brightness control provides the widest dimming range (greater than 20:1). By turning the LEDs ON and OFF using the control signal, the LEDs operate at either zero or full current, but their average current changes with the PWM signal duty cycle. Typically, a 5 kHz to 40 kHz PWM signal is used. A filtered PWM signal is used to dim the LEDs, as shown in Figure 15. In addition to providing the widest dimming range, PWM brightness control also ensures the “purest” white LED color over the entire dimming range. The true color of a white LED changes with operating current, and is the “purest” white at a specific forward current, usually 15 mA or 20 mA. If the LED current is less than or more than this value, the emitted light becomes more blue. Applications involving color LCDs can find the blue tint objectionable. VDC 0.095 V I LED(MIN) 0.095 V I LED(INCREASE) PCB Layout Guidelines The CAT37 is a high−frequency switching regulator where proper PCB board layout and component placement can minimize noise and radiation and increase efficiency. To maximize efficiency, the CAT37 design has fast switch rise and fall times. To prevent radiation and high frequency resonance problems minimize the length and area of all traces connected to the SW pin and use a ground plane under the switching regulator. The SW pin, schottky diode and capacitor C2 signal path should be kept as short as possible. The ground connection for the R1 resistor should be tied directly to the GND pin and not be shared with other components. CAT37 CAT37 CAT37 FB FB FB R3 180 kW R2 10 kW PWM R1 6.3 W Figure 14. Dimming with a DC Voltage 10 kW R2 10 kW R3 180 kW RINCR R1 6.3 W 0.1 mF Figure 15. Dimming with a Filtered PWM Signal http://onsemi.com 8 Logic Signal 2N7002 Figure 16. Dimming with a Logic Signal R1 6.3 W CAT37 TYPICAL APPLICATION CIRCUITS L1 6.8 mH 3 V to 4.2 V 90 D1 VIN VDC Dimming 1 SW 5 VIN 180 kW 15 mA CAT37 4 3 10 kW FB SHDN C2 1 mF EFFICIENCY (%) C1 1 mF VIN = 3.6 V 85 GND 2 VIN = 3.0 V 80 75 70 65 R1 6.34 W 60 0 5 10 15 20 25 30 LED CURRENT (mA) Figure 17. Two LEDs with DC Level Dimming Control L1 6.8 mH 3 V to 4.2 V Figure 18. Efficiency − Two LEDs 90 D1 VDC Dimming C1 1 mF 5 VIN 180 kW CAT37 4 C2 1 mF 1 SW FB SHDN 15 mA 10 kW 3 GND 2 EFFICIENCY (%) VIN 85 VIN = 3.6 V 80 VIN = 3.0 V 75 70 65 R1 6.34 W 60 0 5 10 15 20 25 30 LED CURRENT (mA) Figure 19. Three LEDs with DC Level Dimming Control L1 6.8 mH 3 V to 4.2 V Figure 20. Efficiency − Three LEDs 90 D1 VIN 85 PWM 10 kW 1 SW 5 VIN SHDN 180 kW GND 2 FB 1 mF 0.1 mF CAT37 4 VIN = 3.6 V C2 EFFICIENCY (%) C1 1 mF 3 15 mA 10 kW 80 VIN = 3.0 V 75 70 65 R1 6.34 W 60 0 5 10 15 20 25 LED CURRENT (mA) Figure 21. Four LEDs with PWM Dimming Control Figure 22. Efficiency − Four LEDs http://onsemi.com 9 30 CAT37 PACKAGE DIMENSIONS TSOT−23, 5 LEAD CASE 419AE−01 ISSUE O SYMBOL D MIN NOM A1 0.01 0.05 0.10 A2 0.80 0.87 0.90 b 0.30 c 0.12 A e E1 1.00 0.45 0.15 D 2.90 BSC E 2.80 BSC E1 1.60 BSC E MAX e 0.20 0.95 TYP L 0.30 0.40 L1 0.60 REF L2 0.25 BSC 0º θ 0.50 8º TOP VIEW A2 A b q L A1 c L1 SIDE VIEW END VIEW Notes: (1) All dimensions are in millimeters. Angles in degrees. (2) Complies with JEDEC MO-193. http://onsemi.com 10 L2 CAT37 Example of Ordering Information (Note 5) 3. 4. 5. 6. 7. Prefix Device # Suffix CAT 37 TD I −G T3 Company ID (Optional) Product Number 37 Package TD: TSOT−23 Industrial Temperature I: Range = I: −40°C to +85°C Lead Finish Blank: Matte−Tin G: NiPdAu Tape & Reel (Note 7) T: Tape & Reel 3: 3,000 / Reel All packages are RoHS−compliant (Lead−free, Halogen−free). The standard lead finish is NiPdAu. The device used in the above example is a CAT37TDI−GT3 (TSOT−23, Industrial Temperature, NiPdAu, Tape & Reel, 3,000/Reel). For additional package and temperature options, please contact your nearest ON Semiconductor Sales office. For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specifications Brochure, BRD8011/D. ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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