CAT4237 High Voltage CMOS Boost White LED Driver Description The CAT4237 is a DC/DC step−up converter that delivers an accurate constant current ideal for driving LEDs. Operation at a constant switching frequency of 1 MHz allows the device to be used with small value external ceramic capacitors and inductor. LEDs connected in series are driven with a regulated current set by the external resistor R1. LED currents up to 40 mA can be supported over a wide range of input supply voltages from 2.8 V to 5.5 V, making the device ideal for battery−powered applications. The CAT4237 high−voltage output stage is perfect for driving six, seven or eight white LEDs in series with inherent current matching in LCD backlight applications. 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 “zero” quiescent current. In addition to thermal protection and overload current limiting, the device also enters a very low power operating mode during “Open LED” fault conditions. The device is housed in a low profile (1 mm max height) 5−lead thin SOT23 package for space critical applications. http://onsemi.com 5 1 TSOT−23 TD SUFFIX CASE 419AE PIN CONNECTIONS 1 VIN SW GND SHDN FB (Top View) MARKING DIAGRAMS Features • • • • • • • • • • • • Drives 6 to 8 White LEDs in Series from 3 V Up to 87% Efficiency Low Quiescent Ground Current 0.6 mA Adjustable Output Current (up to 40 mA) High Frequency 1 MHz Operation High Voltage Power Switch Shutdown Current Less than 1 mA Open LED Low Power Mode Automatic Shutdown at 1.9 V (UVLO) Thermal Shutdown Protection Thin SOT23 5−lead (1 mm Max Height) These Devices are Pb−Free, Halogen Free/BFR Free and are RoHS Compliant Applications • • • • • • November, 2011 − Rev. 3 UDYM LT = CAT4237TD−T3 UD = CAT4237TD−GT3 Y = Production Year (Last Digit) M = Production Month (1−9, A, B, C) ORDERING INFORMATION (Note 3) Package Shipping (Note 4) CAT4237TD−T3 (Note 1) TSOT−23 (Pb−Free) 3,000/ Tape & Reel CAT4237TD−GT3 (Note 2) TSOT−23 (Pb−Free) 3,000/ Tape & Reel Device 1. Matte−Tin Plated Finish (RoHS−compliant). 2. NiPdAu Plated Finish (RoHS−compliant) Color LCD and Keypad Backlighting Cellular Phones Handheld Devices Digital Cameras PDAs Portable Game Machine © Semiconductor Components Industries, LLC, 2011 LTYM 3. For detailed information and a breakdown of device nomenclature and numbering systems, please see the ON Semiconductor Device Nomenclature document, TND310/D, available at www.onsemi.com 4. 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. 1 Publication Order Number: CAT4237/D CAT4237 L VIN 3 V to 4.2 V D 33 mH C1 4.7 mF C2 SW VIN VOUT 0.22 mF CAT4237 OFF ON SHDN GND FB VFB = 300 mV 20 mA R1 15 W L: Sumida CDRH3D16−330 D: Central CMDSH05−4 (rated 40 V) C2: Taiyo Yuden UMK212BJ224 (rated 50 V) Figure 1. Typical Application Circuit Table 1. ABSOLUTE MAXIMUM RATINGS Ratings Units VIN, FB voltage Parameters −0.3 to +7 V SHDN voltage −0.3 to +7 V SW voltage −0.3 to +55 V Storage Temperature Range −65 to +160 _C Junction Temperature Range −40 to +150 _C 300 _C Lead Temperature 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 2. RECOMMENDED OPERATING CONDITIONS Parameters VIN SW pin voltage Ambient Temperature Range 6, 7 or 8 LEDs Range Units 2.8 to 5.5 V 0 to 30 V −40 to +85 _C 1 to 40 mA NOTE: Typical application circuit with external components is shown above. 5. Thin SOT23−5 package thermal resistance qJA = 135°C/W when mounted on board over a ground plane. http://onsemi.com 2 CAT4237 Table 3. DC ELECTRICAL CHARACTERISTICS (VIN = 3.6 V, ambient temperature of 25°C (over recommended operating conditions unless otherwise specified)) Symbol Parameter Conditions Min Typ Max Unit IQ Operating Current VFB = 0.2 V VFB = 0.4 V (not switching) 0.6 0.1 1.5 0.6 mA ISD Shutdown Current VSHDN = 0 V 0.1 1 mA VFB FB Pin Voltage 8 LEDs with ILED = 20 mA 300 315 mV IFB FB pin input leakage 1 mA ILED Programmed LED Current VIH VIL R1 = 10 W R1 = 15 W R1 = 20 W SHDN Logic High SHDN Logic Low Enable Threshold Level Shutdown Threshold Level 285 28.5 19 14.25 30 20 15 31.5 21 15.75 mA 0.4 0.8 0.7 1.5 V FSW Switching Frequency 0.8 1.0 1.3 MHz ILIM Switch Current Limit 350 450 600 mA RSW Switch “On” Resistance ISW = 100 mA 1.0 2.0 W ILEAK Switch Leakage Current Switch Off, VSW = 5 V 1 5 mA Thermal Shutdown 150 °C Thermal Hysteresis 20 °C VUVLO Undervoltage Lockout (UVLO) Threshold 1.9 V VOV-SW Overvoltage Threshold 35 V Pin Description VIN is the supply input for the internal logic. The device is compatible with supply voltages down to 2.8 V and up to 5.5 V. It is recommended that a small bypass ceramic capacitor (4.7 mF) be placed between the VIN and GND pins near the device. If the supply voltage drops below 1.9 V, the device stops switching. SHDN is the shutdown logic input. When the pin is tied to a voltage lower than 0.4 V, the device is in shutdown mode, drawing nearly zero current. When the pin is connected to a voltage higher than 1.5 V, the device is enabled. GND is the ground reference pin. This pin should be connected directly to the ground place on the PCB. SW pin is connected to the drain of the internal CMOS power switch of the boost converter. The inductor and the Schottky diode anode should be connected to the SW pin. Traces going to the SW pin should be as short as possible with minimum loop area. An over-voltage detection circuit is connected to the SW pin. When the voltage reaches 35 V, the device enters a low power operating mode preventing the SW voltage from exceeding the maximum rating. FB feedback pin is regulated at 0.3 V. A resistor connected between the FB pin and ground sets the LED current according to the formula: I LED + 0.3 V R1 The lower LED cathode is connected to the FB pin. Table 4. PIN DESCRIPTIONS Pin # Name Function 1 SW 2 GND Ground pin. Connect the pin to the ground plane. 3 FB Feedback pin. Connect to the last LED cathode. 4 SHDN 5 VIN Switch pin. This is the drain of the internal power switch. Shutdown pin (Logic Low). Set high to enable the driver. Power Supply input. http://onsemi.com 3 CAT4237 Block Diagram 33 mH VIN SW C2 C1 4.7 mF 0.22 mF 1 MHz Oscillator 300 mV – A1 + Enable SHDN Thermal Shutdown & UVLO Driver + RC LED Current PWM & Logic A2 – N1 CC RS Current Sense GND – VREF + VIN Over Voltage Protection FB Figure 2. Block Diagram Device Operation The CAT4237 is a fixed frequency (1 MHz), low noise, inductive boost converter that provides a constant current with excellent line and load regulation. The device uses a high-voltage CMOS power switch between the SW pin and ground to energize the inductor. When the switch is turned off, the stored energy in the inductor is released into the load via the Schottky diode. The on/off duty cycle of the power switch is internally adjusted and controlled to maintain a constant regulated voltage of 0.3 V across the feedback resistor connected to the feedback pin (FB). The value of the resistor sets the LED current accordingly (0.3 V/R1). During the initial power-up stage, the duty cycle of the internal power switch is limited to prevent excessive in-rush currents and thereby provide a “soft-start” mode of operation. While in normal operation, the device can deliver up to 40 mA of load current into a string of up to 8 white LEDs. In the event of an “Open LED” fault condition, where the feedback control loop becomes open, the output voltage will continue to increase. Once this voltage exceeds 35 V, an internal protection circuit will become active and place the device into a very low power safe operating mode where only a small amount of power is transferred to the output. This is achieved by pulsing the switch once every 60 ms and keep it on for about 1 ms only. R1 15 W Thermal overload protection circuitry has been included to prevent the device from operating at unsafe junction temperatures above 150°C. In the event of a thermal overload condition the device will automatically shutdown and wait till the junction temperatures cools to 130°C before normal operation is resumed. Light Load Operation Under light load condition (under 4 mA) and with input voltage above 4.2 V, the CAT4237 driving 6 LEDs, the driver starts pulse skipping. Although the LED current remains well regulated, some lower frequency ripple may appear. Figure 3. Switching Waveform VIN = 4.2 V, ILED = 4 mA http://onsemi.com 4 CAT4237 TYPICAL CHARACTERISTICS (VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.) 140 2.0 SUPPLY CURRENT (mA) INPUT CURRENT (mA) 120 100 80 60 VFB = 0.4 V (not switching) 40 1.5 1.0 0.5 20 0 2.7 3.0 3.3 3.6 3.9 4.2 4.5 0 4.8 3.5 4.0 4.5 INPUT VOLTAGE (V) Figure 4. Quiescent Current vs. VIN (Not Switching) Figure 5. Quiescent Current vs. VIN (Switching) 8 LEDs at 20 mA VOUT = 26 V FB PIN VOLTAGE (mV) 310 305 300 295 8 LEDs 305 300 295 290 290 2.7 3.0 3.3 3.6 3.9 4.2 4.5 285 4.8 0 5 10 15 20 25 INPUT VOLTAGE (V) OUTPUT CURRENT (mA) Figure 6. FB Pin Voltage vs. Supply Voltage Figure 7. FB Pin Voltage vs. Output Current FREQUENCY (kHz) 1040 SW pin 20V/div 1020 Inductor Current 100mA/div 1000 980 960 5.0 315 310 FEEDBACK (mV) 3.0 INPUT VOLTAGE (V) 315 285 2.5 VOUT AC coupled 200mV/div 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 0.5 msec/div INPUT VOLTAGE (V) Figure 8. Switching Frequency vs. Supply Voltage Figure 9. Switching Waveforms http://onsemi.com 5 30 CAT4237 TYPICAL CHARACTERISTICS (VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.) 35 1.0 RFB = 10 W CURRENT VARIATION (%) LED CURRENT (mA) 30 25 RFB = 15 W 20 RFB = 20 W 15 10 5 0 2.5 3.0 3.5 4.0 4.5 −0.5 3.0 3.3 3.6 3.9 Figure 10. LED Current vs. Input Voltage (8 LEDs) Figure 11. LED Current Regulation (20 mA) 4.8 90 VIN = 4.2 V EFFICIENCY (%) 75 8 LEDs VOUT ~ 27 V at 20 mA L = 33 mH 5 10 15 20 mA 85 VIN = 3.6 V 70 15 mA 80 75 8 LEDs VOUT ~ 27 V at 20 mA L = 33 mH 70 20 25 65 30 3.0 3.5 4.0 4.5 LED CURRENT (mA) INPUT VOLTAGE (V) Figure 12. 8 LED Efficiency vs. Load Current Figure 13. 8 LED Efficiency vs. Input Voltage 5.0 90 90 VIN = 4.2 V VIN = 4.2 V 85 85 VIN = 3.6 V EFFICIENCY (%) EFFICIENCY (%) 4.5 INPUT VOLTAGE (V) 80 80 75 7 LEDs VOUT ~ 23 V at 20 mA L = 33 mH 70 65 4.2 INPUT VOLTAGE (V) 85 EFFICIENCY (%) 0 −1.0 5.0 90 65 0.5 5 10 15 VIN = 3.6 V 80 75 6 LEDs VOUT ~ 20 V at 20 mA L = 33 mH 70 20 25 65 30 5 10 15 20 25 LED CURRENT (mA) LED CURRENT (mA) Figure 14. 7 LED Efficiency vs. Load Current Figure 15. 6 LED Efficiency vs. Load Current http://onsemi.com 6 30 CAT4237 TYPICAL CHARACTERISTICS (VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.) 2.0 SWITCH RESISTANCE (W) EN 5V/div VOUT 10V/div Input Current 100mA/ div 1.5 1.0 0.5 0 3.5 4.0 INPUT VOLTAGE (V) Figure 16. Power−up with 8 LEDs at 20 mA Figure 17. Switch ON Resistance vs. Input Voltage 4.5 1.0 SHUTDOWN VOLTAGE (V) 302 301 300 299 VIN = 3.6 V, 8 LEDs ILED = 20 mA 298 0 50 100 25°C 85°C 0.6 125°C 0.4 0.2 150 −40°C 0.8 3.0 3.5 4.0 4.5 5.0 TEMPERATURE (°C) INPUT VOLTAGE (V) Figure 18. FB Pin Voltage vs. Temperature Figure 19. Shutdown Voltage vs. Input Voltage 140 MAX OUTPUT CURRENT (mA) FEEDBACK VOLTAGE (mV) 3.0 50 msec/div 303 297 −50 2.5 120 100 VOUT = 15 V 80 60 VOUT = 20 V 40 20 0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) Figure 20. Maximum Output Current vs. Input Voltage http://onsemi.com 7 5.0 CAT4237 Application Information given current. In order to achieve the best efficiency, this forward voltage should be as low as possible. The response time is also critical since the driver is operating at 1 MHz. Central Semiconductor Schottky diode CMDSH05−4 (500 mA rated) is recommended for most applications. External Component Selection Capacitors The CAT4237 only requires small ceramic capacitors of 4.7 mF on the input and 0.22 mF on the output. Under normal condition, a 4.7 mF input capacitor is sufficient. For applications with higher output power, a larger input capacitor of 10 mF may be appropriate. X5R and X7R capacitor types are ideal due to their stability across temperature range. LED Current Setting The LED current is set by the external resistor R1 connected between the feedback pin (FB) and ground. The formula below gives the relationship between the resistor and the current: R1 + Inductor A 33 mH inductor is recommended for most of the CAT4237 applications. In cases where the efficiency is critical, inductances with lower series resistance are preferred. Inductors with current rating of 300 mA or higher are recommended for most applications. Sumida CDRH3D16−330 33 mH inductor has a rated current of 320 mA and a series resistance (D.C.R.) of 520 mW typical. 0.3 V current LED Table 5. RESISTOR R1 AND LED CURRENT LED Current (mA) R1 (W) Schottky Diode The current rating of the Schottky diode must exceed the peak current flowing through it. The Schottky diode performance is rated in terms of its forward voltage at a http://onsemi.com 8 5 60 10 30 15 20 20 15 25 12 30 10 CAT4237 Open LED Protection In the event of an “Open LED” fault condition, the CAT4237 will continue to boost the output voltage with maximum power until the output voltage reaches approximately 35 V. Once the output exceeds this level, the internal circuitry immediately places the device into a very low power mode where the total input power is limited to about 4 mW (about 1 mA input current with a 3.6 V supply). The SW pin clamps at a voltage below its maximum rating of 60 V. There is no need to use an external zener diode between Vout and the FB pin. A 50 V rated C2 capacitor is required to prevent any overvoltage damage in the open LED condition. C1 SW VIN SUPPLY CURRENT (mA) 0.5 2.5 3.0 3.5 4.0 4.5 5.0 Figure 23. Open LED Supply Current vs. VIN without Zener C2 4.7 mF 1.0 INPUT VOLTAGE (V) VOUT 33 mH 1.5 0 Schottky 100 V (Central CMSH1−100) L VIN 2.0 0.22 mF 50 OFF ON SHDN GND FB OUTPUT VOLTAGE (V) CAT4237 VFB = 300 mV R1 15 W Figure 21. Open LED Protection without Zener 45 40 35 30 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V) Figure 24. Open LED Output Voltage vs. VIN without Zener SW PIN 10 V/div 10 msec/div Figure 22. Open LED Switching Waveforms without Zener http://onsemi.com 9 CAT4237 Dimming Control There are several methods available to control the LED brightness. VIN SW CAT4237 PWM Signal on the SHDN Pin SHDN GND FB LED brightness dimming can be done by applying a PWM signal to the SHDN input. The LED current is repetitively turned on and off, so that the average current is proportional to the duty cycle. A 100% duty cycle, with SHDN always high, corresponds to the LEDs at nominal current. Figure 25 shows a 1 kHz signal with a 50% duty cycle applied to the SHDN pin. The recommended PWM frequency range is from 100 Hz to 2 kHz. PWN Signal 2.5 V 3.73 kW 3.1 kW RA RB 0V C1 VFB = 300 mV 1 kW R2 i LED Current R1 15 W 0.22 mF Figure 26. Circuit for Filtered PWM Signal A PWM signal at 0 V DC, or a 0% duty cycle, results in a max LED current of about 22 mA. A PWM signal with a 93% duty cycle or more, results in an LED current of 0 mA. LED CURRENT (mA) 25 20 15 10 5 Figure 25. Switching Waveform with 1 kHz PWM on SHDN 0 0 10 20 30 40 50 60 70 80 90 100 PWM DUTY CYCLE (%) Filtered PWM Signal A filtered PWM signal used as a variable DC voltage can control the LED current. Figure 26 shows the PWM control circuitry connected to the CAT4237 FB pin. The PWM signal has a voltage swing of 0 V to 2.5 V. The LED current can be dimmed within a range from 0 mA to 20 mA. The PWM signal frequency can vary from very low frequency up to 100 kHz. Figure 27. Filtered PWM Dimming (0 V to 2.5 V) http://onsemi.com 10 CAT4237 Board Layout The CAT4237 is a high−frequency switching regulator. The traces that carry the high−frequency switching current have to be carefully layout on the board in order to minimize EMI, ripple and noise in general. The thicker lines on Figure 28 show the switching current path. All these traces have to be short and wide enough to minimize the parasitic inductance and resistance. The loop shown on Figure 28 corresponds to the current path when the CAT4237 internal switch is closed. On Figure 29 is shown the current loop, when the CAT4237 switch is open. Both loop areas should be as small as possible. Capacitor C1 has to be placed as close as possible to the VIN pin and GND. The capacitor C2 has to be connected separately to the top LED anode. A ground plane under the CAT4237 allows for direct connection of the capacitors to ground. The resistor R1 must be connected directly to the GND pin of the CAT4237 and not shared with the switching current loops and any other components. Open Closed Figure 28. Closed−switch Current Loop Figure 29. Open−switch Current Loop http://onsemi.com 11 CAT4237 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 L2 L1 SIDE VIEW END VIEW Notes: (1) All dimensions are in millimeters. Angles in degrees. (2) Complies with JEDEC MO-193. 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. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. 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This literature is subject to all applicable copyright laws and is not for resale in any manner. PUBLICATION ORDERING INFORMATION LITERATURE FULFILLMENT: Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada Email: [email protected] N. American Technical Support: 800−282−9855 Toll Free USA/Canada Europe, Middle East and Africa Technical Support: Phone: 421 33 790 2910 Japan Customer Focus Center Phone: 81−3−5817−1050 http://onsemi.com 12 ON Semiconductor Website: www.onsemi.com Order Literature: http://www.onsemi.com/orderlit For additional information, please contact your local Sales Representative CAT4237/D