Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 DESCRIPTION The PT6937 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The device can drive two, three or four LEDs in series from a Li-Ion cell. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors. The PT6937 switches at 1.2MHz, allowing the use of tiny external components. The output capacitor can be as small as 0.22µF, saving space and cost versus alternative solutions. A low 95mV feedback voltage minimizes power loss in the current setting resistor for better efficiency. The PT6937 is available in a low profile SOT package. FEATURES • • • • • • • High efficiency: 85% Typical Drives up to four LEDs from a 3.3V supply Drives up to ten LEDs from a 5V supply Fast 1.2MHz switching frequency Uses tiny 22µH inductors Requires only 0.22µF output capacitor Low profile SOT package APPLICATIONS • • • • • Cellular phones PDAs, Handheld computers Digital cameras MP3 players GPS receivers PT6937 V1.1 -1- March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 BLOCK DIAGRAM VIN 5 FB 3 1 SW COMPARATOR 95mV DRIVER + Rc A2 + Cc Q R M1 S + VREF 1.25V A1 0.2 RAMP GENERATOR /SHDN 4 PT6937 V1.1 SHUTDOWN 2 GND 1.2MHz OSCILLATOR -2- March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 PIN DESCRIPTION Pin Name SW GND FB /SHDN VIN PT6937 V1.1 Description Pin No. Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI. Ground Pin. Connect directly to local ground plane. Feedback Pin. Reference voltage is 95mV. Connect cathode of lowest LED and resistor here. Calculate resistor value according to the formula: RFB=95mV/ILED Shutdown Pin. Connect to 1.5V or higher to enable device; 0.4V or less to disable device. Input Supply Pin. Must be locally bypassed. -3- 1 2 3 4 5 March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 APPLICATION INFORMATION INDUCTOR SELECTION A 22µH inductor is recommended for most PT6937 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1MHz and low DCR (copper wire resistance). CAPACITOR SELECTION The small size of ceramic capacitors makes them ideal for PT6937 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1µF input capacitor and a 0.22µF output capacitor are sufficient for most PT6937 applications. DIODE SELECTION Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for the PT6937 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while the diode capacitance (CT or CD) represents the switching losses. For diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching losses at the 1.2MHz switching frequency of the PT6937. A Schottky diode rated at 100mA to 200mA is sufficient for most PT6937 applications. LED CURRENT CONTROL The LED current is controlled by the feedback resistor (R1 in Figure 1). The feedback reference is 95mV. The LED current is 95mV/R1. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula and table for R1 selection are shown below. R1=95mV / ILED (1) ILED (mA) 5 10 12 15 20 PT6937 V1.1 -4- R1 (Ω) 19.1 9.53 7.87 6.34 4.75 March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 OPEN-CIRCUIT PROTECTION In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The PT6937 will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed its maximum 33V rating. A zener diode can be used at the output to limit the voltage on the SW pin (Figure 1). The zener voltage should be larger than the maximum forward voltage of the LED string. The current rating of the zener should be larger than 0.1mA. L1 22 µ H D VIN C OUT 0.22 µ F C IN 1µF VIN SW PT6937 /SHDN FB GND R2 1K R1 6.43Ω Figure1. LED driver with open-circuit protection DIMMING CONTROL There are four different types of dimming control circuits: USING A PWM SIGNAL TO /SHDN PIN With the PWM signal applied to the /SHDN pin, the PT6937 is turned on or off by the PWM signal. The LEDs operate at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM signal. A 0% duty cycle will turn off the PT6937 and corresponds to zero LED current; and 100% duty cycle corresponds to full current. The typical frequency range of the PWM signal is 1KHz to 10KHz. The magnitude of the PWM signal should be higher than the minimum /SHDN voltage high. The switching waveforms of the /SHDN pin PWM control are shown in the Figure 2 as below. PT6937 V1.1 -5- March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 USING A DC VOLTAGE For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The dimming control using a DC voltage is shown in the figure below. As the DC voltage increases, the voltage drop on R2 increases and the voltage drop on R1 decreases. Thus, the LED current decreases. The selection of R2 and R3 will make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias current. For VDC range from 0V to 5V, the selection of resistors in Figure 3 gives dimming control of LED current from 0mA to 15mA. L1 22 µ H L1 22 µ H D4 1 N58 19 D4 1N5819 VIN V IN C1 1µ F > 2V 0V 4 5 1 D1 W LED V IN SW D2 W LED P T 6937 /S HD N FB C1 1µF C2 0.22 µ F D3 W LED 3 4 5 1 D1 WLED VIN SW D2 WLED PT6937 3 /SHDN FB 2 R3 51K GND R1 4.7 G ND R2 1K 2 C2 0.22 µ F D3 WLED R1 4.7 DC 0~5V Figure 2. PWM Dimming control Using the /SHDN Figure 3. Dimming Control Using DC Voltage Pin USING A FILTERED PWM SIGNAL The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the variable DC voltage source in dimming control. The circuit is shown in Figure 4. L1 22 µ H D4 1N5819 VIN C1 1µF 4 5 1 D1 WLED VIN SW D2 WLED PT6937 3 /SHDN FB GND 2 R2 5K R3 90K C2 0.22 µ F D3 WLED R1 4.7 C3 0.1 µ F R4 10K PWM 0~5V Figure 4. Dimming Control Using Filtered PWM Signal. PT6937 V1.1 -6- March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 BOARD LAYOUT CONSIDERATION As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To maximize efficiency, switch rise and fall times are made as short as possible. To prevent electromagnetic interference (EMI) problems, proper layout of the high frequency switching path is essential. The voltage signal of the SW pin has sharp rise and fall edges. Minimize the length and area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize inter-plane coupling. In addition, the ground connection for the feedback resistor R1 should be tied directly to the GND pin and not shared with any other component, ensuring a clean, noise-free connection. Recommended component placement is shown in Figure 5 (SOT-23 Package) Figure 5. Recommended Component Placements PT6937 V1.1 -7- March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 TYPICAL APPLICATIONS L1 22 H D1 C1 1 F PTC PT6937 (3 LEDs) C2 0.22 F LED 1 VIN SW LED 2 15mA LED 3 PT6937 /SHDN FB OFF ON R1 6.43 GND EFFICIENCY (%) VIN 3V ~ 5V 90% 80% V IN = 3V V IN = 3.6V 70% 60% 50% 2 D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K 5 10 15 20 LED CURRENT (mA ) Li-Ion Powered Driver for Three White LEDs PTC PT6937 (2 LEDs) L1 22 H C IN 1 F C OUT 1 F VDC DIMMING VIN SW 90K PT6937 /SHDN FB 5K EFFICIENCY (%) VIN 3V ~ 5V D1 85% 80% 75% 70% 65% 60% 55% 50% 2 R1 2.2 GND VIN = 3V VIN = 3.6V 5 10 15 20 LED CURRENT (mA) D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K Li-Ion to Two White LEDs PTC PT6937 (3 LEDs) L1 22 H 90% C IN 1 F C OUT 0.22 F VDC DIMMING VIN SW 90K PT6937 /SHDN FB GND 5K EFFICIENCY (%) VIN 3V ~ 5V D1 80% VIN = 3V VIN = 3.6V 70% 60% 50% R1 4.75 2 5 10 15 20 LED CURRENT (mA) D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K Li-Ion to Three White LEDs PT6937 V1.1 -8- March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 PTC PT6937 (4 LEDs) L1 22 H D1 85% VIN 3V ~ 5V C OUT 0.22 F VDC DIMMING VIN SW 90K PT6937 /SHDN FB 5K R1 3 GND 80% EFFICIENCY (%) C IN 1 F 75% 70% VIN = 3V 65% VIN = 3.6V 60% 55% 50% 2 D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K 5 10 15 20 LED CURRENT (mA) Li-Ion to Four White LEDs L1 22 H PTC PT6937 (5 LEDs) D1 VIN 3V ~ 5V 85% C IN 1 F VDC DIMMING VIN SW 90K PT6937 /SHDN FB GND 5K EFFICIENCY (%) 80% C OUT 0.22 F 75% 70% VIN = 3V 65% VIN = 3.6V 60% 55% R1 3 50% 2 D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K 5 10 15 LED CURRENT (mA) Li-Ion to Five White LEDs PT6937 V1.1 -9- March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 PTC PT6937 (7 LEDs) L1 22 H 85% D1 VIN 5V VDC DIMMING C IN 1 F VIN C OUT 0.22 F SW 90K PT6937 /SHDN FB GND 5K EFFICIENCY (%) 80% 75% 70% VIN = 5V 65% 60% 55% R1 3 50% 2 D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K 5 10 15 LED CURRENT (mA) 5V to Seven White LEDs PT6937 V1.1 - 10 - March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 ABSOLUTE MAXIMUM RATINGS Parameter Symbol VIN Rating 5 33 5 5 Topr -40 to +85 Unit V V V V ℃ Tstg -65 to +150 ℃ Maximum junction temperature 125 ℃ Lead temperature (Soldering, 10 sec.) 300 ℃ Input voltage SW voltage FB voltage /SHDN voltage Operating temperature Storage temperature Note: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. ELECTRICAL CHARACTERISTICS (unless otherwise specified, Ta=25℃, VIN=3V, V /SHDN=3V,. Parameter Minimum operating voltage Conditions Min. Typ. 2.5 ISW=100mA, Duty cycle=66% Unit V Maximum operating voltage Feedback voltage Max. 5.5 V 86 95 104 mV Switching frequency 0.8 5 0.8 0.1 1.2 10 1.2 1.0 1.6 nA mA µA MHz Maximum duty cycle 84 87 90 % Switch current limit 300 350 400 mA 1.3 350 0.1 - 380 1 0.5 30 mV µA V V µA FB pin bias current Supply current /SHDN=0V Switch VCESAT ISW=250mA Switch leakage current /SHDN voltage high /SHDN voltage low /SHDN pin bias current VSW=30V 20 Note: The PT6937 is guaranteed to meet specifications from 0℃ to 70℃. Specifications over the -40℃ to 85℃ operating temperature range are assured by design, characterization and correlation with statistical process controls. PT6937 V1.1 - 11 - March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 ORDER INFORMATION Valid Part Number PT6937 PT6937 V1.1 Package 5 Pins, SOT - 12 - Top Code PT6937 March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 PACKAGE INFORMATION 5 PINS, SOT-23 D e1 4 1 2 e 3 b ( 5x ) A2 0.1 0 C 5X A E E1 5 A1 S EAT ING PL AN E PT6937 V1.1 - 13 - March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 V1.1 PT6937 - 14 - March, 2006 Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw White LED Step-Up Converter PT6937 Symbol A A1 A2 b b1 c c1 D E E1 e e1 L L1 L2 R R1 Min. 0 0.90 0.30 0.30 0.08 0.08 Max. 1.45 0.15 1.30 0.50 0.45 0.22 0.20 0.10 0.10 Typ. 1.15 0.40 0.13 2.90 BSC. 2.80 BSC. 1.60 BSC. 0.95 BSC. 1.90 BSC. 0.45 0.60 REF. 0.25 BSC. - θ 0° 4° 8° θ1 5° 10° 15° 0.30 0.60 0.25 Notes: 1. Dimension and tolerancing per ASME Y14.5M-1994. 2. Dimension in Millimeters. 3. Dimension D does not include mold flash, protrusion or gate burrs. Mold flash, protrusions or gate burrs shall not exceed 0.25mm per end. Dimension E1 does not include interlead flash or protrusion. Interlead flash or protrusion shall not exceed 0.25mm per side. D and E1 dimensions are determined at datum H. 4. The package top may be smaller than the package bottom. Dimensions D and E1 are determined at the outermost extremes of the plastic body exclusive of mold flash, tie bar burrs, gate burrs and interlead flash, but including any mismatch between the top and bottom of the plastic body. D and E1 dimensions are determined at datum H. 5. Datums A & B to be determined at datum H. 6. Package variation “AA” is a 5 lead version of the 6 lead variation “AB” where lead #5 removed from the 6 lead “AB” variation. 7. These dimensions apply to the flat section of the lead between 0.08mm and 0.15mm from the lead tip. 8. Dimension “b” does not include dambar protrusion. Allowable dambar protrusion shall be 0.08mm total in exceed of the “b” dimension at maximum material condition. The dambar cannot be located on the lower radius of the foot. Minimum space between protrusion and an adjacent lead shall not be less than 0.07mm. 9. Details of the pin 1 identifier are optional, but must be located within the zone indicated. 10. Refer to JEDEC MO-178 Variation AA JEDEC is the trademark of JEDEC SOLID STATE TECHNOLOGY ASSOCIATION PT6937 V1.1 - 15 - March, 2006