PT6937 PTC | Alldatasheet

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Technical content

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 1 - March, 2006 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 22 µH inductors
  • Requires only 0.22µF output capacitor
  • Low profile SOT package

APPLICATIONS

  • Cellular phones
  • PDAs, Handheld computers
  • Digital cameras
  • MP3 players
  • GPS receivers

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 2 - March, 2006 White LED Step-Up Converter PT6937 BLOCK DIAGRAM RAMP GENERATOR 1.2MHz OSCILLATOR VIN FB A195mVVREF 1.25V Rc Cc COMPARATOR R S Q DRIVER 0.2 SW GND2 /SHDN SHUTDOWN

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 3 - March, 2006 White LED Step-Up Converter PT6937 PIN DESCRIPTION Pin Name Description Pin No. SW Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI. 1 GND Ground Pin. Connect directly to local ground plane. 2 FB Feedback Pin. Reference voltage is 95mV. Connect cathode of lowest LED and resistor here. Calculate resistor value according to the formula: RFB=95mV/ILED /SHDN Shutdown Pin. Connect to 1.5V or higher to enable device; 0.4V or less to disable device. 4 VIN Input Supply Pin. Must be locally bypassed. 5

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 4 - March, 2006 White LED Step-Up Converter PT6937

APPLICATION INFORMATION

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) R1 (Ω) 5 19.1 10 9.53 12 7.87 15 6.34 20 4.75

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 5 - March, 2006 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. PT6937 VIN SW FB/SHDN GND R1 6.43 Ω 22 Hµ C 0.22 F OUT µ C IN µ VIN D 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.

Figure 5. Recommended Component Placements

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 8 - March, 2006 White LED Step-Up Converter PT6937 TYPICAL APPLICATIONS PT6937 VIN SW FB/SHDN GND LED 1 LED 2 LED 3 6.43 15mA 22 H 0.22 F 1 F VIN OFF ON 3V ~ 5V D1: FAIRCHILD 1N5819 L1: 3L Electronic Cor p. SMTSDR322520C-220K PTC PT6937 (3 LEDs) 50% 60% 70% 80% 90% 2 5 10 15 20 LED CURRENT (mA ) EFFICIENCY (% ) VI N = 3V VI N = 3.6V Li-Ion Powered Driver for Three White LEDs PT6937 VIN SW FB/SHDN GND R1 2.2 22 H C 1 F OUTC 1 F IN VIN 3V ~ 5V D1: FAIRCHILD 1N5819 L1: 3L Electronic Cor p. SMTSDR322520C-220K 90K VDC DIMMING PTC PT6937 (2 LEDs) 50% 55% 60% 65% 70% 75% 80% 85% 2 5 10 15 20 LED CURRENT (mA) EFFICIENCY (%) VIN = 3V VIN = 3.6V Li-Ion to Two White LEDs PT6937 VIN SW FB/SHDN GND R1 4.75 22 H C 0.22 F OUTC 1 F IN VIN 3V ~ 5V D1: FAIRCHILD 1N5819 L1: 3L Electronic Cor p. SMTSDR322520C-220K 90K VDC DIMMING PTC PT6937 (3 LEDs) 50% 60% 70% 80% 90% 2 5 10 15 20 LED CURRENT (mA) EFFICIENCY (%) VIN = 3V VIN = 3.6V Li-Ion to Three White LEDs

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 9 - March, 2006 White LED Step-Up Converter PT6937 PT6937 VIN SW FB/SHDN GND R1 22 H C 0.22 F OUTC 1 F IN VIN 3V ~ 5V D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K 90K VDC DIMMING PTC PT6937 (4 LEDs) 50% 55% 60% 65% 70% 75% 80% 85% 2 5 10 15 20 LED CURRENT (mA) EFFICIENCY (%) VIN = 3V VIN = 3.6V Li-Ion to Four White LEDs PT6937 VIN SW FB/SHDN GND R1 22 H C 0.22 F OUTC 1 F IN VIN 3V ~ 5V D1: FAIRCHILD 1N5819 L1: 3L Electronic Corp. SMTSDR322520C-220K 90K VDC DIMMING PTC PT6937 (5 LEDs) 50% 55% 60% 65% 70% 75% 80% 85% 2 5 10 15 LED CURRENT (mA) EFFICIENCY (%) VIN = 3V VIN = 3.6V Li-Ion to Five White LEDs

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 10 - March, 2006 White LED Step-Up Converter PT6937 PT6937 VIN SW FB/SHDN GND R1 22 H C 0.22 F OUTC 1 F IN VIN D1: FAIRCHILD 1N5819 L1: 3L Electronic Cor p. SMTSDR322520C-220K 90K VDC DIMMING PTC PT6937 (7 LEDs) 50% 55% 60% 65% 70% 75% 80% 85% 2 5 10 15 LED CURRENT (mA) EFFICIENCY (%) VIN = 5V 5V to Seven White LEDs

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 11 - March, 2006 White LED Step-Up Converter PT6937 ABSOLUTE MAXIMUM RATINGS Parameter Symbol Rating Unit Input voltage VIN 5 V SW voltage 33 V FB voltage 5 V /SHDN voltage 5 V Operating temperature Topr -40 to +85 Ċ Storage temperature Tstg -65 to +150 Ċ Maximum junction temperature 125 Ċ Lead temperature (Soldering, 10 sec.) 300 Ċ Note: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired.

ELECTRICAL CHARACTERISTICS

(unless otherwise specified, Ta=25Ċ, VIN=3V, V /SHDN=3V,. Parameter Conditions Min. Typ. Max. Unit Minimum operating voltage 2.5 V Maximum operating voltage 5.5 V Feedback voltage ISW=100mA, Duty cycle=66% 86 95 104 mV FB pin bias current 5 10 nA Supply current /SHDN=0V 0.8 0.1 1.2 1.0 mA µA Switching frequency 0.8 1.2 1.6 MHz Maximum duty cycle 84 87 90 % Switch current limit 300 350 400 mA Switch VCESAT ISW=250mA 350 380 mV Switch leakage current VSW=30V 0.1 1 µA /SHDN voltage high 1.3 - - V /SHDN voltage low 0.5 V /SHDN pin bias current 20 30 µA Note: The PT6937 is guaranteed to meet specifications from 0Ċ to 70Ċ. Specifications over the -40Ċ to 85Ċ operating temperature range are assured by design, characterization and correlation with statistical process controls.

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 12 - March, 2006 White LED Step-Up Converter PT6937 ORDER INFORMATION Valid Part Number Package Top Code PT6937 5 Pins, SOT PT6937

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 13 - March, 2006 White LED Step-Up Converter PT6937

PACKAGE INFORMATION

5 PINS, SOT-23

E D 5 4 321 e b (5x) A 0.10 C SEATING PLANE

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 14 - March, 2006 White LED Step-Up Converter PT6937

Tel: 886-2-66296288 Fax: 886-2-29174598 URL: http://www.princeton.com.tw PT6937 V1.1 - 15 - March, 2006 White LED Step-Up Converter PT6937 Symbol Min. Typ. Max. A - - 1.45 A1 0 - 0.15 A2 0.90 1.15 1.30 b 0.30 - 0.50 b1 0.30 0.40 0.45 c 0.08 - 0.22 c1 0.08 0.13 0.20 D 2.90 BSC. E 2.80 BSC. E1 1.60 BSC. e 0.95 BSC. e1 1.90 BSC. L 0.30 0.45 0.60 L1 0.60 REF. L2 0.25 BSC. R 0.10 - - R1 0.10 - 0.25 θ 0 ° 4 ° 8 ° θ1 5 ° 10 ° 15 ° 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 plasti c 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