RT9271PE

RT9271 White LED Step-Up Converter in Tiny Package General Description The RT9271 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The device can drive one to three 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 RT9271 switches at 1.1 MHz, allowing the use of tiny external components. The input and output capacitor can be as small as 1uF, saving space and cost versus alternative solutions. A low 0.25V feedback voltage minimizes power loss in the current setting resistor for better efficiency. The RT9271 is available in low profile SOT-23-6 package. Features Inherently Matched LED Current Up to 80mA Output Current @ VOUT < 12V High Efficiency : 85% Typical Drives Up to Three LEDs from 2.8V Supply 20V Internal Switch Fast 1.1 MHz Switching Frequency Uses Tiny 1 mm Height Inductors Requires Only 1uF Output Capacitor Low Profile SOT-23-6 Package Optional 15V Over Voltage Protection RoHS Compliant and 100% Lead (Pb)-Free Applications Mobile Phone Digital Still Camera PDAs, Handheld Computers MP3 Players GPS Receivers Ordering Information RT9271 Package Type B : SOT-23-5 E : SOT-23-6 Operating Temperature Range P : Pb Free with Commercial Standard G : Green (Halogen Free with Commercial Standard) Pin Configurations (TOP VIEW) VCC 5 1 LX 2 GND EN 4 3 FB VCC OVP 6 1 LX 5 2 GND EN 4 3 FB Note : RichTek Pb-free and Green products are : RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. 100% matte tin (Sn) plating. SOT-23-5 SOT-23-6 Marking Information For marking information, contact our sales representative directly or through a RichTek distributor located in your area, otherwise visit our website for detail. Note : There is no pin1 indicator on top mark for SOT-23-6 type, and pin1 will be lower left pin when reading top mark from left to right. DS9271-13 March 2007 www.richtek.com 1 RT9271 Typical AppIication Circuit VIN 2.4 to 3.2V C1 1uF LX 10uH D1 SS0520 VCC EN Dimming Control LX OVP FB C3 1uF D2 GND R2 12 Figure 1. RT9271 Drivers 1 WLED Application Circuit C1 1uF LX 10uH D1 SS0520 VIN 2.4 to 5V VCC EN Dimming Control LX OVP FB C3 1uF D2 D3 GND R2 12 Figure 2. RT9271 Drivers 2 Series WLEDs Application Circuit VIN 2.4 to 6V C1 1uF LX 10uH D1 SS0520 D2 VCC EN Dimming Control LX OVP FB C3 1uF D3 D4 GND R2 12 Figure 3. RT9271 Drivers 3 Series WLEDs Application Circuit Note : 1. D1 is Schottky diode (SS0520). 2. D2 ~ D4 are the WLED (HT-S91CW-DT) of HARVATEK. 3. LX is the SH4018 series of ABC TAIWAN ELECTRONICS CORP. Recommended Circuits for Driving LEDs Figure 1 to Figure 3 illustrates the recommended application circuits for driving white LEDs. The series connected LEDs are driven with identical current to emit uniform luminescence, and the 250mV low reference voltage can minimize the efficiency loss across the current-sensing resistor. The recommended current setting for driving white LEDs is 10mA to 20mA, and the dimming control can be implemented by toggling EN pin with 60Hz to 1kHz PWM clock. Please refer to application notes for guidance of component selection and board layout. www.richtek.com 2 DS9271-13 March 2007 RT9271 Functional Pin Description Pin No. −XB 1 2 3 4 − 5 −XE 1 2 3 4 5 6 Pin Name Pin Function LX GND FB EN OVP VCC 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 0.25V. Connect cathode of lowest LED and resistor here. Calculate resistor value according to the formula: RFB = 0.25/ILED Chip Enable Pin. Connect to 1.4V or higher to enable device, 0.4V or less to disable device. Over Voltage Protection Pin. Voltage sensing input to trigger the function of over voltage protection, the trip point is 15.5V. Leave it unconnected to disable this function. Input Voltage Pin. Must be locally bypass with 1uF capacitor to GND. Function Block Diagram OVP 15.5V LX FB + VREF 0.25V A1 + COMPARATOR A2 R S Q DRIVER M1 VCC EN CHIP ENABLE 4μA RAMP GENERATOR 1.1MHz OSCILLATOR Operation The RT9271 is a constant frequency step-up converter with an internal switch. For excellent line and load regulation, the current mode control is adopted. The operations of RT9271 can be understood from block diagram clearly. The oscillator triggers the SET input of SR latch to turn on the power switch M1 at the start of each cycle. A current sense voltage sum with a stabilizing ramp is connected to the positive terminal of the PWM comparator A2. When this voltage exceeds the output voltage of the error amplifier A1, the SR latch is reset to turn off the power switch till next cycle starts. The output voltage of the error amplifier A1 is amplified from the difference between the reference voltage 0.25V and the feedback voltage. In this manner, if the error amplifiers voltage increases, more current is delivered to the output; if it decreases, less current is delivered. A 15.5V Zener diode connects from OVP pin to FB pin internally to provide an optional protection function which prevents LX pin from over-voltage damage. Especially when the case of the feedback loop broken due to component wear-out or improper connection occurs. The behavior of OVP is to clamp the output voltage to 15.5V typically. This function is suitable for the applications while driving white LEDs less than 4 in series. DS9271-13 March 2007 + 0.75Ω GND www.richtek.com 3 RT9271 Absolute Maximum Ratings (Note 1) Supply Voltage, VCC -------------------------------------------------------------------------------------------------- −0.3V to 7V LX, OVP ----------------------------------------------------------------------------------------------------------------- −0.3V to 21V The Other Pins --------------------------------------------------------------------------------------------------------- −0.3V to 7V Power Dissipation, PD @ TA = 25°C SOT-23-6 ---------------------------------------------------------------------------------------------------------------- 0.4W Package Thermal Resistance (Note 4) SOT-23-6, θJA ----------------------------------------------------------------------------------------------------------- 250°C/W Maximum Junction Temperature ----------------------------------------------------------------------------------- 125°C Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260°C Storage Temperature Range ---------------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 2) HBM (Human Body Mode) ------------------------------------------------------------------------------------------ 2kV MM (Machine Mode) -------------------------------------------------------------------------------------------------- 200V Recommended Operating Conditions (Note 3) Supply Voltage, VCC -------------------------------------------------------------------------------------------------- 2.4V to 6V Junction Temperature Range ---------------------------------------------------------------------------------------- −40°C to 125°C Electrical Characteristics (VCC = 3.6V, TA = 25°C, unless otherwise specified.) Parameter System Supply Input Under Voltage Lock Out Maximum Output Voltage Supply Current Quiescent Current Shut Down Current Oscillator Operation Frequency Maximum Duty Cycle Reference Voltage Feedback Voltage MOSFET On Resistance of MOSFET Current Limit Current Limit Control and Protection Shut Down Voltage Enable Voltage EN Pin Pull Low Current OVP Threshold (Note 5) Symbol UVLO ICC1 ICC2 ICC3 FOSC Dmax VFB Rds(on) Imax1 Imax2 VEN1 VEN2 IEN OVP Test Condition Min 1.8 -- Typ 2.2 --90 0.1 1.1 90 0.25 0.75 900 625 0.8 0.8 4 15.5 Max 2.3 20 2 120 1 1.3 -0.263 1.0 1000 750 -1.4 6 20.0 Units V V mA μA μA MHz % V Ω mA mA V V μA V VCC=6V, Continuously Switching VCC=6V, FB=1.3V, No Switching VCC=6V, VEN