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AT6731 White LED Step-Up Converter Immense Advance Tech. FEATURES DESCRIPTION Inherently Matched LED Current The AT6731 is a step-up DC/DC converter Drives Up to 27 LEDs From a 5V Supply specifically designed to drive white LEDs with a 36V Rugged Bipolar Switch constant current. The device can drive up to 27 Fast 1.2MHz Switching Frequency LEDs from a 5V supply. Over Voltage Protection The AT6731 implements a 1.2MHz control scheme. The high frequency operation saves board space by reducing external component sizes. To optimize efficiency, the feedback voltage is set to APPLICATION only 95mV. This reduces the power dissipation in Cellular Phones PDAs, Handheld Computers Digital Cameras Mp3 Players GPS Receivers the current set resistor and allows the lowest total output voltage, hence minimal current draw from the battery. Additional feature include output voltage limiting when LEDs are disconnected. PIN CONFIGURATIONS (TOP VIEW) ORDER INFORMATION AT 6731-KF R IAT Circuit Type Shipping: R: Tape & Reel KF: SOT-26 S8: SOP-8 Rev 1.1 1 AT6731 White LED Step-Up Converter Immense Advance Tech. PIN DESCRIPTIONS Pin Name SW GND FB Pin Description Switch Pin. (Minimize trace area at this pin to reduce EMI.) Ground Pin. Connect directly to local ground plane. Feedback Pin. Reference voltage is 95mV. (Calculate resistor value according to the formula RFB = 95mV / ILED.) EN Enable Pin. (Connect to 1.5V or higher to enable device; 0.4V or less to disable device.) VIN Input Supply Pin. (Must be locally bypassed.) Vout Over Voltage Protection Pin. Voltage sensing input to trigger the function of over-voltage protection TYPICAL APPLICATION CIRCUIS Figure .1 Rev 1.1 2 AT6731 White LED Step-Up Converter Immense Advance Tech. BLOCK DIAGRAM Figure .2 OPERATION The AT6731 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the block diagram in Figure 2. At the start of each oscillator cycle, the RS latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the RS latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 95mV. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. Rev 1.1 3 AT6731 White LED Step-Up Converter Immense Advance Tech. ABSOLUTE MAXIMUM RATINGS (Note 1) Parameter Range Unit Input Voltage (VIN ) 10 V SW Voltage 36 V FB Voltage 10 V EN Voltage 10 V -40 to +85 °C 125 °C -65 to +150 °C 260 °C Operating Temperature Range Maximum Junction Temperature Storage Temperature Range Lead Temperature (Soldering, 5sec) SOT-26 300 TA=25°C (Note 2) SOP8 400 Thermal Resistance SOT-26(Note 3) 333 Junction to Ambient SOP8 160 Power Dissipation Thermal Resistance Junction to Case PD @ SOT-26 106.6 mW °C/W C/W Note 1: Stresses listed as the above “Absolute Maximum Ratings” may cause permanent damage to the device. These are for stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliability. Note 2: Thermal Resistance is specified with the component mounted on a low effective thermal conductivity test board in free air at TA=25°C. Note 3: Thermal Resistance is specified with approximately 1 square of 1 oz copper. Rev 1.1 4 AT6731 White LED Step-Up Converter Immense Advance Tech. ELECTRICAL CHARACTERISTICS TA= 25°C, VIN= 5V, VEN =5V, unless otherwise specified. Parameter Symbol Minimum Operating Voltage VIN_Min Maximum Operating Voltage VIN_Max Feedback Voltage IFB FB Pin Bias Current IFB_BIAS Supply Current ISUPPLY Condition Min Typ Max 2.5 Unit V 10 V ILOAD = 180mA, VIN = 5V 83 95 107 ILOAD= 100mA, VIN = 5V 86 95 104 10 45 100 nA 2.1 3.0 mA 0.1 1.0 μA 1.6 MHz EN = 0V mV Switching Frequency IFREQ 0.8 1.2 Maximum Duty Cycle DCMAX 85 90 % Switch Current Limit ILIMIT 650 mA ISW = 250mA 350 mV VSW = 5V 0.01 Switch VCESAT Switch Leakage Current VCESAT VSW_LEAKAGE EN Voltage High EN_H EN Voltage Low EN_L EN Pin Bias Current OVP Threshold 5 1.5 μA V 0.4 V IEN 65 μA VOVP 29 V Rev 1.1 5 AT6731 White LED Step-Up Converter Immense Advance Tech. TYPICAL OPERATING CHARACTERISTICS Rev 1.1 6 AT6731 White LED Step-Up Converter Immense Advance Tech. APPLICATIONS INFORMATION Inductor Selection Schottky diodes with higher current ratings usually A 10μH inductor is recommended for most AT6731 have lower forward voltage drop and larger diode applications. Although small size and high efficiency capacitance, which can cause significant switching are major concerns, the inductor should have low losses at the 1.2MHz switching frequency of the core losses at 1.2MHz and low DCR (copper wire AT6731. A Schottky diode rated at 1000mA it is resistance). sufficient for most AT6731 applications. Recommended Inductors Rated Rated Part No. Size DCR (WxLxH:mm) (Ω) Dimming Control There are two techniques for dimming control. One Current, Inductance (μH) Irms (A) 10 2.08 is PWM dimming, and the other is continuous dimming. 7.6 x 7.6 x DR73-100-R 0.0656 1. PWM dimming control is implemented by 3.55 applying a PWM signal on EN pin as shown in 5.2 x 5.2 x SD14-100-R 10 1.1 0.1913 Figure 3. The AT6731 is turned on and off by the 1.45 PWM signal. With this method, the LEDs 4.0 x 4.5 x LD1-100-R 10 1.13 0.1820 operate with either zero or full current. The 3.2 COOPER Bussmann average LED current is increased proportionally http://www.cooperbussmann.com to the duty-cycle of the PWM signal. This technique has high-efficiency because the IC Capacitor Selection and the LEDs consume no current during the off The small size of ceramic capacitors makes them cycle of the PWM signal. Typical PWM ideal for AT6731 applications. X5R and X7R types are recommended because they retain frequency should be between 100Hz and their 10KHz. capacitance over wider voltage and temperature 2. Continuous dimming control is implemented by ranges than other types such as Y5V or Z5U. A applying a DC control voltage to the FB pin of 4.7μF input capacitor and a 4.7μF output capacitor the AT6731 through a series resistor as shown are sufficient for most AT6731 applications. in Figure 4. The LED intensity (current) can by dynamically varied applying a DC voltage to the Diode Selection FB pin. The DC voltage can come from a DAC The Schottky diodes, with their low forward voltage signal, or a filtered PWM signal. The advantage drop and fast reverse recovery, are the ideal choices of this approach is a high frequency PWM signal for AT6731 applications. The forward voltage drop of (>10KHz) that can be used to control LED a Schottky diode represents the conduction losses intensity. 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. Rev 1.1 7 AT6731 White LED Step-Up Converter Immense Advance Tech. APPLICATION INFORMATION (CONTINUED) Figure 3. PWM Dimming Method Figure 4. Continuous Dimming Rev 1.1 8 AT6731 White LED Step-Up Converter Immense Advance Tech. PACKAGE OUTLINE DIMENSIONS Rev 1.1 9 AT6731 White LED Step-Up Converter Immense Advance Tech. Note : Information provided by IAT is believed to be accurate and reliable. However, we cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an IAT product; nor for any infringement of patents or other rights of third parties that may result from its use. We reserve the right to change the circuitry and specifications without notice. Life Support Policy: IAT does not authorize any IAT product for use in life support devices and/or systems. Life support devices or systems are devices or systems which, (I) are intended for surgical implant into the body or (II) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. Typical numbers are at 25°C and represent the most likely norm. Rev 1.1 10