IS31LT3352 MR16 EVALUATION BOARD GUIDE DESCRIPTION QUICK START The IS31LT3352 is a continuous mode inductive step-down converter, designed to efficiently drive a single or multiple LEDs connected in series from a voltage source higher than the total LED voltage. The device can operate with an input voltage of 6V to 40V and generate an output current of up to 750mA for a total of 30W. The IS31LT3352 includes an integrated output switch and a high-side output current sensing circuit. An Figure 1: Photo of IS31LT3352 Evaluation Board external resistor is used to set the output current. It also integrates a temperature compensation function in order to maintain stable and reliable operation of the RECOMMENDED EQUIPMENT LEDs. The IS31LT3352 monitors the ambient 30VDC power supply temperature near the LEDs with an external Negative LED panel (1 WLED, 3 LEDs in series) Temperature Coefficient (NTC) thermistor mounted Multi-meter close to LEDs. The output current is automatically reduced if the ambient temperature exceeds the resistance threshold value set at RTH pin. The current RECOMMENDED INPUT AND OUTPUT RATINGS will return to the set value once the ambient temperature goes below the threshold. Input: 12~24VDC or 12VAC Output: 1~3 LEDs in series/333mA The IS31LT3352 can be connected in a serial chain all using the same temperature compensation Note: The input voltage must be 2V higher than the output percentage. In this chain, the device ADJO output pin voltage (total V). f will drive the next IS31LT3352s ADJI input pin with the temperature compensation information. For this ABSOLUTE MAXIMUM RATINGS configuration a single thermistor is required. Input voltage 25VDC The IS31LT3352 MR16 DEMO board is designed for an MR16 lamp. Caution: Do not exceed the conditions listed above otherwise the board will be damaged. FEATURES PROCEDURE Simple low parts count Internal 40V power switch The IS31LT3352 demo board is fully assembled and tested. Follow the steps listed below to verify board Wide input voltage range: 6V to 40V operation. Up to 750mA output current Caution: Do not turn on the power supply until all connections High efficiency (up to 95% ) are completed. 1200:1 dimming rate 1) Connect the terminals of the power supply to the Typical 5% output current accuracy AC1 and AC2 pin. Single pin on/off and brightness control using DC 2) Connect the Cathode (-) of the LED panel (LED voltage or PWM arrays) to the EVB (-) terminal. Up to 1MHz switching frequency 3) Connect the Anode (+) of the LED panel (LED Inherent open-circuit LED protection arrays) to the EVB (+) terminal. Thermal shutdown to protect IC itself 4) Turn on the power supply and the LED panels Temperature compensation to protect LEDs (LED arrays) will be light. Lumissil Microsystems www.lumissil.com 1 Rev. A, 04/03/2013 IS31LT3352 MR16 EVALUATION BOARD GUIDE ORDERING INFORMATION Part No. Temperature Range Package IS31LT3352-V1GRLS2-EB -40C to +85C (Industrial) SOP-8 IS31LT3352-V2GRLS2-EB Table 1: Ordering Information For pricing, delivery, and ordering information, please contacts Lumissils analog marketing team at analog Lumissil.com or (408) 969-6600. DETAILED DESCRIPTION reduce output current as the monitored LED temperature increases. LED Current Control As shown in Figure 2, temperature compensation is The LED(s) output current is determined by the value decided by NTC thermistor R3 and resistor R2. When of the external current sense resistor (R1) connected the LED(s) temperature increases, thermistor between VIN and ISENSE and is calculated by: resistance of R3 starts to decrease. The temperature compensation function is enabled once the NTC (R3) 0.1 (for R > 0.13 ) I S resistance decreases to the point that the total OUT NOM R S resistance (R3 + R4) equals the resistance of R2. The table below lists examples of output current values The equation for Iout current with temperature for various current set resistors (R1): compensation is: In the case that 0.3< V <1.2V: ADJI Nominal Average Output Current R ( ) S (mA) I = 0.083V (R3+R4)/R3R2 OUTdc ADJI 0.13 769 In the case that V >1.2V: ADJI 0.15 667 I = 0.1 (R3+R4)/R2R1 OUTdc 0.27 370 R3 and R4 decide the temperature compensation 0.3 333 slope, if R4 is 0 , the slope is decided by thermistor R3s parameter B-constant. A larger R4 resistance The above values assume the ADJ pin is left floating at helps to smooth out the slope. a nominal voltage of V =1.2V. REF Large R3 and R4 resistor values will require a larger Note that R1=0.13 is the minimum allowed value of R2 to match and vice versa. Too large resistance value sense resistor in order to maintain the switch current for R2 will make the Rth pin more sensitive to noise, below the maximum allowable current. too small R4 resistance will increase IC current Inductor Selection consumption. An R2 resistance value of between 1K to 100k is recommended. Recommended inductor values are in the range of 47 H to 220H. The higher inductor values are recommended for higher supply voltages and low output currents in order to minimize variation in output current over the supply voltage range. Higher inductance values also minimize errors due to switching delays, which cause increased ripple and lower efficiency. The inductor should be mounted as close to LX pin as possible. Temperature Compensation Of Output Current Figure 2 IS31LT3352 Temperature Compensation High luminance LEDs often need to be supplied with a An IS31LT3352 calculator is available from the temperature compensated current in order to maintain Lumissil to assist with temperature compensation stable and reliable operation at all drive levels. The design. LEDs are usually mounted remotely from the chip. If output current compensation is required, it is possible to use an external temperature sensing network - normally using Negative Temperature Coefficient (NTC) thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network can Lumissil Microsystems www.lumissil.com 2 Rev. A, 04/03/2013