IS31LT3352 MR16 DEMO Board Guide Description 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 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 LEDs’. The IS31LT3352 monitors the ambient temperature near the LEDs with an external Negative Temperature Coefficient (NTC) thermistor mounted close to LEDs. The output current is automatically reduced if the ambient temperature exceeds the resistance threshold value set at RTH pin. The current will return to the set value once the ambient temperature goes below the threshold. The IS31LT3352 can be connected in a serial chain all using the same temperature compensation percentage. In this chain, the device ADJO output pin will drive the next IS31LT3352’s ADJI input pin with the temperature compensation information. For this configuration a single thermistor is required. The IS31LT3352 MR16 DEMO board is designed for an MR16 lamp. Features Quick Start Recommended Equipment 30VDC Power supply LED panel (1W LED, 3 LEDs in series) Multi-meter Recommended Input and Output Ratings Input: 12-24VDC or 12VAC Output: 1-3 LEDs in series/333mA Note: The input voltage must be at least 2V higher than the output voltage (total Vf). Absolute Maximum Ratings Input voltage ≤ 25VDC Caution: Do not exceed the conditions listed above, otherwise the board will be damaged. Procedure The IS31LT3352 DEMO Board is fully assembled and tested. Follow the steps listed below to verify board operation. Caution: Do not turn on the power supply until all connections are completed. 1) 2) 3) 4) Connect the terminals of the power supply to the AC1 and AC2 pin. Connect the Cathode (-) of the LED panel (LED arrays) to the EVB (-) terminal. Connect the Anode (+) of the LED panel (LED arrays) to the EVB (+) terminal. Turn on the power supply and the LED panel (LED arrays) will light. Simple low parts count Internal 40V power switch Wide input voltage range: 6V to 40V Up to 750mA output current High efficiency (up to 95% ) 1200:1 dimming rate Typical 5% output current accuracy Single pin on/off and brightness control using DC voltage or PWM Up to 1MHz switching frequency Inherent open-circuit LED protection Thermal shutdown to protect IC itself Temperature compensation to protect LEDs Bill of Materials No. Name Description Ref Des. Qty. 1 AL Capacitor 220uF±10%,25V C1 1 2 SMD Capacitor 100nF±20%,50V,0805 C2,C4 2 3 SMD Capacitor 1uF±10%,50V,0805 1 4 SMD Resistor 0.3Ω±1%,0805 C3 R1 5 SMD Resistor 75KΩ±1%,0603 R2 1 6 NTC Resistor NTC,100K,B=4050 R3 1 7 SMD Resistor NC R4,R5 2 8 Schottky Diode SS16,1A,60V,SMA 5 9 SMD Inductor 47uH,Isat≥600mA D1-D5 L1 10 IC1 IS31LT3352,SOP8 U1 1 Integrated Silicon Solution, Inc. – www.issi.com R1.1, 04/03/2013 Mfr 1 1 ISSI 1 IS31LT3352 MR16 DEMO Board Guide R1 D2 D3 D5 1 AC/DC C1 C2 6 ADJI D1 D4 RTH R2 LX LT3352 SN3352 ADJO RNTC GND 4 7 R3 NTC ISENSE VIN 3 C3 2 L1 8 47uH 5 R2 temperature compensation point 75K 31ºC 33K 51ºC 20K 65ºC C4 Figure 1 IS31LT3352 MR16 DEMO Board Schematic Detailed Description LED Current Control The LED(s) output current is determined by the value of the external current sense resistor (R1) connected between VIN and ISENSE and is calculated by: I OUT nom = 0.1/R1 [for R1>0.13Ω] The table below lists examples of output current values for various current set resistors (R1): R1 (Ω) Output current (mA) 0.13 0.15 0.27 0.3 769 667 370 333 The above values assume the ADJ pin is left floating at a nominal voltage of VREF =1.2V. Note that R1=0.13Ω is the minimum allowed value of sense resistor in order to maintain the switch current below the maximum allowable current. Inductor selection Recommended inductor values are in the range of 47μH to 220μH. 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 High luminance LEDs often need to be supplied with a temperature compensated current in order to maintain stable and reliable operation at all drive levels. The 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) Integrated Silicon Solution, Inc. – www.issi.com R1.1, 04/03/2013 thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network can reduce output current as the monitored LED temperature increases. As shown in Figure 2, temperature compensation is decided by NTC thermistor R3 and resistor R2. When the LED(s) temperature increases, thermistor resistance of R3 starts to decrease. The temperature compensation function is enabled once the NTC (R3) resistance decreases to the point that the total resistance (R3 + R4) equals the resistance of R2. The equation for Iout current with temperature compensation is: In the case that 0.3< VADJI <1.2V: I OUTdc = 0.083*VADJI(R3+R4)/R3*R2 In the case that VADJI >1.2V: I OUTdc = 0.1*(R3+R4)/R2*R1 R3 and R4 decide the temperature compensation slope, if R4 is 0Ω, the slope is decided by thermistor R3’s parameter B-constant. A larger R4 resistance helps to smooth out the slope. Large R3 and R4 resistor values will require a larger R2 to match and vice versa. Too large resistance value for R2 will make the Rth pin more sensitive to noise, too small R4 resistance will increase IC current consumption. An R2 resistance value of between 1K to 100K is recommended. Figure 2 IS31LT3352 Temperature Compensation An IS31LT3352 calculator is available from the ISSI to assist with temperature compensation design. 2 IS31LT3352 MR16 DEMO Board Guide PCB Layout NOTE: Physical dimensions are (L x W x H): 18mm x 14mm x 10mm Figure 2 PCB Layout-Top Layer Figure 4 Component Placement Guide -Top Layer Figure 3 PCB Layout-Bottom Layer Figure 5 Component Placement Guide -Bottom Layer Revision History Date Revision Changes 2012.03.06 R1.0 Initial release 2012.04.12 R1.1 1. BOM: Add Cap tolerance value. 2. BOM: NTC B=3950 change to B=4050. 3. Change the temperature compensation point in the schematic table. Copyright © 2013 Integrated Silicon Solution, Inc. All rights reserved. ISSI reserves the right to make changes to this specification and its products at any time without notice. ISSI assumes no liability arising out of the application or use of any information, products or services described herein. Customers are advised to obtain the latest version of this device specification before relying on any published information and before placing orders for products. Integrated Silicon Solution, Inc. – www.issi.com R1.1, 04/03/2013 3