DEMO CIRCUIT 1285 LT3598 QUICK START GUIDE LT3598 6-String 30mA LED Driver with 1.5% Current Matching DESCRIPTION multiple strings together to achieve higher per WARNING string current is allowed. To paralleling strings, tie the respective LEDn pins together. OPENLED in- Do not look directly at operating LED. dicator flags should any string be left open. To im- This circuit produces light that can damage eyes. prove efficiency, it is recommended that V is be- IN tween 3.2V and 5.5V. The V bias capacitor of the IN Demonstration circuit 1285 is a 6-String 30mA LED demo circuit is rated at 6.3V. PWM dimming ratio Driver with 1.5% Current Matching featuring the can be as high as 3000:1. LT3598. The LT3598 is a step-up DC/DC converter The LT3598 datasheet gives complete descriptions designed to drive up to 6-strings of LEDs at an of the part, operation and application information. output voltage up to 44V. The LED strings are The datasheet must be read in conjunction with connected to internal current sources where the this quick start guide for working on or modifying current level is set with an external resist. the demo circuit 1285. The demonstration circuit drives 20mA through each LED string. Each string can have up to 10 white LEDs. The circuit operation frequency is set Design files for this circuit board are available. to 1MHz. Parameters such as the LED current the Call the LTC factory. switching frequency, etc can be modified easily L, LTC, LTM, LT are trademarks of Linear Technology Corporation. Other product based on the demo circuit. When a string is not names may be trademarks of the companies that manufacture the products. used, tie its LED pin to V . Any unused string is OUT no longer in the regulation loop. Direct paralleling PERFORMANCE SUMMARY Specifications are at TA = 25C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Range 3.2 5.5* V IN PV Power Input Supply Range** 3 40 V IN I Quiescent Current PWM=0V, Not switching 3.5 5 mA Q F Switching Frequency 0.9 1 1.1 MHz SW I LED String Current 19.5 20 20.7 mA LED I LED String Current Matching*** 0.5 1.5 % M Eff Efficiency V = 3.3V PV = 12V I = 20mA 90 % IN IN LED D PWM dimming ratio PVIN = 12V 3000:1 PWM *: Limited by the voltage rating of the bias capacitor, C2, on the demo circuit. The IC rating is 30V. **: MIN PV depends on LED string voltages and the Maximum Switch Duty Cycle (specified on page 3 IN of the datasheet). PV greater than MAX LED string voltage is permitted but power dissipation needs to IN be taken into considerations. ***: Specifications apply over the full operating temperature range. 1 LT3598 QUICK START PROCEDURE Demonstration circuit 1285 is easy to set up to 4. Connect LED strings to LEDn+ and LEDn-. If evaluate the performance of the LT3598. Refer to any string is not used, tie LEDn- to LEDn+, Figure 1 for proper measurement equipment setup where n is the string number (between 1 and 6). and follow the procedure below: 5. Turn on power supplies for VIN and PVIN. 1. With power off, connect the input power supply NOTE. Make sure that VIN does not exceed 5.5V and PVIN to VIN and GND. does not exceed 40V. 2. Connect the power input power supply to PVIN 6. Check for the proper LED voltage and current. and GND. 3. Tie PWM to VIN or a PWM signal. 2