DEMO MANUAL DC2339A LT3744 High Current Synchronous Step-Down LED Driver Description Demonstration circuit 2339A is a high current synchronous The typical efficiency of the demo board is 93.5% from step-down LED driver featuring the LT 3744. The unique a 12V input to 5V, 20A load. The lower the input voltage, drive stage used on the LT3744 allows the anodes of three the higher the efficiency tends to be for a given load. At LEDs to be connected together for better heat sinking. This output power level of 100W, even one percent of efficiency connection will work in both the step-down configuration improvement is a big advantage in minimizing temperature and the inverting buck-boost configuration. The pros and rise. If an efficiency measurement is needed in an applica- cons of each configuration can be found in the data sheet. tion, the output voltage must be measured at the output This demonstration circuit 2339A is for customers to test capacitors instead of the LED load. This prevents cable the step-down configuration only. The inverting buck-boost loss from being counted as a loss of the board. configuration is shown in a separate demonstration circuit. The demo circuit DC2399A achieves fast current rise time The input of the demo board is up to 36V. The components from 0A to 20A in 5s. To see the real rise time, connecting are optimized for the efficiency, thermal and PWM dim- wires between the LED and the board should be as short as ming for a 12V input. Each of the three outputs is up to possible to minimize the wire inductance and resistance. It 5V, 20A with a 6.05V maximum output voltage limit. The is recommended to measure the voltage across R32 with PWM1, PWM2 and PWM3 pins are set to low by default. a short 50 coax cable directly into a BNC connector on A DC or PWM signal is required to connect to at least one the oscilloscope. The current can be calculated from the of the PWM pins to enable the circuit. At any giving time, measured voltage. Figure 1 shows the current rise time. A output current only passes through one LED determined current probe adds more delays to the rise time so using by settings of PWM pins. a current probe is not recommended unless rise time is not a concern. The CTRL1, CTRL2 and CTRL3 pins can be adjusted to provide accurate analog dimming down to 20:1 ratio. The LT3744 data sheet gives a complete description of the part, operation and application information. The data The minimal input voltage for the LT3744 to operate is sheet must be read in conjunction with this quick start 3.3V. However, to maintain the output current regulation guide for demo circuit 2339A. in a step down regulator, the minimum input voltage is determined by the LED voltage and the maximum duty cycle. Design files for this circuit board are available at For a 5V output, the demo board minimum input voltage DEMO MANUAL DC2339A Description PWM 2V/DIV PWM (YELLOW) 2V/DIV LED CURRENT (GREEN) 6.67A/DIV 2.00s/DIV Figure 1. DC2339A Current Rise Time. V = 12V, LED Voltage = 4.2V when ON. Total LED Current = 20A IN p s Specifications are at T = 25C A SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS PV Input Supply Voltage V + = 5V 5.5 36 V IN LED V + Maximum Output Voltage 6.05 V LED I Output Current CTRL1, CTRL2, CTRL3 = 1.5V 19 20 21 A OUT f Switching Frequency 380 425 470 kHz SW EFF Efficiency at DC V = 12V, I = 20A, V = 5V 93.5 % IN OUT OUT Quick s p D Demonstration circuit 2339A is easy to set up to evalu- 2. With power off, connect the LED load to V + and LED ate the performance of the LT3744. Refer to Figure 2 for proper LED1 , LED2 , LED3 according to the setting proper measurement equipment setup and follow the of the PWM pins. procedure below: 3. Turn on the power at the input. Turn on PWM1, PWM2 1. With power off, connect the input power supply to PV and/or PWM3. IN and GND. Note: make sure PV is below 36V. Con- IN 4. Carefully evaluate other design parameters as needed. nect a DC supply or a PWM signal to PWM1, PWM2 and/or PWM3. dc2339af 2 ure roce tart ummary erformance