DEMO MANUAL DC1930A LT3744 High Current Synchronous Inverting Buck-Boost LED Driver DESCRIPTION Demonstration circuit 1930A is a high current synchronous LEDs may not be able to handle the high current, even for inverting buck-boost LED driver featuring the LT 3744. The a short period of time. It is necessary to mount the LED unique drive stage used on the LT3744 allows the anodes of load on a proper heat sink. A fan may become necessary three LEDs to be connected together for better heat sinking to avoid exceeding LEDs maximum temperate rating. in both the step-down configuration and the inverting buck- The typical efficiency of the demo board is 89.3% from a boost configuration. The pros and cons of each configuration 12V input to 4V, 12A load. If an efficiency measurement is can be found in the data sheet. This demonstration circuit needed in an application, the output voltage must be mea- 1930A is for customers to test the inverting buck-boost sured at the output capacitors instead of the LED load. This configuration. The step-down configuration is shown in a prevents cable loss from being counted as loss of the board. separate demonstration circuit DC2339A. The demo circuit DC1930A achieves fast current rise time The components are optimized for the efficiency, thermal from 0A to 12A in 5s. To see the real rise time, wires and PWM dimming for a 12V input. Each of the three outputs between the LED and the board should be as short as is up to 5V, 12A with a 6.05V maximum output voltage possible to minimize the wire inductance and resistance. It limit. The PWM1, PWM2 and PWM3 pins are set to low is recommended to measure the voltage across R32 with by default. A DC or PWM signal is required to connect to a short 50 coax cable directly into a BNC connector on at least one of the PWM pins to enable the circuit. At any the oscilloscope. Figure 1 shows the current rise time. The giving time, output current only passes through one LED current can be calculated from the measured voltage. A determined by settings of PWM pins. current probe adds more delays to the rise time so using The CTRL1, CTRL2 and CTRL3 pins can be adjusted to a current probe is not recommended unless rise time is provide accurate analog dimming down to 20:1 ratio. not a concern. The input voltage range for the LT3744 to operate is from The LT3744 data sheet gives a complete description of 3.3V to 36V. However, to maintain the output current the part, operation and application information. The data regulation in an inverting buck-boost regulator, the actual sheet must be read in conjunction with this quick start input voltage range is determined by the load current, guide for demo circuit 1930A. voltage, the maximum duty cycle, etc. For a 5V, 12A load, Design files for this circuit board are available at the demo board operating input range should limit to 7V DEMO MANUAL DC1930A DESCRIPTION LED CURRENT 5A/DIV SW VOLTAGE 5V/DIV DC1930A F01 Figure 1: DC1930A Current Rise Time: V = 12V, LED Voltage = 4.2V when ON. Total LED Current = 12A IN QUICK START PROCEDURE Demonstration circuit 1930A 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. 1. With power off, connect the input power supply to PV IN 4. Carefully evaluate other design parameters as needed. and GND. Note: make sure PV is below 30V. IN dc1930af 2