DEMO MANUAL DC1470A LT3743 High Current Synchronous Step-Down LED Driver with Three-State Control DESCRIPTION Demonstration circuit 1470A is a high current synchronous may be needed. The typical efficiency of the demo board step-down LED driver with three-state control featuring is 94% from a 12V input to 5V, 20A load. The lower the the LT 3743EUFD. The demo board is optimized for 20A input voltage, the higher the efficiency tends to be with output from a 12V input. Being an LED driver, the output a given load. At output power level of 100W, a couple of current is being regulated until the output voltage reaches percent of efficiency improvement is a huge advantage in a programmed voltage limit. This voltage limit on this minimizing temperature rise. If an efficiency measurement demo board is set to around 6V by R5 and R6. The 6V is is needed in an application, the output voltage must be chosen because of the 6.3V voltage rating of the output measured at the output capacitors instead of at the LED capacitors. The ideal load to be used with this demo board load. This prevents cable loss from being counted as a is a single LED, such as PT120 from Luminus Devices. loss of the board. If efficiency measurement is required Smaller LEDs may not be able to handle the high current, during PWM dimming, the average current and voltage even for a short period of time. At 20A, the demo circuit can values should be measured with proper equipment. All operate continuously. However, it is necessary to mount LT3743 circuits turn off unnecessary circuits during PWM the LED load on a proper heat sink and possibly with a off period to minimize power losses. As a result, efficiency fan to avoid exceeding its maximum temperate rating. in many PWM dimming applications is almost the same as efficiency of the constant current applications. Note: The DC470A will drive a LED with its cathode grounded. Grounding cathode of the LED allows customers The DC1470A uses a split output capacitor configuration to use positive input supply. to achieve <2s current rise time from 0A to 20A. To see the real rise time, connecting wires between the LED and The input voltage range of the LT3743 itself is 6V to 36V. the board should be minimized to no more than 2 inches However, the demo board utilizes 30V MOSFETs to demon- strate best efficiency so the maximum recommended input Design files for this circuit board are available at voltage is 24V for the demo board. When input is above DEMO MANUAL DC1470A DESCRIPTION total to minimize the wire inductance. 1-inch is even better. R15, R16 and C15 help filtering out voltage spikes seen + The cathode of the LED should return to LED with R25 on the SENSE or SENSE pin. It is critical to have those populated with a resistor in a few m. It is recommended components on a board. to measure the voltage on R25 with a short 50 coax The LT3743 has a three-state control. It can change output cable directly into a BNC connector on the oscilloscope. current among zero, a low level and a high level, all in a The current can then be calculated from the measured few s. The low current level of the demo board is set to voltage. To get accurate current, R25 should be measured around 5A. The high current level is 20A. Both levels can before it is mounted on the board. A current probe adds be adjusted by changing the voltage dividers on CTRL H more delays to the rise time so using a current probe is and CTRL L. Applying a PWM signal to CTRL SEL will not recommended. If fast rise time measurement is not toggle the output between 5A to 20A. While applying a PWM necessary, the cathode of the LED can be connected to signal to the PWM pin will toggle the output between either GND as shown in Figure 1. 0A to 5A or 0A to 20A depending on the CTRL SEL level. R27 can be used to slow down the gate drive. Slower gate The LT3743 data sheet gives a complete description of drive helps to reduce ringing on the SW node without the part, operation and application information. The data noticeable effect on the efficiency. A 10 is usually more sheet must be read in conjunction with this demo manual than enough to completely damp any ringing. for DC1470A. QUICK START PROCEDURE Demonstration circuit 1470A is easy to set up to evalu- 4. At this time, the output current will be slightly below ate the performance of the LT3743. Refer to Figure 1 for 5A. If no output current is observed, turn off PV and IN proper measurement equipment setup and follow the check the connections. procedure below: 5. To increase the load current to 20A, turn off PV . Pull IN 1. With power off, connect the input power supply to PV CTRL SEL high either by populating R21 or by using an IN and GND. The input power supply should have a current external voltage source. Make sure the load can handle limit of 10A or more. 20A continuously and the load voltage is not too high for the board. Then turn on PV . IN 2. With power off, connect a LED between V and GND OUT or between V and LED, if fast rise time is to be 6. Both the low current level, 5A and the high current OUT verified. When returning LED to LED, R25 needs to level 20A can be adjusted by changing the voltage on be populated. To check voltage across R25, the best CTRL L and CTRL H terminals between 0V and 2V. set up is to solder a short 50 coax cable across R25. 7. To evaluate the transient between 0A and a non-zero Connect the other end of the cable directly into a BNC current level, apply a PWM signal to the PWM terminal. connector on an oscilloscope. A less perfect but easier The nonzero current level is dictated by the CTRL SEL set up is to touch the probe tip directly across the IS+ and CTRL H or CTRL L voltages. and IS terminals with out the probe ground wire. See 8. To evaluate the transient between two non-zero cur rent Figure 2 for setup. The latter set up tends to have more levels, apply a PWM signal to the CTRL SEL terminal. noise but is ok for non-critical measurements. The LED connecting wires should be as short as possible, 1 inch 9. To modify the demo board for other applications, please total being ideal. contact Linear Applications Group for help. 3. Turn on the power at the input. dc1470af 2