DEMO CIRCUIT 1290A LTC3775EUD QUICK START GUIDE LTC3775EUD HIGH FREQUENCY SYNCHRONOUS STEP-DOWN CONVERTER DESCRIPTION Demonstration circuit 1290A is a high frequency synchronous step-down converter featuring the LTC3775EUD. The circuit is designed as a drop-in layout with the main circuit components fitting in a 1” by ½” area. The package style for the LTC3775EUD is a 16-pin 3mm x 3mm QFN. Two versions of the demonstration board are available. DC-1290A-A has been optimized for 5V to 26V input, while the DC1290A-B is optimized for 5V to 36Vin range. Both circuits are designed for minimal size and maximum step-down ratio to demonstrate the low on-time capability of the LTC3775EUD and not for maximum efficiency. The main features of the board include an internal 5V linear regulator for bias and a Mode selector that allows the converter to run in forced continuous conduction (FCC) or pulse skip mode operation. Synchronization to an external clock is also possible. The LTC3775 datasheet gives a complete description of the part, operation and application information and must be read in conjunction with this quick start guide for demo circuit 1290A Design files for this circuit board are available. Call the LTC factory. L, LTC are registered trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. PERFORMANCE SUMMARY Specifications are at TA = 25°C SYMBOL VIN PARAMETER Input Supply Range VOUT Output Voltage Range IOUT Maximum Load Current fSW Typical Free Running Switching Frequency Efficiency See Figures 3 and 4 for efficiency curves CONDITIONS DC1290A-A DC1290A-B VIN = 5-26V, (DC1290A-A) VIN = 5-36V (DC1290A-B) DC1290A-A DC1290A-B DC1290A-A DC1290A-B DC1290A-A, VIN = 12V, IOUT = 15A DC1290A-B , VIN = 12V, IOUT = 10A MIN 5 5 TYP MAX 26 36 UNITS V 1.164 1.20 1.236 V 15 10 500 330 85.5 86.5 A A kHz kHz % % QUICK START PROCEDURE Demonstration circuit 1290A is easy to set up to evaluate the performance of the LTC3775EUD. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: NOTE. When measuring the input or output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across the Vin or Vout and GND terminals or directly across relevant capacitor. See Figure 2 for proper scope probe technique. 1. Place jumpers in the following positions: JP1 FCC JP2 ON 2. With power off, connect the input power supply to Vin and GND. 1 LTC3775EUD 3. Turn on the power at the input. NOTE. Make sure that the input voltage does not exceed 26V or 36V for DC1290A–A or DC1290A–B demonstration boards respectively. 4. Check for the proper output voltages. VOUT = 1.164V to 1.236V If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 5. Once the proper output voltage is established, adjust the loads within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. A + V - + A IOUT IIN Vin supply + Load Vin - - + V - Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple Across Terminals or Directly Across Bulk Capacitor FREQUENCY SYNCHRONIZATION AND MODE SELECTION Demonstration circuit 1290A’s Mode selector allows the converter to run in FCC operation, pulse skip operation or synchronizing to an external clock by changing 2 the position of JP1 accordingly. Please note that the external synchronizing frequency should be close to (+/-20%) the free running frequency. LTC3775EUD Mode Selection and Synchronized Operation Options CONFIGURATION FCC operation Pulse skip operation Synchronized to ext. clock applied to SYNC pin JP1 ‘FCC’ ‘Pulse_Skip’ ‘Sync’ Figure 3. Typical Efficiency vs. Load Current for DC1290A-A Figure 4. Typical Efficiency vs. Load Current for DC1290A-B 3 LTC3775EUD Top Bottom Figure 5. Thermal images of DC1290A-A, 26VIN, 1.2VOUT @ 15A, 22°C ambient Top Bottom Figure 6. Thermal images of DC1290A-B, 36VIN, 1.2VOUT @ 10A, 22°C ambient 4 LTC3775EUD 5