DEMO CIRCUIT 1290A LTC3775EUD QUICK START GUIDE LTC3775EUD HIGH FREQUENCY SYNCHRONOUS STEP-DOWN CONVERTER DESCRIPTION Demonstration circuit 1290A is a high frequency syn- The main features of the board include an internal 5V chronous step-down converter featuring the linear regulator for bias and a Mode selector that allows LTC3775EUD. The circuit is designed as a drop-in layout the converter to run in forced continuous conduction with the main circuit components fitting in a 1 by (FCC) or pulse skip mode operation. Synchronization to area. The package style for the LTC3775EUD is a 16-pin an external clock is also possible. 3mm x 3mm QFN. The LTC3775 datasheet gives a complete description of Two versions of the demonstration board are available. the part, operation and application information and must DC-1290A-A has been optimized for 5V to 26V input, be read in conjunction with this quick start guide for while the DC1290A-B is optimized for 5V to 36Vin range. demo circuit 1290A Both circuits are designed for minimal size and maxi- Design files for this circuit board are available. Call mum step-down ratio to demonstrate the low on-time the LTC factory. capability of the LTC3775EUD and not for maximum effi- L, LTC are registered trademarks of Linear Technology Corporation. Other product names ciency. may be trademarks of the companies that manufacture the products. PERFORMANCE SUMMARY Specifications are at TA = 25C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Range DC1290A-A 5 26 V IN DC1290A-B 5 36 V Output Voltage Range V = 5-26V, (DC1290A-A) OUT IN 1.164 1.20 1.236 V V = 5-36V (DC1290A-B) IN I Maximum Load Current DC1290A-A 15 A OUT DC1290A-B 10 A f Typical Free Running Switching Frequency DC1290A-A 500 kHz SW DC1290A-B 330 kHz Efficiency DC1290A-A, V = 12V, I = 15A 85.5 % IN OUT See Figures 3 and 4 for efficiency curves DC1290A-B , V = 12V, I = 10A 86.5 % IN OUT QUICK START PROCEDURE See Figure 2 for proper scope probe technique. Demonstration circuit 1290A is easy to set up to evaluate the performance of the LTC3775EUD. Refer to Figure 1 1. Place jumpers in the following positions: for proper measurement equipment setup and follow the FCC JP1 procedure below: JP2 ON 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 2. With power off, connect the input power supply to input or output voltage ripple by touching the probe tip directly across the Vin and GND. Vin or Vout and GND terminals or directly across relevant capacitor. 1 GND VIN LTC3775EUD 3. Turn on the power at the input. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. NOTE. Make sure that the input voltage does not exceed 26V or 36V for DC1290AA or DC1290AB demonstration boards respectively. 5. Once the proper output voltage is established, adjust 4. Check for the proper output voltages. the loads within the operating range and observe the output voltage regulation, ripple voltage, efficiency V = 1.164V to 1.236V OUT and other parameters. A A I I OUT IN + + + + Vin supply Load V V Vin - - - - Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple Across Terminals or Directly Across Bulk Capacitor FREQUENCY SYNCHRONIZATION AND MODE SELECTION Demonstration circuit 1290As Mode selector allows the position of JP1 accordingly. Please note that the the converter to run in FCC operation, pulse skip opera- external synchronizing frequency should be close to tion or synchronizing to an external clock by changing (+/-20%) the free running frequency. 2