DEMO CIRCUIT 947 LT3740EDHC QUICK START GUIDE LT3740EDHC Wide Operating Range, Valley Mode, No R Synchronous Step-Down Controller SENSE DESCRIPTION Demonstration circuit 947 is a Wide Operating control of the output voltage during operation, out- Range, Valley Mode, No R Synchronous put voltage tracking or soft-start. SENSE Step-Down Controller featuring the LT3740EDHC. In addition, the LT3740 has three current limit lev- It produces 1.8VDC for loads up to 10A from an els that can be chosen by connecting the RANGE input range of 3-15VDC, switching at a fixed fre- pin to ground, leaving it open circuit, or tying to the quency of 300KHz. input. In order to operate from an input voltage as low as DC947 is equipped with jumpers that allow the 3V, the circuit must provide adequate voltage to user to select among soft-start options and refer- drive the external power MOSFETs. To this end, ence voltage sources. Unpopulated pads have the LT3740 includes an on-chip step-up converter been added to DC947 in order to facilitate input to provide a bias about 7V above the input voltage voltages higher than 15V and No R opera- SENSE to power the top MOSFET gate driver. The stock tion. configuration of DC947 also uses this bias to power the bottom gate drive circuit, but pads have been added to the circuit board to power the bot- Design files for this circuit board are available. tom gate drive from another source. Call the LTC factory. The LT3740 is equipped with an external reference and LT are registered trademarks of Linear Technology Corporation. No R SENSE is a trademark of Linear Technology Corporation. Other product names may be input that allows the user to override the 0.8V in- trademarks of the companies that manufacture the products. ternal reference with any lower value, allowing full PERFORMANCE SUMMARY Specifications are at TA = 25C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Range 3.0 15.0 V IN V Output Voltage Range V = 3.0 15.0V, I = 0-10A 1.76 1.84 V OUT IN LOAD I Output current V = 3.0 15.0V, V = 1.8V 0 10.0 A OUT IN OUT V Output Ripple V = 3.0 15.0V, I = 0-10A 20 mV RIPPLE IN LOAD QUICK START PROCEDURE Demonstration circuit 947 is easy to set up to Refer to Figure 1 for proper measurement equip- evaluate the performance of the LT3740EDHC. ment setup and follow the procedure below: 1 LT3740EDHC NOTE. When measuring the output voltage ripple, care must be 4. Turn on the power at the input. Make sure that taken to avoid a long ground lead on the oscilloscope probe. the input voltage is between 3 and 15V. Measure the input or output voltage ripple by touching the probe tip directly across the Vin or Vout and GND terminals. See Figure 2 for NOTE. Make sure that the input voltage does not exceed 15V. proper scope probe technique. 5. Check for the proper output voltages. Vout should be 1.8V, nominal. If there is no output, 1. Place jumpers in the following positions: temporarily disconnect the load to make sure that the load is not set too high. JP1 Up (Internal Reference) 6. Once the proper output voltages are estab- JP2 Up (Voltage SS ON) lished, adjust the loads within the operating JP3 Down (Current SS ON) range and observe the output voltage regula- tion, ripple voltage, efficiency and other parame- 2. With power off, connect the input power supply ters. to Vin and GND. 3. Connect the 0-10A load. Figure 1. Proper Measurement Equipment Setup 2