QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1293A MONOLITHIC STEP-DOWN SWITCHING REGULATOR WITH LDO LT1939 Demo circuit 1293A is a monolithic step-down switching regulator with LDO featuring LT1939. The demo circuit is designed for 5.0V and 3.3V outputs from a 7V to 25V input. The LDO output is configured as a postregulator of the switching regulator output. The total current capability is up to 2A. The switching regulator can be synchronized to an external clock input or be resistor-programmed to a 250kHz to 2.2MHz internal oscillator. Programmable frequency allows for optimization between efficiency and external component size. Cycle-by-cycle current limit, frequency foldback and thermal shutdown provide protections against a shorted output. The soft-start feature controls the ramp rate of the output voltage, elimi- Performance Summary ( TA = 25oC ) PARAMETER Minimum Input Voltage Maximum Input Voltage Output Voltage VOUT1 Output Voltage VOUT2 Switching Frequency Maximum Total Output Current (I OUT1 +I OUT2) Voltage Ripple VOUT1 Voltage Ripple VOUT2 nates input current surge during startup, and also provides output tracking. The LT1939’s low current shutdown mode (<12uA) enables easy power management in battery-powered systems. The LT1939 datasheet gives a complete description of the part, operation and application information. The datasheet must be read in conjunction with this quick start guide for demo circuit 1293A. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. CONDITION VIN=12V, IOUT1=1.5A VIN=12V, IOUT2=0.5A VALUE 7V 25V 5.0V ±4% 3.3V ±4% 800kHz 2.0A 10mV 5mV 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1293A MONOLITHIC STEP-DOWN SWITCHING REGULATOR WITH LDO Demo circuit 1293A is easy to set up to evaluate the performance of the LT1939. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 3. Turn on the power at the input. NOTE. Make sure that the input voltage does not exceed 25V. 4. Check for the proper output voltages. NOTE. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 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. See Figure 2 for proper scope probe technique. 5. Once the proper output voltages are established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. 1. Place JP1 on the RUN position. 2. With power off, connect the input power supply to VIN and GND. Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1293A MONOLITHIC STEP-DOWN SWITCHING REGULATOR WITH LDO Install a 20K resistor at R16 to enable SYNC function. Install both R1 and R3 for the UVLO function, otherwise, let SHDN pin float for normal operation. See datasheet “Shutdown and Undervoltage Lockout” for resistor calculation. 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1293A MONOLITHIC STEP-DOWN SWITCHING REGULATOR WITH LDO 4