DC1293A Analog Devices

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1293A MONOLITHIC STEP-DOWN SWITCHING REGULATOR WITH LDO LT1939 Demo circuit 1293A is a monolithic step-down switch- nates input current surge during startup, and also pro- ing regulator with LDO featuring LT1939. The demo vides output tracking. circuit is designed for 5.0V and 3.3V outputs from a 7V The LT1939s low current shutdown mode (<12uA) to 25V input. The LDO output is configured as a post- enables easy power management in battery-powered regulator of the switching regulator output. The total systems. current capability is up to 2A. The LT1939 datasheet gives a complete description of The switching regulator can be synchronized to an ex- the part, operation and application information. The ternal clock input or be resistor-programmed to a datasheet must be read in conjunction with this quick 250kHz to 2.2MHz internal oscillator. Programmable start guide for demo circuit 1293A. frequency allows for optimization between efficiency Design files for this circuit board are available. Call and external component size. Cycle-by-cycle current the LTC factory. limit, frequency foldback and thermal shutdown provide protections against a shorted output. The soft-start fea- , LTC and LT are registered trademarks of Linear Technology Corporation. ture controls the ramp rate of the output voltage, elimi- o Performance Summary ( T = 25 C ) A PARAMETER CONDITION VALUE Minimum Input Voltage 7V Maximum Input Voltage 25V Output Voltage V 5.0V 4% OUT1 Output Voltage V 3.3V 4% OUT2 Switching Frequency 800kHz Maximum Total Output Current (I +I ) 2.0A OUT1 OUT2 Voltage Ripple V V =12V, I =1.5A 10mV OUT1 IN OUT1 Voltage Ripple V V =12V, I =0.5A 5mV OUT2 IN OUT2 1111 GND VIN 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 3. Turn on the power at the input. performance of the LT1939. Refer to Figure 1 for NOTE. Make sure that the input voltage does not exceed 25V. proper measurement equipment setup and follow the 4. Check for the proper output voltages. procedure below: NOTE. If there is no output, temporarily disconnect the load to make NOTE. When measuring the input or output voltage ripple, care must be sure that the load is not set too high. 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 5. Once the proper output voltages are established, ad- V or V and GND terminals. See Figure 2 for proper scope probe IN OUT just the load within the operating range and observe technique. the output voltage regulation, ripple voltage, effi- 1. Place JP1 on the RUN position. ciency and other parameters. 2. With power off, connect the input power supply to V and GND. IN Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple 2222