DC1221B LT3502 QUICK START GUIDE LT3502 1.1MHz, 500mA, Step-down Regulator in 2mm2mm DFN DESCRIPTION Demo circuit DC1221B is a monolithic step-down DC/DC current limit and frequency foldback provide the protec- switching regulator featuring the LT3502. The demo cir- tion against shorted outputs. DA pin can further protect cuit is designed for 3.3V output from a 4.5V to 40V in- the silicon under fault conditions by sensing the diode put. The wide input range of the LT3502 allows a variety current. of input sources including automotive batteries and 24V The LT3502 datasheet gives a complete description of industrial supplies. The high switching frequency allows the part, operation and application information. The data- the use of tiny, low cost external components. The cur- sheet must be read in conjunction with this quick start rent mode control scheme creates fast transient re- guide for demo circuit DC1221B. sponse and good loop stability. The boost diode is inter- Design files for this circuit board are available. Call nal to reduce component count. The SHDN pin can also the LTC factory. be used to program soft start. In this mode, the SHDN pin is driven through an external RC filter to create a , LTC and LT are registered trademarks of Linear Technology Corporation. voltage ramp on this pin. The soft start function reduces the input current surge during start-up. Cycle by cycle o Performance Summary (T = 25 C) A PARAMETER CONDITION VALUE Minimum Input Voltage 4.5V Maximum Input Voltage 40V Output Voltage V 3.328V 2.8% OUT Switching Frequency 1.1MHz 18% Maximum Output Current 500mA Efficiency V =12V, I =500mA 80% IN out Voltage Ripple V =12V, I =500mA 10mV IN out QUICK START PROCEDURE Demonstration circuit 1221B is easy to set up to evaluate 2. With power off, connect the input power supply to Vin the performance of the LT3502. Refer to Figure 1 for and GND. proper measurement equipment setup and follow the 3. Turn on the power at the input. procedure below: NOTE. Make sure that the input voltage does not exceed 40V. 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 4. Check for the proper output voltages. input or output voltage ripple by touching the probe tip directly across the NOTE. If there is no output, temporarily disconnect the load to make Vin or Vout and GND terminals. See Figure 2 for proper scope probe sure that the load is not set too high. technique. 5. Once the proper output voltage is established, adjust 1. Place JP1 on the ON position. the load within the operating range and observe the 1 GND VIN LT3502 output voltage regulation, ripple voltage, efficiency and other parameters. Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple 2