DC1023A LT3505 QUICK START GUIDE LT3505 1.2A, Step-down Switching Regulator in 3mm×3mm DFN DESCRIPTION Demo circuit DC1023A is a monolithic step-dow n DC/DC sw itching regulator featuring the LT3505. The demo circuit is designed for 5V output from a 6V to 36V input. The w ide input range of the LT3505 allow s a variety of input sources. The typical sources are automotive batteries, w all adaptors and industrial supplies. The oscillator can be programmed for high frequency operation allow ing the use of tiny, low cost external components or it can be programmed for low er frequency operation to maximize efficiency. The current mode control scheme creates fast transient response and good loop stability. The gate drive of the internal sw itch is boosted to a voltage that is higher than Vin to ensure saturation of the sw itch. A charge pump consisting of a diode and capacitor on the demo board performs the boost function. The SH DN pin can be used to set the part in shutdow n mode, reducing the supply current to less than 2uA. The SH DN pin can also be used to program soft start. In this mode, the SH DN pin is driven through an external R C filter to create a voltage ramp on this pin. The soft start function reduces the input current surge during start-up. Cycle by cycle current limit, frequency foldback and thermal shutdow n provide the protection against shorted outputs. The LT3505 datasheet gives a complete description of the part,operation and application information. The datasheet must be read in conjunction w ith this quick start guide for demo circuit DC1023A. Design files for this circuit board are available. Call the LTC factory. ,LTC and LT are registered trademarks ofLinear Technology Corporation. Performance Summary ( TA = 25oC ) PAR AM ETER Minimum Input Voltage Maximum Input Voltage O utput Voltage VOUT Sw itching Frequency Sw itching Frequency Maximum O utput Current Efficiency Efficiency Voltage R ipple Voltage R ipple CO N DITIO N JP2 in 2.2MH z position JP2 in 750kH z position VIN =8~32V VIN =12V,Iout=1A,Freq=750kH z VIN =12V,Iout=1A,Freq=2.2MH z VIN =12V,Iout=1A,Freq=2.2MH z VIN =12V,Iout=1A,Freq=750kH z VALU E 6V 36V 5.05V ±4% 2.2MH z ±10% 750kH z ±10% 1.0A 85.0% 83.3% 5mV 20mV 1 LT3505 QUICK START PROCEDURE Demonstration circuit 1023 is easy to set up to evaluate the performance of the LT3505. R efer to Figure 1 for proper measurement equipment setup and follow the procedure below : NOTE. W hen 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. 3. W ith pow er off,connect the input pow er supply to Vin and G N D. 4. Turn on the pow er at the input. NOTE. Make sure that the input voltage does not exceed 36V. 5. NOTE. Ifthere is no output,temporarily disconnect the load to make sure that the load is not set too high. 6. 1. Place JP1 on the R U N position. 2. Place JP2 on the selected frequency Check for the proper output voltages. O nce the proper output voltage is established, adjust the load w ithin the operating range and observe the output voltage regulation,ripple voltage,efficiency and other parameters. Figure 1. Proper M easurem ent Equipm ent Setup GND VIN Figure 2. M easuring Input or Output Ripple 2 LT3505 INPUT VOLTAGE FREQUENCY FOLDBACK W hen the FR EQ jumper (JP2) is selected as 750K, the sw itching frequency is fixed at 750kH z w ithin 6V to 36V input voltage range. W hen FR EQ jumper (JP2) is selected as 2.2M, the sw itching frequency is 2.2MH z w ithin 6V to 16V input voltage range. The sw itching frequency w ill be linearly decreased w hen the input voltage increases. The input voltage frequency foldback curve is show n in Figure 3. 2.5 Freq (MHz) 2 1.5 1 0.5 0 0 10 20 Vin (V) 30 40 Figure 3. Input Voltage Frequency Foldback OUTPUT CAPABILITY VO U T =5V Vin (V) 6 8 32 36 750K 0.7 1 1 1 Io max (A) 2.2M 0.05 1 1 0.4 3 LT3505 4