QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 463 STEP-DOWN REGULATOR WITH LDO LTC3700 DESCRIPTION Demonstration circuit 463 is a step-down (buck) regulator and a Low Drop Out (LDO) regulator using the LTC3700 in the 10-lead MSOP package. The buck regulator is capable of providing 1A at an output voltage of 3.3V from an input supply of 5V to 9V. The LDO can be powered from either the buck regulator output or a separated DC input, and produces an output up to 2.5V at 150mA. The combination of a buck regulator and a LDO, and the use of surface mount components result in a highly efficient application circuit in a very small board space. The demo circuit highlights the capabilities of the LTC3700. A constant operating frequency of 550kHz makes the LTC3700 attractive for noise-sensitive applications while allowing for smaller external components such as output filter inductor and capacitor. In addition, high efficiency operation over a wide load current range makes the LTC3700 ideal for battery-powered applications. Design files for this circuit board are available. Call the LTC factory. Table 1. Performance Summary PARAMETER CONDITION VALUE Input Voltage Range of Buck regulator 5V to 9V Input Voltage Range of LDO 3.3V to 5V VOUT1 VIN = 5V to 9V, IOUT1 = 0A to 1A 3.3V 3% VOUT2 VIN = 3.3V to 5V, IOUT1 = 0A to 150mA 2.5V 4% Typical Output Ripple VOUT1 VIN = 5V, IOUT1 = 1A 30mVP-P Typical Output Ripple VOUT2 VIN = 3.3V, IOUT2 = 150mA 5mVP-P Nominal Switching Frequency Buck Regulator 550kHz QUICK START PROCEDURE Demonstration circuit 463 is easy to set up to evaluate the performance of the LTC3700. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 2. To operate the LDO circuit with a separate, second NOTE: When measuring the input or output voltage rip- 3. Connect the loads. ple, 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 input or output and ground terminals. See Figure 2 for proper scope probe technique. input, set JP1 to the Vin2 position. Connect a second 3.3V to 5V power source to the Vin2 and GND terminals. 4. For single source operation, set the input source se- lector Jumper JP1 to Vout1 position. Remove the connection of power source Vin2. The LDO will be powered from Vout1. 1. Connect a 5V to 9V power source to the Vin1 and GND terminals. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 463 STEP-DOWN REGULATOR WITH LDO - + + - - + - LOAD - -+ + + + + - + - LOAD - - - Figure 1. Proper Measurement Equipment Setup 20mV/DIV GND + + VIN - + Figure 2. Measuring Input or Output Ripple 10mV/DIV 1s/DIV 1s/DIV Figure 3. 3.3V Buck Output Ripple (VIN = 5V, IOUT = 1A) Figure 4. 2.5V LDO Output Ripple (VIN = 3.3V, IOUT = 150mA) 2 A B C D E 4 4 1 3 2 3 4 5 VIN2 ITH/RUN LDO VFB VFB2 PGOOD GND SENSEVIN PGATE 10 3 9 8 7 6 2 2 1 1 A B C D E