QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC645 LTC3408EDD, WCDMA CELLULAR PHONE BUCK REGULATOR LTC3408EDD DESCRIPTION Demonstration circuit DC645 is a constant-frequency The LTC3408EDD is available in a small 8-Pin DFN pack- step-down converter, using the LTC3408EDD monolithic age and has an operating frequency of 1.5MHz (allowing synchronous buck regulator. It is specifically designed the exclusive use of low profile surface mount compo- for WCDMA power amplifier applications. The output nents). voltage can be dynamically controlled from 0.3V to 3.5V. These and other features make the demonstration circuit An 80m internal bypass P-channel MOSFET connects ideal for single-cell Li-ion-powered hand-held applica- the VOUT to VIN reducing the power loss through the tions. buck regulator. For an input voltage range of 2.7V to 5.5V, DC645 is capable of delivering up to 600mA of output current. The efficiency of the DC645 is very high: Design files for this circuit board are available. Call up to 96%. the LTC factory. PowerPath is a trademark of Linear Technology Corporation QUICK START PROCEDURE Demonstration circuit DC645 is easy to set up to evalu- 5. Turn on the VREF power supply. ate the performance of the LTC3408EDD. Refer to NOTE: Make sure that the input voltage VREF does not Figure 1 for proper measurement equipment setup and exceed VIN follow the procedure below: 6. For VREF 1.2V, Check for the output voltages VOUT NOTE: When measuring the input or output voltage rip- = 3 VREF ple, care must be taken to avoid a long ground lead on 7. For VREF>1.2V, VOUT is directly connected to VIN. the oscilloscope probe. Measure the input or output VOUT VIN. voltage ripple by touching the probe tip directly across the VIN or VOUT and GND terminals. See Figure 2 for If there is no output, temporarily disconnect the load proper scope probe technique. to make sure that the load is not set too high. 1. Place jumper JP1 in the ON position. 8. Once the proper output voltages are established, ad- just the loads within 600mA range and observe the 2. With power off, connect the input power supply VIN output voltage regulation, ripple voltage, efficiency to VIN and GND. and other parameters. 3. Connect input power supply VREF to REF and GND. 4. Turn on the power at VIN. NOTE: Make sure that the input voltage VIN does not exceed 5.5V. 1 GND VIN QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC645 LTC3408EDD, WCDMA CELLULAR PHONE BUCK REGULATOR Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple CH2:100mV/DIV 20mV/DIV CH4:500mA/DIV 40s/DIV 200ns/DIV Figure 3. VOUT (100mV / Div) Transient Response Figure 4. Typical Output Ripple for 0 to 600mA load Step. (V = 3.3V, V = 1.5V, I = 600mA) IN OUT OUT (V = 3.3V, V = 1.2V) IN OUT 2