QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 836 NO RSENSE CONSTANT FREQUENCY STEP-DOWN DC/DC CONTROLLER IN 3X2 DFN LTC3772 DESCRIPTION Demonstration circuit 836 is a No RSENSE™ constant frequency step-down DC/DC converter featuring the LTC3772 Controller in a 3mm×2mm DFN package. The demo board generates 2A output current at one of the three output voltages: 1.8V, 2.5V or 3.3V. The output voltage is selectable by JP1. Higher output current is made possible using optional components. The exclusive use of surface mount components results in a highly efficient application in a very small board space. The demo board highlights the capabilities of the LTC3772, which uses current mode architecture to drive an external P-channel power MOSFET. This results in fast loop dynamics and transient response. The ON resistance of the MOSFET is used to sense the switch current. No additional sense resistor is needed. The LTC3772’s internal soft-start reduces the maximum input current during start up. To maximize the runtime from a battery source, the MOSFET is turned on continuously in dropout (100% duty cycle). The peak current sense voltage can be configured into three different voltage thresholds for different loads. The LTC3772 enters Burst Mode™ at light loads, enhancing the efficiency in this mode. If the output is shorted to ground, the frequency is folded back to prevent current runaway. High efficiency over wide load conditions and compact size make the LTC3772 ideal for portable applications. The LTC3772 datasheet gives a complete description of the part, operation and application information. The datasheet must be read in conjunction with this quick start guide for demo circuit 836. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. PERFORMANCE SUMMARY Specifications are at TA = 25°C SYMBOL IQ VIN PARAMETER Input DC Current in Shutdown Mode Input Voltage CONDITIONS VITH/RUN = 0V, VIN = 4.2V VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V VOUT Output Voltage IOUT EFF Output Current Efficiency VPP OUTPUT VOLTAGE RIPPLE ILOAD = 2A, JP1 on 1.8V position ILOAD = 2A, JP1 on 2.5V position ILOAD = 2A, JP1 on 3.3V position VIN = 3V, VOUT = 1.8V, IOUT = 2A VIN = 3V, VOUT = 2.5V, IOUT = 2A VIN = 4V, VOUT = 3.3V, IOUT = 2A VIN = 8V, VOUT = 3.3V, IOUT = 2A MIN 2.75 3 3.5 1.728 2.4 3.168 TYP 8 MAX 20 8* 8* UNITS uA V 8* 1.8 2.5 3.3 1.872 2.6 3.432 2 86% 92% 94% 22 V A mV *: The maximum VIN is limited by the MOSFET gate voltage. The maximum VIN of LTC3772 is rated at 9.8V. 1 LTC3772 QUICK START PROCEDURE Demonstration circuit 836 is easy to set up to evaluate the performance of the LTC3772. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 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 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. 1. NOTE: Make ceed 8V. 4. 2. With power off, connect the input power supply to VIN and GND and the load to VOUT and GND. 3. Turn on the power at the input. Check for the proper output voltages per the JP1 setup. NOTE: If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 5. Once the proper output voltages are established, adjust the loads within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. 6. To evaluate output current higher than 2A, the footprints on the bottom side of the board can be used. Refer to the datasheet for more application examples. Place jumpers in the following positions: One of three positions (1.8V, 2.5V or JP1 3.3V) sure that the input voltage does not ex- Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 2 LTC3772 3