DEMO CIRCUIT 924 LTC3772B QUICK START GUIDE LTC3772B No Rsense Constant Frequency StepDown DC/DC Controller in 3x2 DFN DESCRIPTION Demonstration circuit 924 is a No RSENSE™ constant frequency step-down DC/DC converter featuring the LTC3772B Controller in a 3mm×2mm DFN package. The demo circuit 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 circuit highlights the capabilities of the LTC3772B, which uses current mode architecture to drive an external Pchannel 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 LTC3772B’s internal soft-start reduces the inrush 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. If the output is shorted to ground, the frequency is folded back to prevent current runaway. 550KHz constant frequency operation makes the LTC3772B ideal for low noise applications. The LTC3772B 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 924. 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 ILOAD = 2A, JP1 on 1.8V position ILOAD = 2A, JP1 on 2.5V position ILOAD = 2A, JP1 on 3.3V position IOUT(MAX) Maximum Output Current EFF Efficiency VPP F OUTPUT VOLTAGE RIPPLE Oscillator Frequency 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 VFB = 0.8V MIN 3 3.3 3.9 1.728 2.4 3.168 2 500 TYP 8 MAX 20 8* 8* 1.8 2.5 3.3 1.872 2.6 3.432 UNITS uA V 8* V A 86% 91% 94% 22 550 650 mV KHz *: The maximum VIN is limited by the MOSFET gate voltage. The maximum VIN of LTC3772B is rated at 9.8V. 1 LTC3772B QUICK START PROCEDURE Demonstration circuit 924 is easy to set up to evaluate the performance of the LTC3772B. 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. 4. 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. One of three positions (1.8V, 2.5V or 3.3V) With power off, connect the input power supply to VIN and GND and the load to VOUT and GND. Turn on the power at the input. Note. Make sure that the input voltage does not exceed 8V. Place jumpers in the following positions: JP1 2. 3. Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 2 LTC3772B 3