DEMO CIRCUIT 957 LTC3785 QUICK START GUIDE LTC3785 3.3V, 3.0A Synchronous Buck-Boost Controller DESCRIPTION Demonstration circuit 957 is a wide input range, LTC3785 provides current limit and shutdown dis- 3.3V, 3.0A Synchronous Buck-Boost Converter connect. It offers Burst Mode operation for high featuring the LTC3785. efficiency at light loads. This circuit was designed to demonstrate the high levels of performance, efficiency, and small solu- tion size attainable using this part in a buck-boost Design files for this circuit board are available. power supply. It operates at 500kHz and produces Call the LTC factory. a regulated 3.3V, 3.0A output from an input voltage , LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are range of 2.7 to 10V. It is suitable for portable appli- registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, Module, Micropower cations, and has a footprint area that is 0.36 SwitcherCAD, Multimode Dimming, No Latency , No Latency Delta-Sigma, No square inches. R , Operational Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SENSE SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trade- marks of the companies that manufacture the products. Synchronous rectification helps to attain efficiency up to 96%, depending on line and load. The PERFORMANCE SUMMARY Specifications are at TA = 25C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Range 2.7 10 V IN V Output Voltage 3.3 V OUT I Output Current Range V = 2.7 10V 0 3.0 A OUT IN F Switching (Clock) Frequency 500 kHz SW V Output Ripple V = 4.2V, I = 3.0A (20MHz BW) 20 mV OUT P-P IN OUT PP I Output Regulation Line and Load (2.7-10V, 0-3.0A) 0.36 % REG P /P Efficiency (see Figure 2) V =4.2V, I = 3.0A 92.5 % OUT IN IN OUT OPERATING PRINCIPLES The LTC3785 Synchronous Buck-Boost Controller voltage range. It seamlessly transitions from buck, is employed to produce a 3.3V output from an in- to buck-boost, to boost modes of operation to put ranging from 2.7V to 10V. maintain the output as the input voltage goes from maximum to minimum values. The converter utilizes a proprietary 4-switch topol- ogy to provide an output that is within the input 1 L TC3785 QUICK START PROCEDURE NOTE. Make sure that the input voltage never exceeds 10V. Demonstration circuit 957 is easy to set up to evaluate the performance of the LTC3785. Refer to 4. Check for the proper output voltage of 3.3V. Figure 1 for proper measurement equipment setup Turn off the power at the input. and follow the procedure below: 5. Once the proper output voltages are estab- NOTE. When measuring the output voltage ripple, care must be lished, connect a variable load capable of sink- taken to avoid a long ground lead on the oscilloscope probe. ing 3.0A at 3.3V to the output terminals +Vout Measure the output voltage ripple by touching the probe tip and ground ring directly across the last output capacitor (C15) as shown and Vout. Set the current for 0A. in Figure 1. a. If efficiency measurements are desired, an 1. Set an input power supply that is capable of ammeter or a resistor shunt that is capable 2.7V to 10V to 4.2V. Turn off the supply. of handling 3.0Adc can be put in series with the output load in order to measure the 2. With power off, connect the supply to the input DC957s output current. terminals +Vin and Vin. b. A voltmeter with a capability of measuring at a. Input voltages lower than 2.7V can keep the least 3.3V can be placed across the output converter from turning on due to the under- terminals in order to get an accurate output voltage lockout feature of the LTC3785. voltage measurement. b. If efficiency measurements are desired, an 6. Turn on the power at the input. ammeter capable of measuring 5Adc or a re- sistor shunt can be put in series with the in- NOTE. If there is no output, temporarily disconnect the load to put supply in order to measure the DC957s make sure that the load is not set too high. input current. 7. Once the proper output voltage is again estab- c. A voltmeter with a capability of measuring at lished, adjust the load within the operating range least 10V can be placed across the input and observe the output voltage regulation, ripple terminals in order to get an accurate input voltage, efficiency and other desired parame- voltage measurement. ters. 3. Turn on the power at the input. 2