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Demonstration Circuit 2745A features the LTC3309A 5V, 6A synchronous step-down silent switcher operating as a 2MHz 3.3V to 1.2V 6A buck regulator. The LTC3309A supports adjustable output voltages from 0.5V to VIN, operating frequencies from 1MHz up to 3MHz. The LTC3309A is a compact, ultralow emission, high efficiency, and high speed synchronous monolithic step-down switching regulator. Fast minimum on-time of 22ns enables high VIN to low VOUT conversion at high frequency.

DC2745A operating mode may be selected as Burst Mode® operation, skip or forced continuous (FC) mode. Setting JP1 to the FC/SYNC position will allow the LTC3309A to sync to a clock frequency from 1MHz to 3MHz. The LTC3309A operates in forced continuous mode when syncing to an external clock. RT is connected to VIN through a 0Ω resistor, R9, which sets the operating frequency to 2MHz. The frequency can be easily changed by removing R9 and setting an appropriate resistor in the R4 location to obtain the desired frequency. Refer to the LTC3309A data sheet for the proper RT value.

The DC2745A also has an EMI filter to reduce conducted EMI. This EMI filter can be included by applying the input voltage at the VIN EMI terminal. The EMI performance of the board is shown in the EMI Test Results section. The red lines in the EMI performance graphs illustrate the CISPR25 Class 5 peak limits for the conducted and radiated emission tests.

The LTC3309A data sheet gives a complete description of the part, operation and application information. The data sheet must be read in conjunction with this demo. The LTC3309A is assembled in a 3mm × 3mm LQFN package with exposed pads for low thermal  resistance. The layout recommendations for low EMI operation and maximum thermal performance are available in the data sheet section Low EMI PCB Layout.

The Efficiency vs Load graph shows the efficiency and the power loss of the circuit with a 3.3V input in Burst Mode operation. The LTC3309A is available in a 12-lead 2mm × 2mm LGA package.

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