Evaluation circuit EVAL-LT8641A-AZ is a 65V, 3.5A/5A peak synchronous step-down Silent Switcher® with spread spectrum frequency modulation featuring the LT8641A. The demo board is designed for 5V output from a 5.5V to 65V input. The wide input range allows a variety of input sources, such as automotive batteries and industrial supplies. The LT8641A is a compact, ultralow emission, high efficiency, high speed synchronous monolithic step-down switching regulator. Top and bottom power switches, compensation components, and other necessary circuits inside the LT8641A minimize external components and simplify design. The special Silent Switcher architecture minimizes EMI emissions. The selectable spread spectrum mode can further improve EMI performance. Ultralow 2.5μA quiescent current in Burst Mode® operation achieves high efficiency at very light loads. Fast minimum on-time of 35ns enables high VIN to low VOUT conversion at high frequency.
The LT8641A switching frequency can be programmed either through oscillator resistor or external clock over a 200kHz to 3MHz range. The default frequency of the evaluation board is 2MHz. The SYNC pin on the evaluation board is grounded by JP1 for low ripple burst mode operation. Move JP1 to PULSE SKIPPING position
to change the operation mode to pulse-skipping operation. Once JP1 is on SPREAD SPECTRUM position, VCC is applied to the SYNC pin for low EMI spread spectrum operation. To synchronize to an external clock, move JP1 to SYNC and apply the external clock to the SYNC turret. Figure 1 in the user guide shows the efficiency of the circuit at 12V and 24V input in Burst Mode operation.
Figure 2 in the user guide shows EMI performances of the board (with EMIfilter). The red lines in Figure 2 are CISPR25 Class5 peak limit. The figure shows that the circuit passes the test with a wide margin. To achieve EMI/EMC performance, as shown in Figure 2, the input EMI filter is required, and the input voltage should be applied at the VEMI turret.
The LT8641A data sheet gives a complete description of the part, operation, and application information. Read the data sheet in conjunction with the user guide for EVAL-LT8641A-AZ. The LT8641A is assembled in a 3mm × 4mm plastic QFN package with exposed pad for low thermal resistance. The layout recommendations for low EMI operation and maximum thermal performance are available in the “Low EMI PCB Layout and Thermal Considerations” and “Peak Output Current” sections of the data sheet.
