Demonstration circuit 2918A is a 42V, 5A synchronous step-down SILENT SWITCHER with spread spectrum frequency modulation featuring the LT8636. The demo board is designed for 5V output from a 5.8V to 42V input. The wide input range allows a variety of input sources, such as automotive batteries and industrial supplies. The LT8636 is a compact, low emission, high efficiency, and high frequency synchronous monolithic step-down switching regulator. The proprietary silent switcher architecture minimizes electromagnetic emissions with simplified filter and reduced layout sensitivity. Selectable spread spectrum mode further improves EMI performance, making it perfect solution to the noise sensitive applications. The regulator’s ultralow 2.5μA quiescent current with the output in full regulation enables applications requiring highest efficiency at very light load currents, such as automotive and battery powered portable instruments.
Peak current mode control with minimum on-time of as small as 30ns allows high step-down conversion even at high frequency. The LT8636 switching frequency can be programmed either via oscillator resistor or external clock over a 200kHz to 3MHz range. The default frequency of demo circuit 2918A is 2MHz.
The SYNC/MODE pin on the demo board 2918A is grounded (JP1 at BURST position) by default for low ripple Burst Mode® operation. To synchronize to an external clock, move the Jump JP1 to SYNC/FCM and apply the external clock to the SYNC terminal on the 2918A. In sync mode, the part runs in forced continuous mode. Without external clock applied, the SYNC/MODE pin is floating, and the part runs in forced continuous mode. This mode offers fast transient response and full frequency operation over a wide load range. Alternatively, move the Jump JP1 to the SPREAD-SPECTRUM, and the SYNC/MODE is tied to INTVCC, the part runs in forced continuous mode with spread spectrum function enabled.
To get accurate efficiency measurement, measure the input voltage at the VIN_SENSE and GND terminals, which are Kelvin connected to the input cap C2 through signal traces.
The LT8636 is assembled in a 4mmx3mm LQFN package with exposed pads for low thermal resistance. The rated maximum load current is 5A, while derating is necessary for certain input voltage and thermal conditions.
The demo board has an EMI filter installed. The red line in Radiated EMI Performance is CISPR25 Class 5 peak limit. The circuit passes the test with a wide margin. To achieve EMI/EMC performance, the input EMI filter is required and the input voltage should be applied at VEMI terminal, and the setup can be referred to the CISPR 25 standards.
The LT8636 datasheet gives a complete description of the part, operation and application information. The datasheet must be read in conjunction with this demo manual for demo circuit 2918A. The layout recommendations for low EMI operation and best thermal performance are available in the datasheet section Low EMI PCB Layout and Thermal Considerations and Peak Output Current. Contact ADI applications engineer for support.
