Demonstration Circuit 2841A is a 42V, 15A synchronous step-down Silent Switcher 2 with spread spectrum frequency modulation featuring the LT8648S. The demo board is designed for 5V output from a 5.7V to 42V input. The wide input range allows a variety of input sources, such as automotive batteries and industrial supplies. The LT8648S is a compact, ultralow emission, high efficiency, and high speed synchronous monolithic stepdown switching regulator. The integrated bypass capacitors optimize the fast current loops and make it easier to minimize EMI emissions by reducing layout sensitivity. Selectable spread spectrum mode further improves EMI performance, making it perfect solution to the noise sensitive applications.
Peak current mode control with minimum on-time of as small as 25ns allows high step-down conversion even at high frequency. The LT8648S switching frequency can be programmed either via oscillator resistor or external clock over a 200kHz to 2.2MHz range. The default frequency of demo circuit 2841A is 400kHz.
The SYNC/MODE pin on the demo board DC2841A is grounded (JP1 at BURST position) by default for low ripple Burst Mode® operation. It can be configured into different operation modes through JP1 and SYNC terminal (Table 1).
Figure 1 shows the efficiency of the circuit at 12V input and 24V input in force continuous mode without spread spectrum. To get accurate efficiency measurement, measure the input voltage at the VIN SENSE terminal and measure the output voltage at the VO SENSE terminal.
Figure 2 shows the LT8648S temperature rising on DC2841A demo board under different load conditions. The LT8648S is assembled in a 7mm × 4mm LQFN package with exposed pads for low thermal resistance. The rated maximum load current is 15A, while derating is necessary for certain input voltage and thermal conditions.
The demo board has an EMI filter installed. The EMI performance of the board is shown on Figure 3. The red line in Radiated EMI Performance is CISPR25 Class 5 peak limit. The figure shows that the circuit passes the test with a wide margin. To achieve EMI performance as shown in Figure 3, the input EMI filter is required and the input voltage should be applied at VIN_EMI terminal, and the test setup can be referred to the CISPR25 standards. If the input is applied to VIN terminal, the EMI filter is bypassed.
The LT8648S data sheet gives a complete description of the part, operation and application information. The data sheet must be read in conjunction with this demo manual for DC2841A. The layout recommendations for low EMI operation and maximum thermal performance are available in the data sheet section Low EMI PCB Layout and Thermal Considerations and Peak Output Current. Contact ADI applications engineer for support.
