Demonstration circuit 2929A is a 42V, 10A (12A peak) synchronous step-down Silent Switcher 2 with spread spectrum frequency modulation featuring the LT8638S. The demo board is designed for 3.3V output from a 3.9V to 42V input. The wide input range allows a variety of input sources, such as automotive batteries and industrial supplies. The LT8638S is a compact, ultralow emission, high efficiency, and high speed synchronous monolithic step-down 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 LT8638S switching frequency can be programmed either via oscillator resistor or external clock over a 200kHz to 3MHz range. The default frequency of demo circuit 2929A is 2MHz.
The SYNC/MODE pin on the demo board DC2929A 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 in the demo manual shows the efficiency of the circuit at 12V input and 24V input in BURST mode. 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 LT8638S temperature rise on DC2929A demo board at 12V input and 24V input in BURST mode, with continuous load current. LT8638S can deliver 12A of peak output current for short periods of time. Figure 3 shows its case temperature rise vs the duty cycle of a 1kHz pulsed 12A load. The LT8638S is in a 5mmx4mm LQFN package with exposed pads for low thermal resistance. The rated maximum continuous load current is 10A, while derating may be necessary for certain input voltage and thermal conditions.
Table 1. Operation Mode Configuration
| JP1 POSITION
|
SYNC (E5) INPUT
|
OPERATION MODE
|
| SPREAD-SPECTRUM |
N/A |
Forced Continuous Mode w/Spread-Spectrum |
| BURST |
N/A |
Low Ripple Burst Mode® |
| FCM/SYNC |
Floating |
Forced Continuous Mode w/o Spread-Spectrum |
| FCM/SYNC |
External Clock |
Forced Continuous Mode w/Synchronization |
The demo board has an EMI filter installed. The EMI performance of the board is shown in Figure 4 in the demo manual. The red lines in EMI Performance are CISPR25 Class 5 limits. The figure shows that the circuit passes the test with a wide margin. To achieve EMI performance as shown in Figure 4, the input EMI filter is required and the input voltage should be applied to 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 LT8638S datasheet gives a complete description of the part, the operation and application information. The datasheet must be read in conjunction with the demo manual for demo circuit 2929A. The layout recommendations for low EMI operation and maximum 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.
