Analog Devices Inc.

Demonstration circuit EVAL-LT8685S-AZ is a 4-rail power supply featuring the LT8685S, a 42V quad monolithic synchronous step-down silent switcher. The demo circuit is designed for 5V, 3.3V, 1.8V, and 1.2V outputs from a nominal 12V input. The 5V and 3.3V rails are powered from the 12V input, while the 1.8V and 1.2V rails are powered from one of the high voltage buck regulators of 5V or 3.3V. The current capability is 2.5A for the channels 1 and 2. The current ratings for the low voltage channels are 4A. The two 42V regulators may be combined to provide up to 5A of output current using a single inductor. Similarly, the two 8V regulators may be combined to provide up to 8A of output current using a single inductor. Individual track/soft-start, current limit, independent enable and power good for each channel simplify the complex design of quad-output power converters. All regulators can be synchronized to a common external clock input or internal oscillator that can be programmed with a single resistor for switching frequency from 350kHz to 3MHz, allowing for optimal system design per specific applications. The Performance Summary table in the demo manual summarizes the performance of the demo board at room temperature. The circuit can be easily modified for different applications. The LT8685S data sheet gives a complete description of the part, operation and application information. The data sheet must be read in conjunction with the quick start guide for demo circuit EVAL-LT8685S-AZ.

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