Demonstration circuit 3126A is a quad power supply featuring the LT8686S. The LT8686S is a 42V quad channel current mode monolithic synchronous step-down regulator. The LT8686S combines two 42V capable 2A buck regulators with two 8V capable 2A buck regulators. The demo board is designed for 5V, 3.3V, 1.8V, and 1.2V outputs from a nominal 12V input with switching frequency set at 2MHz. The 5V and 3.3V converters are powered from a wide range of 6V to 42V. The 1.8V and 1.2V converters are powered from the 5V output by default or from 3V to 8V supply alternatively. The current capability is 2A for all 4 outputs when running individually.
The DC3126A provides two 42V regulators that can be combined to deliver up to 4A of output current using a singular inductor. Similarly, the two 8V regulators can be combined to deliver up to 4A of output current using a singular inductor. The allowed channel combinations are given in the data sheet.
The independent track/soft-start and power good for each output simplify the complex design of quad-output power converters. Each output can be independently disabled into low quiescent current shutdown mode with its own TRK/SS pin.
A user-selectable SYNC/MODE pin on the demo board provides two primary modes of operation: pulse-skipping mode and low ripple Burst Mode® operation, plus the option of selecting frequency spread-spectrum for each to improve the EMI/EMC performance. Burst Mode delivers higher efficiency at light load than pulse-skipping mode. In pulse-skipping mode, full switching frequency is maintained to lower load currents than Burst Mode. The SYNC/MODE pin can also be used to synchronize the switching frequency to an external clock. The switching frequency for all regulators can be programmed either via an oscillator resistor or an external clock over a 350kHz to 3MHz range. At all frequencies, a 180° phase shift is maintained between channel 1 and channel 2, channel 3 and channel 4, reducing the input peak current and voltage ripple.
The demo board has an EMI filter installed on the bottom layer. The conducted and radiated EMI performance of the board is shown on Figure 4 in the user guide. The red line in Figure 4 is CISPR25 Class 5 peak limit. The figure shows that the circuit passes the test with a wide margin. To achieve EMI/EMC performance as shown in Figure 4, the input EMI filter is required, and the input voltage should be applied at VEMI turret.
The LT8686S 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 DC3126A. The LT8686S is assembled in a 5mm × 5mm LQFN package with exposed pads for low thermal resistance. Proper board layout is essential for both low EMI operation and best thermal performance.
