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Demonstration circuit 2958A is a 42V, 3A micropower synchronous step-down regulator featuring the LT8609A/09A-5/09A-3.3. There are three assembly versions; DC2958A-A is for the LT8609A adjustable output, DC2958A-B is for the LT8609A-5 with a fixed 5V output voltage and DC2958A-C is for the LT8609A-3.3 with a fixed 3.3V output voltage.

The LT8609A/09A-5/09A-3.3 is a compact, high efficiency, high speed synchronous monolithic step-down switching regulator that consumes only 2.5μA of quiescent current. Top and bottom power switches, compensation components and other necessary circuits are inside of the IC to minimize external components and simplify design.

The SYNC pin on the demo board is grounded by default for low ripple Burst Mode operation. Moving JP1 to PULSE SKIPPING position changes the operation mode to pulse-skipping operation. Once JP1 is placed on SPREAD SPECTRUM position, VCC is applied to the SYNC pin for low EMI spread spectrum operation. To synchronous to an external clock, move JP1 to SYNC and apply the external clock to the SYNC turret.

The demo board is designed for 5V or 3.3V output with up to 42V input. The wide input range allows a variety of input sources, such as automotive batteries and industrial supplies. The minimum input voltage can be as low as 3V for drop out mode operation. Under this mode, the output voltage is below the regulation point and follow the input voltage. To maintain the output under regulation, input voltage should be above certain value. For DC2958A-A (adjustable) and DC2958A-B (5V), this value is 5.5V while 3.8V for DC2958A-C (3.3V). The rated maximum load current is 3A, while derating is necessary for certain input voltage and thermal conditions. Figure 1 shows the efficiency of the circuit. Figure 2 shows the IC top temperature rising on DC2958A demo board under different load conditions.

The demo board has an EMI filter installed. The radiated EMI performances of the board (with EMI filter) are shown in Figure 3 and Figure 4. The red lines in Figure 3 and Figure 4 are CISPR25 Class 5 peak limit. To use the EMI filter, the input should be tied to VEMI, not VIN. An inductor L2, which is shorted on the board by default, can be added in the EMI filter to further reduce the conducted emission.

The LT8609A/09A-5/09A-3.3 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 DC2958A.


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