The circuit in Figure 6-1 serves as both a block diagram of the LM5574 and a typical application board schematic for the LM5574. In a buck regulator, there are two loops where currents are switched very fast. The first loop starts from the input capacitors, to the regulator VIN pin, to the regulator SW pin, to the inductor, and then out to the load. The second loop starts from the output capacitor ground, to the regulator PGND pins, to the regulator IS pins, to the diode anode, to the inductor, and then out to the load. Minimize the loop area of these two loops to reduce the stray inductance and minimize the noise and possible erratic operation. TI recommends a ground plane in the printed-circuit board (PCB) as a means to connect the input filter capacitors to the output filter capacitors and the PGND pins of the regulator. Connect all of the low power ground connections (C_SS, R_T, C_RAMP) directly to the regulator AGND pin. Connect the AGND and PGND pins together through the topside copper trace. Place several vias in this trace to the ground plane.

The two highest power dissipating components are the re-circulating diode and the LM5574 regulator IC. The easiest method to determine the power dissipated within the LM5574 is to measure the total conversion losses (Pin – Pout) then subtract the power losses in the Schottky diode, output inductor and snubber resistor. Use Equation 18 to calculate an approximation for the Schottky diode loss.

Use Equation 19 to calculate an approximation for the output inductor power is

where

• R is the DC resistance of the inductor
• and the 1.1 factor is an approximation for the AC losses.

If a snubber is used, use Equation 20 to calculate an approximation for the damping resistor power dissipation.

where

• Fsw is the switching frequency
• and Csnub is the snubber capacitor.