In a buck regulator, the primary switching loop consists of the input capacitor, MOSFETs, and current sense resistor. Minimizing the area of this loop reduces the stray inductance and minimizes noise and possible erratic operation. The input capacitor must be placed as close as possible to the MOSFETs, with the VIN side of the capacitor connected directly to the high-side MOSFET drain, and the GND side of the capacitor connected as close as possible to the low-side source or current sense resistor ground connection. TI recommends a ground plane in the PC board as a means to connect the quiet end (input voltage ground side) of the input filter capacitors to the output filter capacitors and the PGND pin 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 to a topside copper area covering the entire underside of the device. Place several vias in this underside copper area to the ground plane.

The highest power dissipating components are the two power MOSFETs. The easiest way to determine the power dissipated in the MOSFETs is to measure the total conversion losses (P_IN – P_OUT), then subtract the power losses in the output inductor and any snubber resistors. The resulting power losses are primarily in the switching MOSFETs.

If a snubber is used, the power loss can be estimated with an oscilloscope by observation of the resistor voltage drop at both turn-on and turn-off transitions. Assuming that the RC time constant is << 1 / f_SW.

The regulator has an exposed thermal pad to aid power dissipation. Selecting MOSFETs with exposed pads aids the power dissipation of these devices. Careful attention to R_DS(ON) at high temperature must be observed. Also, at 250 kHz, a MOSFET with low gate capacitance results in lower switching losses.