9.1 Thermal Considerations

The highest power dissipating components are the two power MOSFETs, the recirculating diode, and the output diode. 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 Schottky diodes, output inductor and any snubber resistors. An approximation for the recirculating Schottky diode loss is:

The boost diode loss is

If a snubber is used, the power loss can be estimated with an oscilloscope by observation of the resistor voltage drop at both turnon and turnoff transitions. The LM5118 package has an exposed thermal pad to aid power dissipation. Selecting diodes with exposed pads will aid the power dissipation of the diodes as well. When selecting the MOSFETs, pay careful attention to R_DS(ON) at high temperature. Also, selecting MOSFETs with low gate charge will result in lower switching losses.

9.2 Bias Power Dissipation Reduction

Buck or Buck-boost regulators operating with high input voltage can dissipate an appreciable amount of power while supplying the required bias current of the IC. The VCC regulator must step-down the input voltage VIN to a nominal VCC level of 7 V. The large voltage drop across the VCC regulator translates into high power dissipation in the VCC regulator. There are several techniques that can significantly reduce this bias regulator power dissipation. Figure 27 and Figure 28 depict two methods to bias the IC, one from the output voltage and one from a separate bias supply. In the first case, the internal VCC regulator is used to initially bias the VCC pin. After the output voltage is established, the VCC pin bias current is supplied through the VCCX pin, which effectively disables the internal VCC regulator. Any voltage greater than 4.0 V can supply VCC bias through the VCCX pin. However, the voltage applied to the VCCX pin should never exceed 15 V. The voltage supplied through VCCX must be large enough to drive the switching MOSFETs into full saturation.

Figure 27. VCC Bias From VOUT 4 V < VOUT < 15 V

Figure 28. VCC Bias With Additional Bias Supply

In a buck-boost regulator, there are two loops where currents are switched very fast. The first loop starts from the input capacitors, and then to the buck switch, the inductor, the boost switch then back to the input capacitor. The second loop starts from the inductor, and then to the output diode, the output capacitor, the recirculating diode, and back to the inductor. Minimizing the PCB area of these two loops reduces the stray inductance and minimizes noise and the possibility of erratic operation. A ground plane in the PCB is recommended as a means to connect the input filter capacitors to the output filter capacitors and the PGND pins of the LM5118. Connect all of the low current ground connections (C_SS, R_T, C_RAMP) directly to the regulator AGND pin. Connect the AGND and PGND pins together through topside copper area covering the entire underside of the device. Place several vias in this underside copper area to the ground plane of the input capacitors.