The on-resistance, R_DSON , is the inherent resistance of the High-Side Switch device. This value depends on the temperature of the device, as well as whether the switch is in the saturation or linear region. Since R_DSON is a factor in calculating an accurate power pulse and calculating the junction temperature relies heavily on inputting an accurate power pulse, varying R_DSON as it varies due to temperature is an important factor to consider. The power dissipated by the switch, P_DISS, can be calculated as the product of the current through the switch, I_DS, and the voltage across the switch, V_DS.

Equation 4.

However, in the linear region it is easier to calculate the power dissipated by the switch, P_DISS, as the product of the squared current through the switch, I_DS, and the on-resistance of the switch, R_DSON.

Equation 5.

R_DSON varies as a function of temperature, as Figure 1-2 from the TPS1H100 data sheet shows in the Typical Characteristics plot of R_DSON.

Figure 1-2 Characteristic Plot of R_DSON of TPS1H100

To account for this change in temperature, the PSpice model must recursively take into account the maximum temperature, and then recalculate using the corresponding R_DSON for that temperature. Since the R_DSON decreases for decreasing temperature, the R_DSON will eventually converge onto a realistic R_DSON value instead of the worst-case R_DSON in the data sheet. The equation of the plot can be approximated as a line as Equation 6 shows.

Equation 6.

Where m is the slope of the line and b is the value of R_DSON at 0°C. To account for this change in R_DSON, the PSpice thermal model recursively measures the simulated junction temperature. First, the model inputs the worst-case R_DSON, located in the electrical characteristics section of the high-side switch data sheet. After this initial junction temperature value is calculated, the model iterates until the temperature and R_DSON converges. Figure 1-3 illustrates this behavior.

Figure 1-3 Block Diagram of Model

With the implementation of the thermal behavior implemented within the PSpice model, the recursive calculation is automatically taken care of in the software.