2 Device Thermals

Thermal limits of any IC should always be kept in mind, and this is especially true for high-side switches which are often asked to switch large currents to the load.

We can estimate the total power dissipated by the device itself by summing power dissipation due to the FET R_ON, switching losses, and quiescent power.

We can calculate power dissipated in the HSS FET as in Equation 1. This power is proportional to the on-resistance (specified in the device datasheet) and the average current through the FET.

As PWM operation switches the HSS FET on and off every cycle, we also need to consider how much energy it takes to charge and discharge the gate. We can use Equation 2 to calculate switching loss from switching energy losses E_ON, E_OFF if defined in the device datasheet. Notably, this scales with frequency and is a large component of total power dissipation in the HSS.

If E_ON, E_OFF are not available for a device, we can approximate the switching losses from how long it takes the device to slew the output low or high using Equation 3.

We then substitute our results from Equation 3 into Equation 2 to get switching power. A safe junction temperature must be maintained whether operating in PWM or DC driving. The maximum limit is specified in the HSS datasheet—generally 150°C. TI HSS datasheets contain the thermal characterization parameters listed in Table 2-1

TI HSSs utilize a thermal pad which is recommended to be soldered directly to the PCB. This benefits devices thermals by utilizing the large surface area of the attached thermal plane as a heatsink, which closely couples with board temperature. Board temperature, however, is difficult to measure accurately and is highly dependent on board construction and solder coverage/quality on the high side switch. Ambient temperature, on the other hand, is easier to measure and straightforward to use in junction temperature estimation.

For a given ambient temperature the operating junction temperature may be estimated as below.

The PWM frequency is set tightly by the maximum sustainable power dissipation of the HSS, as switching losses are dominant. This power limit can be estimated from Absolute max junction temperature specification and maximum ambient temperature.