Before examining how to simulate various loading conditions using the thermal PSpice models provided by Texas Instruments, it is important to know the capabilities and limitations of the model. Models with thermal modeling capabilities can provide a power insight on how the device will behave under various ambient and loading conditions; however, in real world applications various factors such as layout and solder coverage will affect the thermal performance of a high-side switch. All high-side switches are designed to perform optimally when using the JESD 51-7 board layout standard and the supported thermal models have been made to most closely match the JEDEC standard. When designing a schematic and engineering the board layout, it is important to follow data sheet recommendations to ensure that the thermal performance of the real world device on the PCB will closely match that of the board itself. It is also important to note that currently only a subset of PSpice models published on ti.com currently have thermal modeling capabilities. Refer to Section 2.3 for determining if a published model has the appropriate ambient temperature parameters for modeling thermal behavior.

TI’s Smart High-Side Switches [HSS] provide reliable off-board load protection by utilizing three main protective features: current sensing, current limiting, and thermal shutdown. As a result of these features, the HSS monitors output current and limits it to a specified value to allow for optimal design for specific capacitive, inductive, or LED loads. This output current can be clamped at either an internal current limit inherent to the device, or a lower current limit, controlled by an external resistor. If the device approaches a certain thermal threshold, the device will shut off, cool down, and auto restart once the temperature has decreased significantly.

With the selection of a HSS, the thermal behavior of the HSS is a factor that should be taken into account. At the core of the thermal protection is the thermal shutdown limit, a specified temperature within the data sheet of the device. When the internal junction temperature of the device reaches the thermal shutdown limit, the device will enter thermal shutdown to protection off-board loads. Devices with a thermal pin [THER] allow for control over the thermal shutdown behavior while devices without the THER pin will either shutdown or reduce the output current limit. The configuration and behavior of the devices with THER pins to enable latching, shutdown, or are found in Fault and Normal Diagnostic Considerations for Smart High Side Switches.

TI’s high-side switches that have thermal modeling implemented will model the behavior of the device when it enters thermal shutdown and provide a look into how the internal junction temperature changes with operation. This takes out all the math calculations needed and puts it in a convenient package to use and model device behavior.