Often in systems central modules provide power to off-board loads in a number of different form factors. This occurs in situations such as a central module powering an automotive head-light, a PLC system powering a robotic arm, and a household appliance powering the indicators on the front panel. Situations where off-board loads must be driven are common in the vast majority of electrical systems and introduce specific challenges to the system designer. While it can be simple to switch enough DC power to meet the system requirements, it is much more challenging to ensure robust protection against short circuits and open circuits, provide fault indication, power up the load quickly, and enable predictive maintenance. These additional features are being increasingly requested by designs, so an engineer needs to select an output topology that enables this functionality. The best way to accomplish this is to use a Smart High Side Switch which can reliably drive off-board loads and enable numerous diagnostic and failure prevention mechanisms.

Not all off-board loads are the same. Each load profile will interact differently with the Smart High Side Switch and require different considerations to ensure robust protection. Whether the load is resistive, capacitive, inductive, or does not fall neatly into one of those categories such as LEDs will change how driving the load must be approached and designed. A proper output power protection designer needs to understand what load profile will be expected,and then understand how that impacts the design of the output stage. This document will analyze a few common load profiles and discuss the specific challenges and considerations for those loads. The load profiles that will be investigated in this document are:

1. Section 2: Driving Resistive Loads
2. Section 3: Driving Capacitive Loads
3. Section 4: Driving Inductive Loads
4. Section 5: Driving LED Loads

For each of these load types this document will give example applications with the given profile, discuss why a Smart High Side Switch offers advantages compared to traditional discrete solutions, go in depth on the technical challenges unique to that load type, and then offer guidelines for selecting the proper Smart High Side Switch for a given application.

Through a proper and thorough understanding of the impacts of a load profile on an output power stage it is possible to significantly improve functionality and reliability for a system. As designs continue to get smarter and more robust this understanding is critical for all designers.

Resistive loads are the simplest loads to drive as they follow Ohm’s Law.

Equation 1.

It's simple because the designer knows the voltage (typically 13.5V for a car battery) and the resistance of the load (by measuring it with an Ohm meter). With these two parameters they can calculate the maximum current that will be flowing through the circuit. Knowing this information is the first step in selecting the correct device to drive this load since each high side switch has an associated ON resistance that limits the amount of nominal current allowed through the device without hitting thermal shutdown. In typical applications the current through the load needs to be varied to provide the intended output. It is also important to have features such as current sensing that can correlate back to the microcontroller what current is actually going through the load. The most basic way to vary the current through the load is through pulse width modulating (PWM) the enable pin. This introduces more complications with regard to the thermal calculations.

In this section we will look into the application of resistive loads and show what relevant features are useful when driving them. We will also see how TI's Smart High Side Switches' feature set aligns well with the requirements for loads. Finally, in order to pick the correct high side switch we must learn how to calculate the power dissipation of the switch and relate that to the junction temperature and set the current limit appropriately so that the high side switch will be able to properly drive the resistive load.