2 Automotive Dashboard and ADAS Loads

The idea of advanced driver assistance systems is to take advantage of more than one sensory input of the user. Since the user and or driver is the intended audience, the loads are usually lower wattage systems. Dashboard and ADAS loads can range from buzzers to motors to lighting cluster warnings taking advantage of hearing, touch and sight respectively. Not all loads are integrated into a vehicle with the goal of safety. Loads like steering wheel heaters and steering column movement motors or pedal box movement motors are integrated to increase driver comfort and overall ease of use. Some systems like gauge lighting cluster can perform double duty of adding interior lighting for comfort but also respond to external sensor stimuli. Therefore, ADAS and dashboard loads can be resistive, capacitive and inductive in nature. A high side switch like TPS1HC100-Q1 is tailor made to drive lower power ADAS loads since it is compatible with resistive, capacitive and inductive loads. Examples of these loads are shown in Figure 2-1

Figure 2-1 Examples of Some Conventional Low-Power Automotive Dashboard Loads

Resistive Loads

Resistive loads are the most common loads for a high side switch. They are the simplest loads to drive as they are linear systems. A common resistive load is a steering wheel heater. A long resistive coil is placed around the steering wheel or patches of resistive elements are placed on certain touch points. When current is passed over these coils or elements they heat up with respect to the square of the DC current multiplied with the specified resistance of the element. Since the voltage of the system is known and the on resistance of the element is specified, the current passing through the coil is easily calculated using Ohm’s Law.

In this situation, using a TI high side switch like TPS1HC100-Q1 is beneficial not only because of its in-built current limit protection but more so due to the integrated high accuracy current sensing of the system. This system measures the current flowing through the power FET and by using an internal current mirror, it outputs a proportional current through the SNS pin of the device. Using an accurate sense resistor from the SNS pin to GND, it is possible to convert this proportional current output into a measurable proportional voltage signal. This voltage can then be read into a controller or an ADC and further processing can be performed.

An application example taking full advantage of this system is to change how much heat is dissipated across the steering wheel heater. The heater can be held in the DC on state initially to heat up the wheel to a comfortable temperature and then after a while the switch can be PWM’d to drop the average current flowing through the heater. The accurate current sense is then able to tell the controller what the temperature is and if there is any fault in the system. All this is performed without the need of a dedicated current sense circuit saving valuable PCB space.

Capacitive Loads

TI’s high side switch offerings like TPS1HC100-Q1 can be used to drive both bulk as well as hold up capacitive loads. It is common to see speakers, buzzers, displays and lighting clusters often have a noticeable amount of input capacitance. Depending on the rise time at power up when connected to battery voltage, it is possible to see high inrush currents in these systems as the current is only limited by cable parasitic inductance and resistance. The inrush current can be significantly higher than the DC current required by the system and this results in accounting for larger cables, larger traces and a switch that can handle higher inrush currents.

TPS1HC100-Q1 circumvents these issues by offering high accuracy current limiting. This feature is also useful to protect the load and source during a short event but is extremely useful to turn on capacitive loads. The device includes an ILIM pin from which the user connects a current limiting resistor to ground. Similar to the current sensing system, there exists an internal current mirror that pushes a proportion of the load current out of the pin and over this resistor. When the voltage at this pin is higher than the internal threshold for current limiting, the device moves away from a linear mode of operation to a resistive mode to hold current at the specified value. The amount of current limit resistance is related to the value at which the device limits.

An application example of this is turning on a capacitive display or lighting cluster. Rather than experiencing a large inrush spike during turn on, the current through the device is held constant and the output rises slowly as the capacitor is charged up. This allows for the internal cabling, trace width and PCB area to be designed for lower currents. In the case that the capacitor to be charged is very large or in the event that there exists a failure more like a short to GND, the device will limit current to the set level. As this involves power dissipation across the FET, the device will heat up and eventually hit thermal shutdown which turns off the device and protects the system.

Inductive Loads

Inductive loads are loads that store energy in the form of a magnetic field. These loads are very common for high side switches especially in high power systems as the length of cable that connects the switch board to the off-board loads can more often have sufficient parasitic inductance for it to hold high energy. For ADAS systems, the power levels are lower and the length of cable is insufficient and as such dedicated inductive loads like motors and relays are of particular concern.

An inductor tries to resist against any change of current that flows through it. This means that during a turn on event, the inductor will result in slower charging time and slower turn on. Once the DC current is reached, the inductor is seen as a small resistive element. The issue arises during the turn off event of an inductive load. The inductor will once again resist the change of current flowing through it even though the switch has turned off. The magnetic energy stored within the inductor is turned into potential energy and causes a large transient voltage to be generated across the inductor. The switch therefore sees a large unregulated voltage spike on the output which can damage the switch.

TI high side switches like TPS1HC100-Q1 are built to withstand such events. When the circuit is turned off, the voltage on the output of the high side switch begins to drop instantaneously. The device integrates a voltage clamp across the drain and the source of the FET which keeps an active discharge path over which the inductive energy can be demagnetized. The voltage drop is therefore regulated to the clamp voltage and large negative spikes are not seen in the system. The current is able to recirculate through the clamp and the energy is dissipated away as heat. This integrated clamp means that there is no requirement for a flyback or high-power recirculation diode which saves on board and PCB cost.

For more information on driving resistive, capacitive and inductive loads please view this application note here. For more information on setting current limit and current sensing resistors for TPS1HC100-Q1, please view the data sheet here.