The microcontroller unit is meant to either relay signals and commands to or take in signals and measurements from different components in the PTC heater control module. They will take signals indicating how much current is to be supplied to the PTC, what temperature the PTC load is be at, and relay commands to the respective components. The MCUs also take in measurements such as temperature, voltage, current and so forth and commit to appropriate actions with respect to those measurements and conditions. The specific signals it sends or receives depends on how many MCUs are present in the design, and whether the MCU is placed on the high-voltage side or low-voltage side.

If the designer chooses to use 2 MCUs, one on the low-voltage side and one on the high-voltage side, the MCU on the low-voltage side is in charge of communication, while the MCU on the high-side is in charge of PTC load control. This topology results in minimizing the number of isolated signals needed.

If the designer decides to use one MCU and place it on the high-voltage side, it is recommended to then have a LIN transceiver without a dedicated WAKE pin to limit signals that need to transferred from the high-voltage side to the low-voltage side. However, if the designer wants the low-voltage rail to be monitored by the MCU to sense if it goes outside of the operating range, that signal can go through a high precision analog isolator, then feed into the MCU. A suitable analog isolator for voltage sensing this application would be the AMC1336-Q1 with its great DC performance of ±0.5mV maximum offset error. It also comes in small DWV (5.85mm × 11.5mm) package. Another option would be to use a discrete comparator circuit to detect when the input voltage rail goes outside of the operating range, and have that signal go through a digital isolator, then to the MCU. The TLV3201-Q1 could be suitable for this purpose. It is recommended that the MCU process enough pins available for connections to the digital isolator, the switch drivers, the temperature sensor(s), the load current monitoring circuits and voltage follower. There is no direct impact on MCU selection when designing with either a 400V or 800V battery. This topology can be used to save cost given the limited isolated signals needed and the use of only one MCU.

Table 3-1 lays out an example of what signals the MCU could send or receive depending on topology and placement. Some signals can be added or removed based on the discretion of the designer.

Interlock is a current and voltage loop mechanism that connects all high-voltage subsystems in the vehicle (battery management systems, traction inverters, on-board chargers, and so forth). It monitors any interference or tampering done on these subsystems or to the service disconnect switch. If this condition is detected, then the high-voltage subsystems in the vehicle, such as the PTC heater module, will shut down. Including the PTC heater module in the interlock mechanism increases safety by decreasing the risk of damage to the user and other subsystems. For guidance on how to design the interlock mechanism, see TIDA-01445.

The choice of using one or two MCUs varies from designer to designer. Either of these topologies are used in PTC heater modules across the market, so there is no right or wrong choice. The designer just needs to understand the advantages and drawbacks of each, and select the topology that aligns best with the goals and requirements of their system. An MCU suitable for PTC heaters is the TMS320F2800153-Q1, which comes with 64KB of flash size in 32-pin RHB (5mm × 5mm) or 48-pin PHP (9mm × 9mm) packages. The 48-pin version has more analog and GPIO pins than the 32-pin version, so it is up to the designer to decide which version to use. It may make more sense to use the 48-pin version if the design calls for 1 MCU. However, a 2 MCU approach may make more sense to use 32-pin MCUs. Another viable option would be the MSPM0L1305-Q1, a 32-pin RHB (5mm × 5mm) MCU with a 32-Mhz Arm® Cortex®-M0+ processor core. Having an Arm Cortex-M0+ core enables optimized power efficiency and high performance. This device also fits well within a PTC heater due to its LIN communication support.