SLUAAY5 December   2024 UCC21551-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Automotive PTC Heater Module Overview
    1. 2.1 Automotive Heating Architectures
      1. 2.1.1 Positive Temperature Coefficient Heaters
      2. 2.1.2 Heat Pumps
    2. 2.2 Automotive Heating Architectures
    3. 2.3 PTC Heater Topologies
  6. 3Design of Automotive PTC Heater Controller
    1. 3.1  Block Diagrams
    2. 3.2  Designing the Power Supplies
    3. 3.3  Picking Low-Dropout Regulators
    4. 3.4  Designing of the Communication Interface
    5. 3.5  Implementation of the Digital Isolator
    6. 3.6  Implementation of the Microcontroller Unit
    7. 3.7  Designing of the Switch Driver Stage
    8. 3.8  Selection of the Power Switches
    9. 3.9  Considerations of the PTC Load
    10. 3.10 Designing the Load Current Monitoring
    11. 3.11 Selection of the Temperature Sensing
  7. 4Summary

3.3 Picking Low-Dropout Regulators

The low dropout (LDOs) regulators in this system are meant to step down voltage from the low-voltage and high-voltage rails and provide clean power rails to the components on both sides at the correct respective voltages. Specifically, for PTCs heater designs, they can power the communication interface, digital isolator and MCU on the low-voltage side, as well as the switch driver(s), load current monitoring circuit, temperature sensors and MCU on the high-voltage side.

If the designer wants an LDO connected to the input voltage rail, this device must be able to handle the input rail’s absolute maximum voltage range (around 4V to 42V) in the cases of cold crank or load dump. In regard to powering an MCU, TI recommends that the LDO has low quiescent current (IQ) to optimize power efficiency. A suitable LDO for PTC heaters would be the TPS7B84-Q1, with its wide input range of 3V to 40V (-0.3V to 42V absolute maximum range) and maximum IQ of 35µA. For more guidance on designing with LDOs, the LDO Basics (Rev. A) eBook is a great resource.

The designer may have to use an off-board temperature sensor to get a more accurate reading of the PTC load temperature. If the power supply used for this temperature sensor is different than the reference voltage for the analog to digital converter (ADC) used to take in the sensor reading, then variation of the sensor power supply and the ADC reference voltage can cause significant ADC output variance, resulting in errors. One way to resolve this issue is by tying the temperature sensor power supply and ADC reference voltages together, but because this involves long cable connections, this method introduces an increased likelihood of fault conditions. The designer can eliminate these issues by using a tracking LDO so that its output to the off-board temperature sensor and the ADC reference voltage are within a specified margin of each other. Figure 3-11 shows an example block diagram of its implementation.

Off-Board Temperature Sensor Implementation Figure 3-11 Off-Board Temperature Sensor Implementation

A suitable tracking LDO for this application would be the TPS7B4254-Q1, as it includes this ratiometric tracking feature with ±4mV output-tracking tolerance. It is also designed with integrated protection features for system robustness against fault conditions, including reverse current protection, reverse polarity protection, and more.