TIDUF26 june   2023 BQ24072 , LMR36520 , TLV62568 , TPS2116

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 24 VAC to DC Rectification
      2. 2.2.2 eFuse Protection
      3. 2.2.3 5-V Rails
        1. 2.2.3.1 LMR36520 Voltage Rail
        2. 2.2.3.2 USB Power Input
      4. 2.2.4 Power Source ORing
      5. 2.2.5 Battery Management
      6. 2.2.6 3.3-V Power Rail
      7. 2.2.7 Power Rail Current Sensing
      8. 2.2.8 Backlight LED Driver
      9. 2.2.9 BoosterPack Overview
    3. 2.3 Highlighted Products
      1. 2.3.1 LMR36520
      2. 2.3.2 TPS2116
      3. 2.3.3 TLV62568
      4. 2.3.4 INA2180
      5. 2.3.5 TPS92360
      6. 2.3.6 TPS2640
      7. 2.3.7 BQ24072
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1  24-VAC Start-Up and Shutdown
      2. 3.3.2  USB Start-Up and Shutdown
      3. 3.3.3  ORing
      4. 3.3.4  LMR36520
      5. 3.3.5  TLV62568 Transient Response
      6. 3.3.6  BM24072 Transient Response
      7. 3.3.7  TLV62568 (3V3 Power Rail)
      8. 3.3.8  LMR36520 (LMOut Power Rail)
      9. 3.3.9  BM24072 (BMOut Power Rail)
      10. 3.3.10 Reference
        1. 3.3.10.1 TLV62568
        2. 3.3.10.2 LMR36520
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

1 System Description

A standard home typically uses a 24-VAC power supply to the HVAC system controls. Thermostats, as well as many other home automation equipment, use this 24-VAC for power. Therefore, a power stage is needed to rectify the 24-V AC and supply a DC voltage at the levels required by the internal components of the thermostat. Low cost is typically a priority for thermostat designs because board space is often plentiful enough to avoid small-footprint, high-cost parts. For thermostats implementing a chargeable battery backup system, high efficiency also becomes a priority to allow a smaller, and therefore lower cost, battery to be used. The TIDA-010932 focuses on these priorities and can be easily adjusted to meet specific needs.

In addition to the 24-V AC and battery backup, this reference design allows a USB power supply to be used for charging and powering the system. Having two supply options, 24-V AC and USB, requires an ORing device. This reference design provides a TI-based ORing strategy based on the TPS2116. If a USB is not desired, the ORing device can be removed with very minimal changes to the design.

The 24-V AC is rectified and stepped down to a 5-V rail using an ultra-low IQ, wide VIN, 2-A buck converter. The wide VIN of the buck converter helps handle transients, thus eliminating the need for a TVS diode and other protection circuitry. Smaller capacitors can be used as the input voltage ripple can be higher when using a wide VIN buck.

The TIDA-010932 uses a battery-management device that allows independent current paths and monitoring for system power and battery charging. This device increases the cycle life of the battery. This reference design features a seamless transition to battery power in case the main supply fails, as well as battery power assist should the load requirement surpass the rating of the main supply. A very efficient, low cost, and low BOM count buck converter is used to step down the battery management voltage to 3.3 V for use by the general system. Both DC-DC buck converters in this design feature a low-load power saving feature to provide high efficiency even at light loads.