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

2.2.1 24 VAC to DC Rectification

In this design a full-bridge rectifier is used for DC rectification. To prevent a significantly large inrush current during initial connection of 24 VAC, a soft-start circuit is implemented.

The schematic shown in Figure 2-2 shows the rectification and soft-start process. C1, C2, and C3 function as high-frequency bypass capacitors. R1, R2, C4, and C5 provide the soft-start time constant for the gate of the N-channel MOSFET (T1). Q1 has a gate threshold voltage range of 1.0 V to 2.5 V and a 92-mΩ RDS(on)max at VGS = 10 V. The values of R2 and R3 are chosen to voltage divide a maximum of 42 V (the peak of the 24 VAC at the high end of the tolerance) down to approximately 10 V once steady state has been reached. Calculating R2 and R3 is shown in Equation 1.

Equation 1. 42   V R 2 R 2 + R 3 = 42 150   k Ω 150   k Ω + 453   k Ω = 10.4   V

The Zener diode (D2) is used as a protective device for the MOSFET gate. R4 is used to provide an initial current path while T1 is still open. The use of R4 prevents significantly differing soft-start times due to variances in the 24-VAC transformer and the gate threshold voltage of Q1. The resulting circuit provides a relatively consistent soft-start time regardless of the 24-VAC source variances.

C6 and C7 function as the smoothing capacitors of the rectifier. The TIDA-010932 has a maximum power output of 10 W. The output ripple is a function of the load current. TINA-TI™ simulation shows a maximum worst-case ripple of 11.7 V. That worst-case condition is typically unlikely to occur depending on application; a more reasonable use case of a 100-mA output from the 3.3- V rail and a nominal 24-VAC transformer provides a rectification ripple of approximately 1.4 V. These ripple voltages must be checked in each application. Even at the worse-case scenario as previously outlined, the LMR36520 buck converter is capable of handling those ripple voltages and voltage ranges.

Figure 2-3 shows a simulation example of the schematic detailed in Figure 2-2 under an input power of approximately 750 mW. The time from applied input power to the output of the rectifier reaching steady-state is approximately 300 ms.

GUID-20230608-SS0I-7MGR-NM4H-QBKZJDBLFZJ2-low.png Figure 2-2 Soft-Start Schematic
GUID-20230608-SS0I-NNM2-N7TS-CTMX1DW56VDR-low.png Figure 2-3 Soft-Start TINA-TI Simulation Example