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

2.2.3.1 LMR36520 Voltage Rail

The LMR36520 device is the best choice for this design. The device has a very wide input voltage range (4.2 V to 65 V), and has a 2-A load capacity. The LMR36520 also has low no-load quiescent current and high efficiency at light loads.

The LMR36520 is used to convert the rectified 24-VAC source to 5 VDC and is used as the primary source of power in the TIDA-010932.

GUID-20230530-SS0I-BRZK-G1M9-TXWW4ZXW5X5W-low.png Figure 2-5 LMR36520 Implementation

The output voltage of LMR36520 is externally adjustable using a resistor divider network. The range of recommended output voltage is found in the Recommended Operating Conditions of the data sheet. The divider network is comprised of RFBT and RFBB, and closes the loop between the output voltage and the converter. The converter regulates the output voltage by holding the voltage on the FB pin equal to the internal reference voltage, VREF. The resistance of the divider is a compromise between excessive noise pickup and excessive loading of the output. Smaller values of resistance reduce noise sensitivity but also reduce the light-load efficiency. The recommended value for RFBT is 100 kΩ with a maximum value of 1 MΩ. If a 1 MΩ is selected for RFBT, then a feedforward capacitor must be used across this resistor to provide adequate loop phase margin.

Equation 5. R F B B = R F B T V O u t V R e f - 1 = 100   k Ω 5   V 1   V - 1 = 25   k Ω

For this design, RFBB = 25 kΩ and RFBT = 100 kΩ.

The parameters for selecting the inductor are the inductance and saturation current. The inductance is based on the desired peak-to-peak ripple current and is normally chosen to be in the range of 20% to 40% of the maximum output current. Experience shows that the best value for inductor ripple current is 30% of the maximum load current. Note that when selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, use the maximum device current. Equation 6 can be used to determine the value of inductance. The constant K is the percentage of inductor current ripple. This design uses K = 0.37 and with input voltage of 42 V, an inductance of L ≅ 15 μH can be calculated.

Equation 6. L = V I N - V O U T f S W × K × I O U T   m a x × V O U T V I N = 42   V - 5   V 400   k H z × 0.37 × 2   A × 5   V 42   V = 15   μ H

Inductors with a ferrite core material have very hard saturation characteristics, but usually have lower core losses than powdered iron cores. Powered iron cores exhibit a soft saturation, allowing some relaxation in the current rating of the inductor. However, powered iron cores have more core losses at frequencies above about 1 MHz. In any case, the inductor saturation current must not be less than the device low-side current limit, ILIMIT. To avoid subharmonic oscillation, the inductance value must not be less than that given in Equation 7:

Equation 7. L M I N ≥ M × V O U T f S W = 0.42 × 5   V 400   k H z = 5.25   μ H

where

  • LMIN = minimum inductance (H)
  • M = 0.42
  • fSW = switching frequency (Hz)

The value of the output capacitor and the respective ESR determine the output voltage ripple and load transient performance. The output capacitor bank is usually limited by the load transient requirements rather than the output voltage ripple. Equation 8 can be used to estimate a lower bound on the total output capacitance, and an upper bound on the ESR that is required to meet a specified load transient.

Equation 8. C O U T ≥ ∆ I O U T f S W × ∆ V O U T × K × 1 - D × 1 + K + K 2 12 × ( 2 - D )
Equation 9. E S R ≤ ( 2 + K ) × ∆ V O U T 2 × 1 + K + K 2 12 × 1 + 1 ( 1 - D )
Equation 10. D = V O U T V I N

where

  • ΔVOUT = output voltage transient
  • ΔIOUT = output current transient
  • K = ripple factor from Inductor Selection

Once the output capacitor and ESR have been calculated, use Equation 11 to check the output voltage ripple.

Equation 11. V r ≅ ∆ I L × E S R 2 + 1 ( 8 × f S W × C O U T ) 2

The ceramic input capacitors provide a low impedance source to the regulator in addition to supplying the ripple current and isolating switching noise from other circuits. A minimum ceramic capacitance of 4.7 μF is required on the input of the LMR36520. This must be rated for at least the maximum input voltage that the application requires; preferably twice the maximum input voltage. This capacitance can be increased to help reduce input voltage ripple and maintain the input voltage during load transients. Most of the input switching current passes through the ceramic input capacitors. The approximate RMS value of this current can be calculated from Equation 12 and must be checked against the maximum ratings of the manufacturer.

Equation 12. I R M S ≅ I O U T 2

In some cases, a feedforward capacitor can be used across RFBT to improve the load transient response or improve the loop-phase margin. This is especially true when values of RFBT > 100 kΩ are used. Large values of RFBT, in combination with the parasitic capacitance at the FB pin, can create a small signal pole that interferes with the loop stability. A CFF can help mitigate this effect. Use Equation 13 to estimate the value of CFF.

Equation 13. C F F < V O U T × C O U T 120 × R F B T × V R E F V O U T