SLVSDJ0E May   2016  – January 2021 TPS25940-Q1

PRODMIX  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Timing Requirements
    7. 7.7 Typical Characteristics
  8. Parametric Measurement Information
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Enable and Adjusting Undervoltage Lockout
      2. 9.3.2 Overvoltage Protection (OVP)
      3. 9.3.3 Hot Plug-In and In-Rush Current Control
      4. 9.3.4 Overload and Short Circuit Protection
        1. 9.3.4.1 Overload Protection
        2. 9.3.4.2 Short Circuit Protection
        3. 9.3.4.3 Start-Up with Short on Output
        4. 9.3.4.4 Constant Current Limit Behavior During Overcurrent Faults
      5. 9.3.5 FAULT Response
      6. 9.3.6 Current Monitoring
      7. 9.3.7 Power Good Comparator
      8. 9.3.8 IN, OUT and GND Pins
      9. 9.3.9 Thermal Shutdown
    4. 9.4 Device Functional Modes
      1. 9.4.1 DevSleep Mode
      2. 9.4.2 Shutdown Control
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Step by Step Design Procedure
        2. 10.2.2.2 Programming the Current-Limit Threshold: R(ILIM) Selection
        3. 10.2.2.3 Undervoltage Lockout and Overvoltage Set Point
        4. 10.2.2.4 Programming Current Monitoring Resistor—RIMON
        5. 10.2.2.5 Setting Output Voltage Ramp Time (tdVdT)
          1. 10.2.2.5.1 Case1: Start-Up Without Load: Only Output Capacitance C(OUT) Draws Current During Start-Up
          2. 10.2.2.5.2 Case 2: Start-Up With Load: Output Capacitance C(OUT) and Load Draws Current During Start-Up
        6. 10.2.2.6 Programing the Power Good Set Point
        7. 10.2.2.7 Support Component Selections—R6, R7 and CIN
      3. 10.2.3 Application Curves
      4. 10.2.4 System Examples
        1. 10.2.4.1 VBUS Short-to-Battery, Short-to-Ground Protection of USB Port in Automotive Systems
        2. 10.2.4.2 Power Failure Protection for Holdup Power
        3. 10.2.4.3 Overload Detection Using TPS25940xx-Q1
  11. 11Power Supply Recommendations
    1. 11.1 Transient Protection
    2. 11.2 Output Short-Circuit Measurements
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Documentation Support
      1. 13.1.1 Related Documentation
    2. 13.2 Support Resources
    3. 13.3 Trademarks
    4. 13.4 Electrostatic Discharge Caution
    5. 13.5 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information
Case 2: Start-Up With Load: Output Capacitance C(OUT) and Load Draws Current During Start-Up

When load draws current during the turn-on sequence, there is additional power dissipated. Considering a resistive load RL(SU) during start-up, load current ramps up proportionally with increase in output voltage during tdVdT time. Typical ramp-up of output voltage, Load current and power dissipation in the device is shown in Figure 10-4 and power dissipation with respect to time is plotted in Figure 10-5. The additional power dissipation during start-up phase is calculated as shown in Equation 17 and Equation 18.

Equation 17. GUID-98EB938F-C8B4-484A-9D34-B6E6194AF9DA-low.gif
Equation 18. GUID-BF2AF5C3-96D2-4A59-8014-C3F9CD03576D-low.gif

Where RL(SU) is the load resistance present during start-up. Average energy loss in the internal FET during charging time due to resistive load is given by Equation 19.

Equation 19. GUID-B4C26D0B-ABAC-4F30-9D6C-C4997AF2F800-low.gif
GUID-2E0C9E96-4337-4340-8448-01BB8472B67C-low.png
V(IN) = 12 VC(dVdT) = 1 nFRL(SU) = 4.8 Ω
Figure 10-4 Start-Up With Load
GUID-E0D2F1B2-297D-4707-B01D-ADE926BA3994-low.png
V(IN) = 12 V C(dVdT) = 1 nF RL(SU) = 4.8 Ω
Figure 10-5 PD(LOAD) in Load During Start-Up

On solving Equation 19 the average power loss in the internal FET due to load is shown in Equation 20.

Equation 20. GUID-93A16914-0FAB-47BB-B2C2-BC0FBBE30148-low.gif

Total power dissipated in the device during startup is shown is Equation 21.

Equation 21. GUID-B4B15A51-FBDA-4876-A702-542BEC3798D8-low.gif

Total current during startup is given by Equation 22.

Equation 22. GUID-E9E14F70-B6B3-4CA4-999A-84991A4EA071-low.gif

If I(STARTUP) > I(LIM), the device limits the current to I(LIM) and the current limited charging time is determined by Equation 23.

Equation 23. GUID-DD1E1F73-B222-4B76-8D7A-6F2169C6DA03-low.gif

The power dissipation, with and without load, for selected start-up time must not exceed the shutdown limits as shown in Figure 10-6.

GUID-014E00EE-2C38-43C8-B777-417685583B09-low.gif
Taken on 2-Layer board, 2oz.(0.08-mm thick) with GND plane area: 14 cm2 (Top) and 20 cm2 (bottom)
Figure 10-6 Thermal Shutdown Limit Plot

For the design example under discussion,

Select ramp-up capacitor C(dVdT) = 1nF, using Equation 24.

Equation 24. GUID-EFC3FB7D-6271-447E-B594-A957CAF5E6D2-low.gif

The inrush current drawn by the load capacitance (C(OUT)) during ramp-up using Equation 25.

Equation 25. GUID-D111FB7F-200C-4CF5-BAB2-7010B70D2354-low.gif

The inrush Power dissipation is calculated, using Equation 26.

Equation 26. GUID-D13FEF37-EFD2-4048-9B4B-37CC05972A6F-low.gif

For 7.2 W of power loss, the thermal shut down time of the device must not be less than the ramp-up time tdVdT to avoid the false trip at maximum operating temperature. From thermal shutdown limit graph Figure 10-6 at TA = 85°C, for 7.2 W of power the shutdown time is approximately 60 ms. So it is safe to use 1 ms as start-up time without any load on output.

Considering the start-up with load 4.8 Ω, the additional power dissipation, when load is present during start up is calculated, using Equation 27.

Equation 27. GUID-434B1D91-7788-4ECA-9DBE-BF6C4D503EA1-low.gif

The total device power dissipation during start up is given by Equation 28.

Equation 28. GUID-042C2629-FF41-4F32-B75F-C18EADC9D2CC-low.gif

From thermal shutdown limit graph at TA = 85°C, the thermal shutdown time for 12.2 W is close to 7.5 ms. It is safe to have 30% margin to allow for variation of system parameters such as load, component tolerance, and input voltage. So it is well within acceptable limits to use the 1 nF capacitor with start-up load of 4.8 Ω.

If there is a need to decrease the power loss during start-up, it can be done with increase of C(dVdT) capacitor.

To illustrate, choose C(dVdT) = 1.5 nF as an option and recalculate:

Equation 29. GUID-AA7A980E-B800-41F3-9855-5326ABC1C6F9-low.gif
Equation 30. GUID-5AD699F4-F3D3-4E9F-AA3B-EBF1C3882C4B-low.gif
Equation 31. GUID-5C0992BA-043C-44EF-8CC3-E9430918D675-low.gif
Equation 32. GUID-45E6E853-756D-4527-9B50-69AAEEC36FF2-low.gif
Equation 33. GUID-F70E977B-8465-4555-AFE2-54C3BC403662-low.gif

From thermal shutdown limit graph at TA = 85°C, the shutdown time for 10-W power dissipation is approximately 17 ms, which increases the margins further for shutdown time and ensures successful operation during start up and steady state conditions.

The spreadsheet tool available on the web can be used for iterative calculations.