SLVSHH5C August   2023  – May 2026 TPS25983

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Switching Characteristics
    8. 6.8 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Undervoltage Protection (UVLO and UVP)
      2. 7.3.2 Overvoltage Protection (OVP)
      3. 7.3.3 Inrush Current, Overcurrent, and Short-Circuit Protection
        1. 7.3.3.1 Slew Rate and Inrush Current Control (dVdt)
        2. 7.3.3.2 Circuit Breaker
        3. 7.3.3.3 Active Current Limiting
        4. 7.3.3.4 Short-Circuit Protection
      4. 7.3.4 Overtemperature Protection (OTP)
      5. 7.3.5 Analog Load Current Monitor (IMON)
      6. 7.3.6 Power Good (PG)
      7. 7.3.7 Reverse Current Blocking FET Driver
      8. 7.3.8 Fault Response
    4. 7.4 Device Functional Modes
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application: Standby Power Rail Protection in Datacenter Servers
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Device Selection
        2. 8.2.2.2 Setting the Current Limit Threshold: RILIM Selection
        3. 8.2.2.3 Setting the Undervoltage and Overvoltage Lockout Set Point
        4. 8.2.2.4 Selecting the Current Monitoring Resistor: RIMON
        5. 8.2.2.5 Setting the Output Voltage Ramp Time (TdVdt)
          1. 8.2.2.5.1 Case 1: Start-Up Without Load: Only Output Capacitance COUT Draws Current
          2. 8.2.2.5.2 Case 2: Start-Up With Load: Output Capacitance COUT and Load Draw Current
        6. 8.2.2.6 Setting the Transient Overcurrent Blanking Interval (tITIMER)
        7. 8.2.2.7 Setting the Auto-Retry Delay and Number of Retries
      3. 8.2.3 Application Curves
    3. 8.3 System Examples
      1. 8.3.1 Optical Module Power Rail Path Protection
        1. 8.3.1.1 Design Requirements
        2. 8.3.1.2 Device Selection
        3. 8.3.1.3 External Component Settings
        4. 8.3.1.4 Voltage Drop
        5. 8.3.1.5 Application Curves
      2. 8.3.2 Input Protection for 12V Rail Applications: PCIe Cards, Storage Interfaces, and DC Fans
      3. 8.3.3 Priority Power MUXing
    4. 8.4 Power Supply Recommendations
      1. 8.4.1 Transient Protection
      2. 8.4.2 Output Short-Circuit Measurements
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
      2. 8.5.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Documentation Support
      1. 9.1.1 Related Documentation
    2. 9.2 Receiving Notification of Documentation Updates
    3. 9.3 Support Resources
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information
8.2.2.5.2 Case 2: Start-Up With Load: Output Capacitance COUT and Load Draw Current

When the load draws current during the turn-on sequence, there is additional power dissipated. Considering a resistive load during start-up RL(SU), load current ramps up proportionally with increase in output voltage during TdVdt time. Equation 17 shows the average power dissipation in the internal FET during charging time due to resistive load.

Equation 17. P D ( L O A D ) = ( 1 6 ) × V I N 2 R L ( S U )

Equation 18 gives the total power dissipated in the device during start-up.

Equation 18. P D ( S T A R T U P ) = P D ( I N R U S H ) + P D ( L O A D )

The power dissipation, with and without load, for selected start-up time must not exceed the start-up thermal shutdown limits as shown in Thermal Shutdown Plot During Start-up.

TPS25983 Thermal Shutdown Plot During Start-up Figure 8-2 Thermal Shutdown Plot During Start-up

For the design example under discussion, the output voltage has to be ramped up in 20ms, which mandates a slew-rate of 0.6V/ms for a 12V rail.

The required CdVdt capacitance on dVdt pin to set 0.6V/ms slew rate can be calculated using Equation 19.

Equation 19. C d V d t ( p F ) = 4600 S R ( V / m s ) = 7666 p F

The dVdt capacitor is subjected to typically VIN + 4V during startup. The high voltage bias leads to a drop in the effective capacitor value. So, it is suggested to select 20% higher than the calculated value, which gives 9.2nF. Select the closest 10% standard value: 10nF

The 10nF CdVdt capacitance sets a slew-rate of 0.46V/ms and output ramp time TdVdt of 26ms.

The inrush current drawn by the load capacitance COUT during ramp-up can be calculated using Equation 20.

Equation 20. I I N R U S H = 1.4 m F × 12 V 26 m s = 0.65 A

The inrush power dissipation can be calculated using Equation 21.

Equation 21. P D ( I N R U S H ) = 0.5 × 12 × 0.65 = 3.9 W

For 3.9W of power loss, the thermal shutdown time of the device must be greater than the ramp-up time TdVdt to establish a successful start-up. Figure 8-2 shows the start-up thermal shutdown limit. For 3.9W of power, the shutdown time is approximately 100ms. So it is safe to use 26ms as the start-up time without any load on the output.

The additional power dissipation when a 10Ω load is present during start-up is calculated using Equation 22.

Equation 22. P D ( L O A D ) = ( 1 6 ) × 12 2 10 = 2.4 W

The total device power dissipation during start-up can be calculated using Equation 23.

Equation 23. P D ( S T A R T U P ) = 3.9 + 2.4 = 6.3 W

From Figure 8-2, the thermal shutdown time for 6.3W is approximately 40ms. 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 10nF for CdVdt capacitor with start-up load of 10Ω.

When COUT is large, there is a need to decrease the power dissipation during start-up. This can be done by increasing the value of the CdVdt capacitor. A spreadsheet tool TPS25983xx Design Calculator available on the web can be used for iterative calculations.