SLUS714E January   2007  – June 2019 TPS40077

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Application Diagram
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Electrical Characteristics
    5. 6.5 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Minimum Pulse Duration
      2. 7.3.2  Slew Rate Limit On VDD
      3. 7.3.3  Setting The Switching Frequency (Programming The Clock Oscillator)
      4. 7.3.4  Loop Compensation
      5. 7.3.5  Shutdown and Sequencing
      6. 7.3.6  Boost and LVBP Bypass Capacitance
      7. 7.3.7  Internal Regulators
      8. 7.3.8  Power Dissipation
      9. 7.3.9  Boost Diode
      10. 7.3.10 Synchronous Rectifier Control
    4. 7.4 Programming
      1. 7.4.1 Programming The Ramp Generator Circuit and UVLO
      2. 7.4.2 Programming Soft Start
      3. 7.4.3 Programming Short-Circuit Protection
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Buck Regulator 8-V to 16-V Input, 1.8-V Output at 10 A
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Power Train Components
            1. 8.2.1.2.1.1  Output Inductor, LOUT
            2. 8.2.1.2.1.2  Output Capacitor, COUT, ELCO and MLCC
            3. 8.2.1.2.1.3  Input Capacitor, CIN ELCO and MLCC
            4. 8.2.1.2.1.4  Switching MOSFET, QSW
            5. 8.2.1.2.1.5  Rectifier MOSFET, QSR
            6. 8.2.1.2.1.6  Timing Resistor, RT
            7. 8.2.1.2.1.7  Feed-Forward and UVLO Resistor, RKFF
            8. 8.2.1.2.1.8  Soft-Start Capacitor, CSS
            9. 8.2.1.2.1.9  Short-Circuit Protection, RILIM and CILIM
            10. 8.2.1.2.1.10 Boost Voltage, CBOOST and DBOOST (Optional)
            11. 8.2.1.2.1.11 Closing the Feedback Loop, RZ1, RP1, RPZ2, RSET1, RSET2, CZ2, CP2, and CPZ1
        3. 8.2.1.3 Application Curves
    3. 8.3 Additional System Examples
  9. 9 Layout
    1. 9.1 Layout Guidelines
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Third-Party Products Disclaimer
    2. 10.2 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Receiving Notification of Documentation Updates
    4. 10.4 Community Resources
    5. 10.5 Trademarks
    6. 10.6 Electrostatic Discharge Caution
    7. 10.7 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

8.2.1.2.1.9 Short-Circuit Protection, RILIM and CILIM

Short-circuit protection is programmed using the RILIM resistor. Selection of this resistor depends on the RDS(on) of the switching MOSFET selected and the required short-circuit current trip point, ISCP. The minimum ISCP is limited by the inductor peak current, the output voltage, the output capacitor, and the soft-start time. Their relationship is given by Equation 38. A short-circuit current trip point greater than that calculated by Equation 38 should be used.

Equation 38. TPS40077 q21_iscp_lus714.gif

The minimum short-circuit current trip point for this design is 12.25 A. This value is used in Equation 39 to calculate the minimum RILIM value.

Equation 39. TPS40077 q22_rilim_lus714.gif

RILIM is calculated to be 1.17 kΩ, and a 1.2-kΩ resistor is used to verify that the short-circuit current requirements are met. The minimum and maximum short-circuit current can be calculated using Equation 40 and Equation 41.

Equation 40. TPS40077 q23_iscpmin_lus714.gif
Equation 41. TPS40077 q24_iscpmax_lus714.gif

where: VILIM(MAX) and VILIM(MIN) are maximum and minimum voltages across the high side FET when it is turned on, taking into account temperature variations.

The minimum ISCP is 12.25 A, and the maximum is 34 A.

It is also recommended to add a small capacitor, CILIM, across RILIM. The value of this capacitor should be about half the value calculated in Equation 42.

Equation 42. TPS40077 q25_cilim_lus714.gif

This equation yields a maximum CILIM as 55 pF. A smaller value of 27 pF is chosen is chosen.