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

8.2.1.2.1.5 Rectifier MOSFET, QSR

Similar criteria to the foregoing can be used for the rectifier MOSFET. There is one significant difference: due to the body diode conducting, the rectifier MOSFET switches with zero voltage across its drain and source, so effectively with zero switching losses. However, there are some losses in the body diode. These are minimized by reducing the delay time between the transition from the switching MOSFET turnoff to rectifier MOSFET turnon and vice-versa. The TPS40077 incorporates TI's proprietary Predictive Gate Drive circuitry (PGD), which helps reduce these delays to around 10 ns.

To calculate the losses in the rectifier MOSFET, use Equation 30 through Equation 33.

Equation 30. TPS40077 q13_pqsr_lus714.gif
Equation 31. TPS40077 q14_pcon2_lus714.gif
Equation 32. TPS40077 q15_pbd_lus714.gif
Equation 33. TPS40077 q16_pgate_lus714.gif

where

  • PBD = body diode losses
  • t1 = body diode conduction prior to turnon of channel = 12 ns for PGD
  • t2 = body diode conduction after turnoff of channel = 12 ns for PGD
  • Vf = body diode forward voltage

Estimating the body diode losses based on a forward voltage of 1 V gives 0.072 W. The gate losses are unknown at this time, so assume 0.1-W gate losses. This leaves 0.428 W for conduction losses. Using this figure, a target RDS(on) of 5 mΩ was calculated.

The Si7336ADP from Vishay was chosen. Using the parameters from its data sheet, the actual expected power losses are calculated. Conduction loss is 0.317 W, body diode loss is 0.072 W, and the gate loss is 0.136W. This totals 0.525 W associated with the rectifier MOSFET.

Two other criteria should be verified before finalizing on the rectifier MOSFET. One is the requirement to ensure that predictive gate drive functions correctly. The turnoff delay of the Si7336ADP is 97 ns. The minimum turnoff delay of the Si7860DP is 25 ns. Together these devices meet the 130-ns requirement.

Secondly, the ratio between Cgs and Cgd should be greater than 1. The Si7336ADP easily meets this criterion. This helps reduce the risk of dv/dt-induced turnon of the rectifier MOSFET. If this is likely to be a problem, a small resistor may be added in series with the boost capacitor, CBOOST.