SNVSB32B August   2018  – June 2021 LM5146-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Pin Configuration and Functions
    1. 6.1 Wettable Flanks
  7. 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 Switching Characteristics
    7. 7.7 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Input Range (VIN)
      2. 8.3.2  Output Voltage Setpoint and Accuracy (FB)
      3. 8.3.3  High-Voltage Bias Supply Regulator (VCC)
      4. 8.3.4  Precision Enable (EN/UVLO)
      5. 8.3.5  Power Good Monitor (PGOOD)
      6. 8.3.6  Switching Frequency (RT, SYNCIN)
        1. 8.3.6.1 Frequency Adjust
        2. 8.3.6.2 Clock Synchronization
      7. 8.3.7  Configurable Soft Start (SS/TRK)
        1. 8.3.7.1 Tracking
      8. 8.3.8  Voltage-Mode Control (COMP)
      9. 8.3.9  Gate Drivers (LO, HO)
      10. 8.3.10 Current Sensing and Overcurrent Protection (ILIM)
      11. 8.3.11 OCP Duty Cycle Limiter
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Standby Mode
      3. 8.4.3 Active Mode
      4. 8.4.4 Diode Emulation Mode
      5. 8.4.5 Thermal Shutdown
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Design and Implementation
      2. 9.1.2 Power Train Components
        1. 9.1.2.1 Inductor
        2. 9.1.2.2 Output Capacitors
        3. 9.1.2.3 Input Capacitors
        4. 9.1.2.4 Power MOSFETs
      3. 9.1.3 Control Loop Compensation
      4. 9.1.4 EMI Filter Design
    2. 9.2 Typical Applications
      1. 9.2.1 Design 1 – 12-A High-Efficiency Synchronous Buck DC/DC Regulator for Automotive Applications
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Custom Design With WEBENCH® Tools
        4. 9.2.1.4 Custom Design With Excel Quickstart Tool
        5. 9.2.1.5 Application Curves
      2. 9.2.2 Design 2 – High Density, 12-V, 8-A Rail From 48-V Automotive Battery Power
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Power Stage Layout
      2. 11.1.2 Gate Drive Layout
      3. 11.1.3 PWM Controller Layout
      4. 11.1.4 Thermal Design and Layout
      5. 11.1.5 Ground Plane Design
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
      2. 12.1.2 Development Support
        1. 12.1.2.1 Custom Design With WEBENCH® Tools
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
        1. 12.2.1.1 PCB Layout Resources
        2. 12.2.1.2 Thermal Design Resources
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Support Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

9.1.2.4 Power MOSFETs

The choice of power MOSFETs has significant impact on DC/DC regulator performance. A MOSFET with low on-state resistance, RDS(on), reduces conduction loss, whereas low parasitic capacitances enable faster transition times and reduced switching loss. Normally, the lower the RDS(on) of a MOSFET, the higher the gate charge and output charge (QG and QOSS respectively), and vice versa. As a result, the product RDS(on) × QG is commonly specified as a MOSFET figure-of-merit. Low thermal resistance ensures that the MOSFET power dissipation does not result in excessive MOSFET die temperature.

The main parameters affecting power MOSFET selection in a LM5146-Q1 application are as follows:

  • RDS(on) at VGS = 7.5 V
  • Drain-source voltage rating, BVDSS, typically 60 V, 80 V, or 100 V, depending on maximum input voltage
  • Gate charge parameters at VGS = 7.5 V
  • Output charge, QOSS, at the relevant input voltage
  • Body diode reverse recovery charge, QRR
  • Gate threshold voltage, VGS(th), derived from the Miller plateau evident in the QG versus VGS plot in the MOSFET data sheet. With a Miller plateau voltage typically in the range of 2 V to 5 V, the 7.5-V gate drive amplitude of the LM5146-Q1 provides an adequately-enhanced MOSFET when on and a margin against Cdv/dt shoot-through when off.

The MOSFET-related power losses are summarized by the equations presented in Table 9-1, where suffixes 1 and 2 represent high-side and low-side MOSFET parameters, respectively. While the influence of inductor ripple current is considered, second-order loss modes, such as those related to parasitic inductances and SW node ringing, are not included. Consult the LM5146-Q1 Quickstart Calculator to assist with power loss calculations.

Table 9-1 Buck Regulator MOSFET Power Losses
POWER LOSS MODEHIGH-SIDE MOSFETLOW-SIDE MOSFET
MOSFET conduction(2)(3)GUID-6B479E60-EF14-436F-B1F8-93B8AF626552-low.gifGUID-3C64A1B4-3335-48D9-9558-3D632DA634D0-low.gif
MOSFET switchingGUID-4C4704BA-0590-4CB9-943C-9E9E057F1E85-low.gifNegligible
MOSFET gate drive(1)GUID-A59FF61E-213E-436F-81E4-D456CCEFD379-low.gifGUID-8ED20F4E-6D39-46CD-8466-3FE94ED9ED61-low.gif
MOSFET output charge(4)GUID-14930524-D155-409D-99E6-2791C56DBBC0-low.gif
Body diode conductionN/AGUID-1C353694-5B9D-48C5-B292-FB4CF04E3769-low.gif
Body diode reverse recovery(5)GUID-D3FA1ABC-B792-43CC-96E9-3F318BC4E1F6-low.gif
(1) Gate drive loss is apportioned based on the internal gate resistance of the MOSFET, externally-added series gate resistance and the relevant gate driver resistance of the LM5146-Q1.
(2) MOSFET RDS(on) has a positive temperature coefficient of approximately 4500 ppm/°C. The MOSFET junction temperature, TJ, and its rise over ambient temperature is dependent upon the device total power dissipation and its thermal impedance. When operating at or near minimum input voltage, ensure that the MOSFET RDS(on) is rated at VGS = 4.5 V.
(3) D' = 1–D is the duty cycle complement.
(4) MOSFET output capacitances, Coss1 and Coss2, are highly non-linear with voltage. These capacitances are charged losslessly by the inductor current at high-side MOSFET turn-off. During turn-on, however, a current flows from the input to charge Coss2 of the low-side MOSFET. Eoss1, the energy of Coss1, is dissipated at turnon, but this is offset by the stored energy Eoss2 on Coss2.
(5) MOSFET body diode reverse recovery charge, QRR, depends on many parameters, particularly forward current, current transition speed, and temperature.

The high-side (control) MOSFET carries the inductor current during the PWM on-time (or D interval) and typically incurs most of the switching losses. It is therefore imperative to choose a high-side MOSFET that balances conduction and switching loss contributions. The total power dissipation in the high-side MOSFET is the sum of the losses due to conduction, switching (voltage-current overlap), output charge, and typically two-thirds of the net loss attributed to body diode reverse recovery.

The low-side (synchronous) MOSFET carries the inductor current when the high-side MOSFET is off (or 1–D interval). The low-side MOSFET switching loss is negligible as it is switched at zero voltage – current just commutates from the channel to the body diode or vice versa during the transition deadtimes. The LM5146-Q1, with its adaptive gate drive timing, minimizes body diode conduction losses when both MOSFETs are off. Such losses scale directly with switching frequency.

In high step-down ratio applications, the low-side MOSFET carries the current for a large portion of the switching period. Therefore, to attain high efficiency, it is critical to optimize the low-side MOSFET for low RDS(on). In cases where the conduction loss is too high or the target RDS(on) is lower than available in a single MOSFET, connect two low-side MOSFETs in parallel. The total power dissipation of the low-side MOSFET is the sum of the losses due to channel conduction, body diode conduction, and typically one-third of the net loss attributed to body diode reverse recovery. The LM5146-Q1 is well suited to drive TI's portfolio of NexFET™ power MOSFETs.