SNVSCU2A August   2024  – August 2024 LM5137-Q1

ADVANCE INFORMATION  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
    1. 5.1 Wettable Flanks
  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 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  Input Voltage Range (VIN)
      2. 7.3.2  Bias Supply Regulator (VCC, BIAS1/VOUT1, VDDA)
      3. 7.3.3  Precision Enable (EN1, EN2)
      4. 7.3.4  Switching Frequency (RT)
      5. 7.3.5  Pulse Frequency Modulation and Synchronization (PFM/SYNC)
      6. 7.3.6  Synchronization Out (SYNCOUT)
      7. 7.3.7  Dual Random Spread Spectrum (DRSS)
      8. 7.3.8  Configurable Soft Start (RSS)
      9. 7.3.9  Output Voltage Setpoints (FB1, FB2)
      10. 7.3.10 Minimum Controllable On-Time
      11. 7.3.11 Error Amplifier and PWM Comparator (FB1, FB2, COMP1, COMP2)
        1. 7.3.11.1 Slope Compensation
      12. 7.3.12 Inductor Current Sense (ISNS1+, BIAS1/VOUT1, ISNS2+, VOUT2)
        1. 7.3.12.1 Shunt Current Sensing
        2. 7.3.12.2 Inductor DCR Current Sensing
      13. 7.3.13 MOSFET Gate Drivers (HO1, HO2, LO1, LO2)
      14. 7.3.14 Output Configurations (CNFG)
        1. 7.3.14.1 Independent Dual-Output Operation
        2. 7.3.14.2 Single-Output Interleaved Operation
        3. 7.3.14.3 Single-Output Multiphase Operation
    4. 7.4 Device Functional Modes
      1. 7.4.1 Sleep Mode
      2. 7.4.2 PFM Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Power Train Components
        1. 8.1.1.1 Power MOSFETs
        2. 8.1.1.2 Buck Inductor
        3. 8.1.1.3 Output Capacitors
        4. 8.1.1.4 Input Capacitors
        5. 8.1.1.5 EMI Filter
      2. 8.1.2 Error Amplifier and Compensation
    2. 8.2 Typical Applications
      1. 8.2.1 Design 1 – Dual 5V and 3.3V, 20A Buck Regulator for 12V Automotive Battery Applications
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 8.2.1.2.2 Custom Design With Excel Quickstart Tool
          3. 8.2.1.2.3 Inductor Calculations
          4. 8.2.1.2.4 Shunt Resistors
          5. 8.2.1.2.5 Ceramic Output Capacitors
          6. 8.2.1.2.6 Ceramic Input Capacitors
          7. 8.2.1.2.7 Feedback Resistors
          8. 8.2.1.2.8 Input Voltage UVLO Resistors
          9. 8.2.1.2.9 Compensation Components
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Design 2 – Two-Phase, Single-Output Buck Regulator for Automotive ADAS Applications
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
      3. 8.2.3 Design 3 – 12V, 20A, 400kHz, Two-Phase Buck Regulator for 48V Automotive Applications
        1. 8.2.3.1 Design Requirements
        2. 8.2.3.2 Detailed Design Procedure
        3. 8.2.3.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Power Stage Layout
        2. 8.4.1.2 Gate Drive Layout
        3. 8.4.1.3 PWM Controller Layout
        4. 8.4.1.4 Thermal Design and Layout
        5. 8.4.1.5 Ground Plane Design
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
      2. 9.1.2 Development Support
        1. 9.1.2.1 Custom Design With WEBENCH® Tools
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
        1. 9.2.1.1 PCB Layout Resources
        2. 9.2.1.2 Thermal Design Resources
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information
    1. 11.1 Tape and Reel Information

8.1.1.1 Power MOSFETs

The choice of power MOSFETs has an outsized 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 of RDS(on) and QG is commonly specified as a MOSFET figure-of-merit. Low thermal resistance of a given package makes sure that the MOSFET power dissipation does not result in excessive MOSFET die temperature.

The main parameters affecting power MOSFET selection in an LM5137-Q1 application are as follows:

  • RDS(on) at VGS = 5V
  • Drain-source voltage rating, BVDSS, is typically 40V, 60V or 80V, depending on the maximum input voltage.
  • Gate charge parameters at VGS = 5V
  • 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 2.5V to 3.2V, the 5V gate drive amplitude of the LM5137-Q1 provides an adequately-enhanced MOSFET when on and a margin against Cdv/dt shoot-through when off.

The MOSFET-related power losses for one channel are summarized by the equations presented in Table 8-1, 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 switch-node voltage ringing, are not included. A comprehensive Quickstart Calculator available from the LM5137-Q1 product folder provides power loss calculations based on the entered MOSFET parameters, including RDS(on) and QG.

Table 8-1 MOSFET Power Losses
POWER LOSS MODE HIGH-SIDE MOSFET LOW-SIDE MOSFET
MOSFET conduction(2)(3) LM5137-Q1 LM5137-Q1
MOSFET switching LM5137-Q1 Negligible
MOSFET gate drive(1) LM5137-Q1 LM5137-Q1
MOSFET output charge(4) LM5137-Q1
Body diode conduction N/A LM5137-Q1
Body diode reverse recovery(5) LM5137-Q1
(1) Gate drive loss is apportioned based on the internal gate resistance of the MOSFET, externally added series gate resistance, and the relevant driver resistance of the LM5137-Q1.
(2) MOSFET RDS(on) has a positive temperature coefficient of approximately 4500ppm/°C. The MOSFET junction temperature, TJ, and the rise over ambient temperature is dependent upon the device total power dissipation and the thermal impedance. When operating at or near minimum input voltage, make sure that the MOSFET RDS(on) is rated for the available gate drive voltage.
(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 turnoff. During turn-on, however, a current flows from the input to charge the output capacitance of the low-side MOSFET. Eoss1, the energy of Coss1, is dissipated at turn-on, but this dissipation is offset by the stored energy Eoss2 on Coss2. For more detail, refer to Comparison of deadtime effects on the performance of DC-DC converters with GaN FETs and silicon MOSFETs, ECCE 2016.
(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, so make sure 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 following:

  • Losses due to conduction
  • Switching (voltage-current overlap)
  • Output charge
  • 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 is switched at zero voltage – current just communicates from the channel to the body diode or vice versa during the transition dead times. The LM5137-Q1, with the 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, optimizing the low-side MOSFET for low RDS(on) is critical. 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 LM5137-Q1 is well suited to drive TI's portfolio of power MOSFETs.