SNVSAH5A September   2017  – May 2020 LM73605 , LM73606

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency versus Load Current VOUT = 5 V, fSW = 500 kHz, Auto Mode
  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 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 System Characteristics
    9. 6.9 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  Synchronous Step-Down Regulator
      2. 7.3.2  Auto Mode and FPWM Mode
      3. 7.3.3  Fixed-Frequency Peak Current-Mode Control
      4. 7.3.4  Adjustable Output Voltage
      5. 7.3.5  Enable and UVLO
      6. 7.3.6  Internal LDO, VCC_UVLO, and BIAS Input
      7. 7.3.7  Soft Start and Voltage Tracking
      8. 7.3.8  Adjustable Switching Frequency
      9. 7.3.9  Frequency Synchronization and Mode Setting
      10. 7.3.10 Internal Compensation and CFF
      11. 7.3.11 Bootstrap Capacitor and VBOOT-UVLO
      12. 7.3.12 Power-Good and Overvoltage Protection
      13. 7.3.13 Overcurrent and Short-Circuit Protection
      14. 7.3.14 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Standby Mode
      3. 7.4.3 Active Mode
        1. 7.4.3.1 CCM Mode
        2. 7.4.3.2 DCM Mode
        3. 7.4.3.3 PFM Mode
        4. 7.4.3.4 Fault Protection Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  Output Voltage Setpoint
        3. 8.2.2.3  Switching Frequency
        4. 8.2.2.4  Input Capacitors
        5. 8.2.2.5  Inductor Selection
        6. 8.2.2.6  Output Capacitor Selection
        7. 8.2.2.7  Feedforward Capacitor
        8. 8.2.2.8  Bootstrap Capacitors
        9. 8.2.2.9  VCC Capacitor
        10. 8.2.2.10 BIAS
        11. 8.2.2.11 Soft Start
        12. 8.2.2.12 Undervoltage Lockout Setpoint
        13. 8.2.2.13 PGOOD
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Layout For EMI Reduction
      2. 10.1.2 Ground Plane
      3. 10.1.3 Optimize Thermal Performance
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Development Support
        1. 11.1.2.1 Custom Design With WEBENCH® Tools
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Related Links
    4. 11.4 Receiving Notification of Documentation Updates
    5. 11.5 Support Resources
    6. 11.6 Trademarks
    7. 11.7 Electrostatic Discharge Caution
    8. 11.8 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.2.2.5 Inductor Selection

The first criterion for selecting an output inductor is the inductance. In most buck converters, this value is based on the desired peak-to-peak ripple current in the inductor, ILripple. An inductance that gives a ripple current of 10% to 30% of the maximum output current (5 A or 6 A) is a good starting point. The inductance can be calculated from Equation 26:

Equation 26. LM73605 LM73606 eq-L-selection-snvsah5.gif

where

  • ILripple = (0.1 to 0.3) × IL_MAX
  • IL_MAX = 5 A for LM73605 and 6 A for LM73606
  • D = VOUT / VIN

The selected ILripple is between 10% to 30% of the rated current of the device.

As with switching frequency, the selection of the inductor is a tradeoff between size, cost, and performance. Higher inductance gives lower ripple current and hence lower output voltage ripple. With peak current mode control, the current ripple is the input signal to the control loop. A certain amount of ripple current is needed to maintain the signal-to-noise ratio of the control loop. Within the same series (same size/height), a larger inductance has a higher series resistance (ESR). With similar ESR, size, height, or both are greater. Larger inductance also has slower current slew rate during large load transients.

Lower inductance usually results in a smaller, less expensive component; however, the current ripple will be higher, thus more output capacitor is needed to maintain the same amount of output voltage ripple. The RMS current is higher with the same load current due to larger ripple. The switching loss is higher because the switch current, which is the peak current, is higher when the HS switch turns off and LS switch turns on. Core loss of the inductor is also larger with higher ripple. Core loss needs to be considered, especially with higher switching frequencies. Check the ripple current over VIN_MIN to VIN_MAX range to make sure current ripple is reasonable over entire supply voltage range.

For applications with large VOUT and typical VOUT / VIN > 50%, subharmonic oscillation can be a concern in peak current-mode-controlled buck converters. Select inductance so that:

Equation 27. L ≥ VOUT / (N × fSW)

where

  • N = 3 with LM73605
  • N = 3.6 with LM73606

The second criterion is inductor saturation current rating. Because the maximum inductor current is limited by the high-side switch current limit, it is advised to select an inductor with a saturation current higher than the ILIMIT-HS. TI recommends selection of soft saturation inductors. A power inductor can be the major source of radiated noise. When EMI is a concern in the application, select a shielded inductor, if possible.

For this design, 20% ripple of 5 A yields 5.8-µH inductance. A 4.7-µH inductor is selected, which gives 25% ripple current.