SNVS817B June   2012  – June 2019 LMR12015 , LMR12020

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Typical Application Circuit
  4. Revision History
  5. Pin Configuration and Functions
    1.     Pin Descriptions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 Recommended Operating Ratings
    3. 6.3 Electrical Characteristics
    4. 6.4 Typical Performance Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Boost Function
      2. 7.3.2  Low Input Voltage Considerations
      3. 7.3.3  High Output Voltage Considerations
      4. 7.3.4  Frequency Synchronization
      5. 7.3.5  Current Limit
      6. 7.3.6  Frequency Foldback
      7. 7.3.7  Soft Start
      8. 7.3.8  Output Overvoltage Protection
      9. 7.3.9  Undervoltage Lockout
      10. 7.3.10 Thermal Shutdown
    4. 7.4 Device Operation Modes
      1. 7.4.1 Enable Pin / Shutdown Mode
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Detailed Design Procedure
        1. 8.2.1.1  Custom Design With WEBENCH® Tools
        2. 8.2.1.2  Inductor Selection
          1. 8.2.1.2.1 Inductor Calculation Example
          2. 8.2.1.2.2 Inductor Material Selection
        3. 8.2.1.3  Input Capacitor
        4. 8.2.1.4  Output Capacitor
        5. 8.2.1.5  Catch Diode
        6. 8.2.1.6  Boost Diode (Optional)
        7. 8.2.1.7  Boost Capacitor
        8. 8.2.1.8  Output Voltage
        9. 8.2.1.9  Feedforward Capacitor (Optional)
        10. 8.2.1.10 Calculating Efficiency and Junction Temperature
          1. 8.2.1.10.1 Schottky Diode Conduction Losses
          2. 8.2.1.10.2 Inductor Conduction Losses
          3. 8.2.1.10.3 MOSFET Conduction Losses
          4. 8.2.1.10.4 MOSFET Switching Losses
          5. 8.2.1.10.5 IC Quiescent Losses
          6. 8.2.1.10.6 MOSFET Driver Losses
          7. 8.2.1.10.7 Total Power Losses
          8. 8.2.1.10.8 Efficiency Calculation Example
          9. 8.2.1.10.9 Calculating the LMR2015/20 Junction Temperature
      2. 8.2.2 Application Curves
      3. 8.2.3 LMR12015/20 Circuit Examples
  9. Layout
    1. 9.1 Layout Considerations
      1. 9.1.1 Compact Layout
      2. 9.1.2 Ground Plane and Shape Routing
      3. 9.1.3 FB Loop
      4. 9.1.4 PCB Summary
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Third-Party Products Disclaimer
      2. 10.1.2 Development Support
        1. 10.1.2.1 Custom Design With WEBENCH® Tools
    2. 10.2 Related Links
    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

Package Options

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

Inductor Selection

Inductor selection is critical to the performance of the LMR12015/20. The selection of the inductor affects stability, transient response and efficiency. A key factor in inductor selection is determining the ripple current (ΔiL) (see Figure 22).

The ripple current (ΔiL) is important in many ways.

First, by allowing more ripple current, lower inductance values can be used with a corresponding decrease in physical dimensions and improved transient response. On the other hand, allowing less ripple current will increase the maximum achievable load current and reduce the output voltage ripple (see Output Capacitor section for more details on calculating output voltage ripple). Increasing the maximum load current is achieved by ensuring that the peak inductor current (ILPK) never exceeds the minimum current limit of 2 A minimum (LMR12015) or 2.5 A minimum (LMR12020) .

Equation 8. ILPK = IOUT + ΔiL / 2

Secondly, the slope of the ripple current affects the current control loop. The LMR12015/20 has a fixed slope corrective ramp. When the slope of the current ripple becomes significantly less than the converter’s corrective ramp (see ), the inductor pole will move from high frequencies to lower frequencies. This negates one advantage that peak current-mode control has over voltage-mode control, which is, a single low frequency pole in the power stage of the converter. This can reduce the phase margin, crossover frequency and potentially cause instability in the converter. Contrarily, when the slope of the ripple current becomes significantly greater than the converter’s corrective ramp, resonant peaking can occur in the control loop. This can also cause instability (sub-harmonic oscillation) in the converter. For the power supply designer this means that for lower switching frequencies the current ripple must be increased to keep the inductor pole well above crossover. It also means that for higher switching frequencies the current ripple must be decreased to avoid resonant peaking.

With all these factors, how is the desired ripple current selected? The ripple ratio (r) is defined as the ratio of inductor ripple current (ΔiL) to output current (IOUT), evaluated at maximum load:

Equation 9. LMR12015 LMR12020 30197022.gif

A good compromise between physical size, transient response and efficiency is achieved when we set the ripple ratio between 0.2 and 0.4. The recommended ripple ratio vs. duty cycle shown below (see Figure 27) is based upon this compromise and control loop optimizations. Note that this is just a guideline. See Application note AN-1197 AN-1197 Selecting Inductors for Buck Converters for further considerations.

LMR12015 LMR12020 30197027.pngFigure 27. Recommended Ripple Ratio vs Duty Cycle

The duty cycle (D) can be approximated quickly using the ratio of output voltage (VOUT) to input voltage (VIN):

Equation 10. LMR12015 LMR12020 30197038.gif

Use the application's lowest input voltage to calculate the ripple ratio. The catch diode forward voltage drop (VD1) and the voltage drop across the internal NFET (VDS) must be included to calculate a more accurate duty cycle. Calculate D by using the following formula:

Equation 11. LMR12015 LMR12020 30197021.gif

VDS can be approximated by:

Equation 12. VDS = IOUT × RDS(ON)

The diode forward drop (VD1) can range from 0.3 V to 0.5 V depending on the quality of the diode. The lower VD1 is, the higher the operating efficiency of the converter.

Now that the ripple current or ripple ratio is determined, the required inductance is calculated by:

Equation 13. LMR12015 LMR12020 30197023.gif

where

  • DMIN is the duty cycle calculated with the maximum input voltage
  • ƒSW is the switching frequency
  • IOUT is the maximum output current of 2 A

Using IOUT = 2 A minimizes the inductor's physical size.