SNVSB12B November   2017  – May 2021 LM73605-Q1 , LM73606-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison
  6. 6 Pin Configuration and Functions
  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 Timing Characteristics
    7. 7.7 Switching Characteristics
    8. 7.8 System Characteristics
    9. 7.9 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  Synchronous Step-Down Regulator
      2. 8.3.2  Auto Mode and FPWM Mode
      3. 8.3.3  Fixed-Frequency Peak Current-Mode Control
      4. 8.3.4  Adjustable Output Voltage
      5. 8.3.5  Enable and UVLO
      6. 8.3.6  Internal LDO, VCC_UVLO, and BIAS Input
      7. 8.3.7  Soft Start and Voltage Tracking
      8. 8.3.8  Adjustable Switching Frequency
      9. 8.3.9  Frequency Synchronization and Mode Setting
      10. 8.3.10 Internal Compensation and CFF
      11. 8.3.11 Bootstrap Capacitor and VBOOT-UVLO
      12. 8.3.12 Power-Good and Overvoltage Protection
      13. 8.3.13 Overcurrent and Short-Circuit Protection
      14. 8.3.14 Thermal Shutdown
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Standby Mode
      3. 8.4.3 Active Mode
        1. 8.4.3.1 CCM Mode
        2. 8.4.3.2 DCM Mode
        3. 8.4.3.3 PFM Mode
        4. 8.4.3.4 Fault Protection Mode
  9. 9 Layout
    1. 9.1 Layout Guidelines
      1. 9.1.1 Layout For EMI Reduction
      2. 9.1.2 Ground Plane
      3. 9.1.3 Optimize Thermal Performance
    2. 9.2 Layout Example
  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 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Receiving Notification of Documentation Updates
    4. 10.4 Receiving Notification of Documentation Updates
    5. 10.5 Support Resources
    6. 10.6 Support Resources
    7. 10.7 Trademarks
    8. 10.8 Electrostatic Discharge Caution
    9. 10.9 Glossary

Package Options

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

8.3.2 Auto Mode and FPWM Mode

The LM73605-Q1/6-Q1 pin-configurable auto mode or FPWM options.

In auto mode, the device operates in diode emulation mode (DEM) at light loads. In DEM, inductor current stops flowing when it reaches 0 A. This is also referred to as discontinuous conduction mode (DCM). This is the same behavior as the non-synchronous regulator, with higher efficiency. At heavier load, when the inductor current valley is above 0 A, the device operates in continuous conduction mode (CCM), where the switching frequency is fixed and set by RT pin.

In auto mode, the peak inductor current has a minimum limit, IPEAK_MIN, in the LM73605-Q1/6-Q1. When peak current reaches IPEAK_MIN, the switching frequency reduces to regulate the required load current. Switching frequency lowers when load reduces. This is when the device operates in pulse frequency modulation (PFM). PFM further improves efficiency by reducing switching losses. Light load efficiency is especially important for battery-operated systems.

In forced PWM (FPWM) mode, the device operates in CCM regardless of load with the frequency set by RT pin or synchronization input. Inductor current can go negative at light loads. At light loads, the efficiency is lower than auto mode, due to higher conduction losses and switching losses. In FPWM, the device has fixed switching frequency over the entire load range, which is beneficial to noise sensitive applications.

Figure 8-2 shows the inductor current waveforms in each mode with heavy load, light load, and very light load. The difference between the two modes is at lighter loads where inductor current valley reaches zero.

GUID-293291E0-76AB-4D84-AA04-F1B4A83477DF-low.gifFigure 8-2 Inductor Current Waveforms at Auto Mode and FPWM Mode with Different Loads

In CCM, the inductor current peak-to-peak ripple can be estimated by Equation 1:

Equation 1. GUID-FDCF82A3-E2A1-4596-8D25-6787EB28C987-low.gif

The average or DC value of the inductor current equals the load current, or output current IOUT, in steady state. Peak inductor current can be calculated by Equation 2:

Equation 2. IPEAK = IOUT + ILripple / 2

Valley inductor current can be calculated by Equation 3:

Equation 3. IVALLEY = IOUT – ILripple / 2

In auto mode, the CCM-to-DCM boundary condition is when IVALLEY = 0 A. When ILripple ≥ IPEAK_MIN, the load current at the DCM boundary condition can be found by Equation 4. When the peak-to-peak ripple current is smaller than ILripple ≥ IPEAK-MIN, the PFM boundary is reached first.

Equation 4. IOUT_DCM = ILripple / 2

when

  • ILripple ≥ IPEAK_MIN

In auto mode, the PFM operation boundary condition is when IPEAK = IPEAK_MIN. Frequency foldback occurs when peak current drops to IPEAK_MIN, regardless of whether it is in CCM or DCM operation. When current ripple is small, ILripple < IPEAK_MIN, the peak current reaches IPEAK_MIN when it is still in CCM. The output current at CCM PFM boundary can be found by Equation 5:

Equation 5. IOUT_CCM_PFM = IPEAK_MIN – ILripple / 2

when

  • ILripple < IPEAK_MIN

The current ripple increases with reduced frequency if load reduces. When valley current reaches zero, the frequency continues to fold back with constant peak current and discontinuous current.

In FPWM mode, there is no IPEAK-MIN limit. The peak current is defined by Equation 2 at light loads and heavy loads.

Mode setting only affects operation at light loads. There is no difference if load current is above the DCM and PFM boundary conditions discussed above.

See the Section 8.3.9 section for mode setting options in the LM73605-Q1/6-Q1.