SNVSCL2A December   2024  – August 2025 LM51770 , LM517701

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 Handling Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Typical Characteristics
  8. 7 Parameter Measurement Information
    1. 7.1 Gate Driver Rise Time and Fall Time
    2. 7.2 Gate Driver Dead (Transition) Time
  9. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Power-On Reset (POR System)
      2. 8.3.2  Buck-Boost Control Scheme
        1. 8.3.2.1 Boost Mode
        2. 8.3.2.2 Buck Mode
        3. 8.3.2.3 Buck-Boost Mode
      3. 8.3.3  Power Save Mode
      4. 8.3.4  Supply Voltage Selection – VMAX Switch
      5. 8.3.5  Enable and Undervoltage Lockout
      6. 8.3.6  Oscillator Frequency Selection
      7. 8.3.7  Frequency Synchronization
      8. 8.3.8  Voltage Regulation Loop
      9. 8.3.9  Output Voltage Tracking
      10. 8.3.10 Slope Compensation
      11. 8.3.11 Configurable Soft Start
      12. 8.3.12 Peak Current Sensor
      13. 8.3.13 Current Monitoring and Current Limit Control Loop
      14. 8.3.14 Short Circuit - Hiccup Protection
      15. 8.3.15 nFLT Pin and Protections
      16. 8.3.16 Device Configuration Pin
      17. 8.3.17 Dual Random Spread Spectrum – DRSS
      18. 8.3.18 Gate Driver
    4. 8.4 Device Functional Modes
  10. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1  Custom Design with WEBENCH Tools
        2. 9.2.2.2  Frequency
        3. 9.2.2.3  Feedback Divider
        4. 9.2.2.4  Inductor and Current Sense Resistor Selection
        5. 9.2.2.5  Slope Compensation
        6. 9.2.2.6  Output Capacitor
        7. 9.2.2.7  Input Capacitor
        8. 9.2.2.8  UVLO Divider
        9. 9.2.2.9  Soft-Start Capacitor
        10. 9.2.2.10 MOSFETs QH1 and QL1
        11. 9.2.2.11 MOSFETs QH2 and QL2
        12. 9.2.2.12 Output Voltage Frequency Compensation
        13. 9.2.2.13 External Component Selection
      3. 9.2.3 Application Curves
    3. 9.3 System Examples
      1. 9.3.1 Bi-Directional Power Backup
      2. 9.3.2 Parallel (Multiphase) Operation
      3. 9.3.3 External Gate Driver with Logic Level High Side Gate Signals
    4. 9.4 Power Supply Recommendations
    5. 9.5 Layout
      1. 9.5.1 Layout Guidelines
        1. 9.5.1.1 Power Stage Layout
        2. 9.5.1.2 Gate Driver Layout
        3. 9.5.1.3 Controller Layout
      2. 9.5.2 Layout Example
  11. 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 Receiving Notification of Documentation Updates
    3. 10.3 Support Resources
    4. 10.4 Trademarks
    5. 10.5 Electrostatic Discharge Caution
    6. 10.6 Glossary
  12. 11Revision History
  13. 12Mechanical, Packaging, and Orderable Information
    1.     86

8.3.13 Current Monitoring and Current Limit Control Loop

The LM51770x features two high voltage current sensors. The first one maintains the peak current sensing between the CSA and CSB pins. The second current sensor inputs are connected to the ISNSP and ISNSN pins.

This second and optional current sensing provides the capability to monitor or to limit either the average input or the output current of the DC/DC converter. If the current sense amplifier is not used, disable it to reduce the bias current consumption of the whole device by connecting the IMONOUT pin to VCC. Do not do this dynamical during the operation of the device because the configuration gets latched at start-up of the converter. Use the CFG pin to select one of the following desired operation modes.

Current Monitor Operation:

In case the current sense amplifier is configured as a monitor, the output voltage on the IMONOUT pin has a linear relation between the sense voltage between ISNSP and ISNSN pins of the current sense transconductance amplifier as well as the resistor placed on the IMONOUT pin:

Equation 17. V ( I M O N O U T ) = ( V ( I S N S P ) - V ( I S N S N ) ) × g m ( I M O N ) × R ( I M O N O U T )

The output voltage of the IMONOUT pin is clamped to the values given in specifications section.

To reduce the bandwidth of the current monitor, place an optional capacitor in parallel to the IMONOUT pin.

Current Limit Operation:

In this configuration, the current sense gm(ILIM) amplifier monitors the voltage across the sense resistor and compares it with an internal reference voltage. If the drop across the sense resistor is greater than the reference threshold the gm(ILIM) amplifier gradually reduces the peak current capability of the DC/DC converter until the differential voltage is equal to the reference voltage. This function of the LM51770x is used to do the following:

  • Regulate the current into the load from the power stage
  • Regulate the current from the output into the power stage
  • Regulate the current from the input supply to the power stage
  • Regulate the current from the power stage into the input supply

If the Hiccup protection of the device is enabled the device Hiccup operation is triggered by the current limit operation threshold.

To select a negative current limit direction, the SYNC pin needs to be pulled low when EN/UVLO rises above the EN rising threshold until the soft-start ramp starts the converter operation. The configuration gets latched and the SYNC pin is used for the synchronization afterward. If the synchronization function is not used it is pulled low continuously. For a positive current limit protection the SYNC pull the pin high or connect the pin to a valid synchronization signal when EN/UVLO rises above the EN rising threshold until the soft-start ramp starts the converter operation

For the current limit operation mode, place an RC compensation network on the IMONOUT pin. For most applications, a compensation bandwidth with a factor of 3× to 5× faster than the compensation of the output voltage loop has shown good results.

LM51770 LM517701 Current Monitor Functional Block DiagramFigure 8-18 Current Monitor Functional Block Diagram