SLVSHB3 November   2025 LM51251A-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Timing Requirements
    7. 5.7 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1  Device Configuration (CFG-pin)
      2. 6.3.2  Device and Phase Enable/Disable (UVLO/EN, EN2)
      3. 6.3.3  Dual Device Operation
      4. 6.3.4  Switching Frequency and Synchronization (SYNCIN)
      5. 6.3.5  Dual Random Spread Spectrum (DRSS)
      6. 6.3.6  Operation Modes (BYPASS, DEM, FPWM)
      7. 6.3.7  VCC Regulator, BIAS (BIAS-pin, VCC-pin)
      8. 6.3.8  Soft Start (SS-pin)
      9. 6.3.9  VOUT Programming (VOUT, ATRK, DTRK)
      10. 6.3.10 Protections
        1. 6.3.10.1 VOUT Overvoltage Protection (OVP)
        2. 6.3.10.2 Thermal Shutdown (TSD)
      11. 6.3.11 Fault Indicator (nFAULT-pin)
      12. 6.3.12 Slope Compensation (CSP1, CSP2, CSN1, CSN2)
      13. 6.3.13 Current Sense Setting and Switch Peak Current Limit (CSP1, CSP2, CSN1, CSN2)
      14. 6.3.14 Input Current Limit and Monitoring (ILIM, IMON, DLY)
      15. 6.3.15 Maximum Duty Cycle and Minimum Controllable On-time Limits
      16. 6.3.16 Signal Deglitch Overview
      17. 6.3.17 MOSFET Drivers, Integrated Boot Diode, and Hiccup Mode Fault Protection (LOx, HOx, HBx-pin)
      18. 6.3.18 I2C Features
        1. 6.3.18.1 Register VOUT (0x0)
        2. 6.3.18.2 Register Configuration 1 (0x1)
        3. 6.3.18.3 Register Configuration 2 (0x2)
        4. 6.3.18.4 Register Configuration 3 (0x3)
        5. 6.3.18.5 Register Operation State (0x4)
        6. 6.3.18.6 Register Status Byte (0x5)
        7. 6.3.18.7 Register Clear Faults (0x6)
    4. 6.4 Device Functional Modes
      1. 6.4.1 Shutdown State
    5. 6.5 Programming
      1. 6.5.1 I2C Bus Operation
  8. 7 LM51251A-Q1 Registers
  9. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Feedback Compensation
      2. 8.1.2 Non-synchronous Application
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Determine the Total Phase Number
        2. 8.2.2.2  Determining the Duty Cycle
        3. 8.2.2.3  Timing Resistor RT
        4. 8.2.2.4  Inductor Selection Lm
        5. 8.2.2.5  Current Sense Resistor Rcs
        6. 8.2.2.6  Current Sense Filter RCSFP, RCSFN, CCS
        7. 8.2.2.7  Low-Side Power Switch QL
        8. 8.2.2.8  High-Side Power Switch QH
        9. 8.2.2.9  Snubber Components
        10. 8.2.2.10 Vout Programming
        11. 8.2.2.11 Input Current Limit (ILIM/IMON)
        12. 8.2.2.12 UVLO Divider
        13. 8.2.2.13 Soft Start
        14. 8.2.2.14 CFG Settings
        15. 8.2.2.15 Output Capacitor Cout
        16. 8.2.2.16 Input Capacitor Cin
        17. 8.2.2.17 Bootstrap Capacitor
        18. 8.2.2.18 VCC Capacitor CVCC
        19. 8.2.2.19 BIAS Capacitor
        20. 8.2.2.20 VOUT Capacitor
        21. 8.2.2.21 Loop Compensation
      3. 8.2.3 Application Curves
        1. 8.2.3.1 Efficiency
        2. 8.2.3.2 Steady State Waveforms
        3. 8.2.3.3 Step Load Response
        4. 8.2.3.4 Sync Operation
        5. 8.2.3.5 AC Loop Response Curve
        6. 8.2.3.6 Thermal Performance
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Documentation Support
      1. 9.1.1 Related Documentation
    2. 9.2 Receiving Notification of Documentation Updates
    3. 9.3 Support Resources
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

6.3.2 Device and Phase Enable/Disable (UVLO/EN, EN2)

During shutdown the UVLO/EN-pin is pulled low by the internal resistor REN. When VUVLO/EN rises above VEN-RISING, REN is disabled and the IUVLO/EN (typically 10μA) current source is enabled to provide the UVLO functionality. The device boots up, reads the configuration and enters STANDBY state (see Functional State Diagram). The I2C Interface is activated when the device reaches the STANDBY state. When VUVLO/EN rises above VUVLO-RISING the IUVLO/EN current source is disabled and the device enters START PHASE 1 and 2 state executing the soft-start ramping up VOUT in DEM operation. A hysteresis VEN-HYS and VUVLO-HYS is implemented. Select the external UVLO resistor voltage divider (RUVLOT and RUVLOB) according to Equation 1 and Equation 2.

Equation 1. RUVLOT= (VON-VUVLO-RISINGVUVLO-FALLING×VOFF)IUVLO-HYS
Equation 2. RUVLOB= VUVLO-FALLING×RUVLOTVOFF-VUVLO-FALLING

The UVLO/EN-pin can be overwritten by I2C programming. The UVLO-bit selects if the UVLO/EN-pin voltage is used to enter START PHASE 1 and 2 state and the device starts switching or if the device enters START PHASE 1 and 2 state when UVLO is set to "1".

A UVLO capacitor (CUVLO) is required in case VI drops below VOFF momentarily during startup or a load transient at low VI. If the required UVLO capacitor is large, an additional series UVLO resistor (RUVLOS) can be used to quickly raise the voltage at the UVLO-pin when IUVLO-HYS is disabled.

The 2nd phase is enabled when VEN2 rises above VEN2_H and disabled when VEN2 falls below VEN2_L. The 2nd phase can also be enabled by I2C setting the EN2-bit to "1". Enable and disable the 2nd phase at or before startup as well as during operation. The 2nd phase is 180° phase shifted towards phase 1 for lowest input and output ripple.

The UVLO/EN-pin voltage is not allowed to exceed the BIAS-pin voltage +0.3V (see Absolute Maximum Ratings) as the ESD-diode between UVLO/EN-pin and BIAS-pin gets conducting. However, a higher voltage up to 42V (Recommended Operating Conditions) is applicable at the UVLO/EN-pin when the current is limited to maximum 100μA with a series resistor.

LM51251A-Q1 Functional Block Diagram UVLO and EN Figure 6-2 Functional Block Diagram UVLO and EN
LM51251A-Q1 Dual-phase Operation Figure 6-3 Dual-phase Operation