SNVS954H February   2013  – June 2017 LM5122


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings: LM5122, LM5122Z
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Undervoltage Lockout (UVLO)
      2. 7.3.2  High Voltage VCC Regulator
      3. 7.3.3  Oscillator
      4. 7.3.4  Slope Compensation
      5. 7.3.5  Error Amplifier
      6. 7.3.6  PWM Comparator
      7. 7.3.7  Soft-Start
      8. 7.3.8  HO and LO Drivers
      9. 7.3.9  Bypass Operation (VOUT = VIN)
      10. 7.3.10 Cycle-by-Cycle Current Limit
      11. 7.3.11 Clock Synchronization
      12. 7.3.12 Maximum Duty Cycle
      13. 7.3.13 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 MODE Control (Forced-PWM Mode and Diode-Emulation Mode)
      2. 7.4.2 MODE Control (Skip-Cycle Mode and Pulse-Skipping Mode)
      3. 7.4.3 Hiccup-Mode Overload Protection
      4. 7.4.4 Slave Mode and SYNCOUT
  8. Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Feedback Compensation
      2. 8.1.2 Sub-Harmonic Oscillation
      3. 8.1.3 Interleaved Boost Configuration
      4. 8.1.4 DCR Sensing
      5. 8.1.5 Output Overvoltage Protection
      6. 8.1.6 SEPIC Converter Simplified Schematic
      7. 8.1.7 Non-Isolated Synchronous Flyback Converter Simplified Schematic
      8. 8.1.8 Negative to Positive Conversion
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1.  Custom Design With WEBENCH® Tools
        2.  Timing Resistor RT
        3.  UVLO Divider RUV2, RUV1
        4.  Input Inductor LIN
        5.  Current Sense Resistor RS
        6.  Current Sense Filter RCSFP, RCSFN, CCS
        7.  Slope Compensation Resistor RSLOPE
        8.  Output Capacitor COUT
        9.  Input Capacitor CIN
        10. VIN Filter RVIN, CVIN
        11. Bootstrap Capacitor CBST and Boost Diode DBST
        12. VCC Capacitor CVCC
        13. Output Voltage Divider RFB1, RFB2
        14. Soft-Start Capacitor CSS
        15. Restart Capacitor CRES
        16. Low-Side Power Switch QL
        17. High-Side Power Switch QH and Additional Parallel Schottky Diode
        18. Snubber Components
        19. Loop Compensation Components CCOMP, RCOMP, CHF
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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


Layout Guidelines

In a boost regulator, the primary switching loop consists of the output capacitor and N-channel MOSFET power switches. Minimizing the area of this loop reduces the stray inductance and minimizes noise. Especially, placing high quality ceramic output capacitors as close to this loop earlier than bulk aluminum output capacitors minimizes output voltage ripple and ripple current of the aluminum capacitors.

In order to prevent a dv/dt induced turn-on of high-side switch, connect HO and SW to the gate and source of the high-side synchronous N-channel MOSFET switch through short and low inductance paths. In FPWM mode, the dv/dt induced turnon can occur on the low-side switch. Connect LO and PGND to the gate and source of the low-side N-channel MOSFET, through short and low inductance paths. All of the power ground connections must be connected to a single point. Also, all of the noise sensitive low power ground connections must be connected together near the AGND pin, and a single connection must be made to the single point PGND. CSP and CSN are high-impedance pins and noise sensitive. Route CSP and CSN traces together with kelvin connections to the current sense resistor as short as possible. If needed, place 100-pF ceramic filter capacitor close to the device. MODE pin is also high impedance and noise sensitive. If an external pullup or pulldown resistor is used at MODE pin, place the resistor close to the device. VCC, VIN and BST capacitor must be as physically close as possible to the device.

The LM5122 has an exposed thermal pad to aid power dissipation. Adding several vias under the exposed pad helps conduct heat away from the device. The junction to ambient thermal resistance varies with application. The most significant variables are the area of copper in the PC board, the number of vias under the exposed pad and the amount of forced air cooling. The integrity of the solder connection from the device exposed pad to the PC board is critical. Excessive voids greatly decrease the thermal dissipation capacity. The highest power dissipating components are the two power switches. Selecting N-channel MOSFET switches with exposed pads aids the power dissipation of these devices.

Layout Example

LM5122 lm5122_layout_snvs954.gif Figure 49. Power Path Layout