SNVS714F April   2011  – August 2015 LM25117 , LM25117-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings (LM25117)
    3. 6.3 ESD Ratings (LM25117-Q1)
    4. 6.4 Recommended Operating Conditions
    5. 6.5 Thermal Information
    6. 6.6 Electrical Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  High Voltage Startup Regulator and VCC Disable
      2. 7.3.2  UVLO
      3. 7.3.3  Oscillator And Sync Capability
      4. 7.3.4  Ramp Generator and Emulated Current Sense
      5. 7.3.5  Error Amplifier and PWM Comparator
      6. 7.3.6  Soft-Start
      7. 7.3.7  Cycle-By-Cycle Current Limit
      8. 7.3.8  Hiccup Mode Current Limiting
      9. 7.3.9  HO and LO Drivers
      10. 7.3.10 Current Monitor
      11. 7.3.11 Maximum Duty Cycle
      12. 7.3.12 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Diode Emulation
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
    3. 8.3 Detailed Design Procedure
      1. 8.3.1  Feedback Compensation
      2. 8.3.2  Sub-Harmonic Oscillation
      3. 8.3.3  Design Requirements
      4. 8.3.4  Timing Resistor RT
      5. 8.3.5  Output Inductor LO
      6. 8.3.6  Diode Emulation Function
      7. 8.3.7  Current Sense Resistor RS
      8. 8.3.8  Current Sense Filter RCS and CCS
      9. 8.3.9  Ramp Resistor RRAMP and Ramp Capacitor CRAMP
      10. 8.3.10 UVLO Divider RUV2, RUV1 and CFT
      11. 8.3.11 VCC Disable and External VCC Supply
      12. 8.3.12 Power Switches QH and QL
      13. 8.3.13 Snubber Components RSNB and CSNB
      14. 8.3.14 Bootstrap Capacitor CHB and Bootstrap Diode DHB
      15. 8.3.15 VCC Capacitor CVCC
      16. 8.3.16 Output Capacitor CO
      17. 8.3.17 Input Capacitor CIN
      18. 8.3.18 VIN Filter RVIN, CVIN
      19. 8.3.19 Soft-Start Capacitor CSS
      20. 8.3.20 Restart Capacitor CRES
      21. 8.3.21 Output Voltage Divider RFB2 and RFB1
      22. 8.3.22 Loop Compensation Components CCOMP, RCOMP and CHF
    4. 8.4 Application Curves
      1. 8.4.1 Constant Current Regulator
      2. 8.4.2 Constant Voltage and Constant Current Regulator
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guideline
      1. 10.1.1 PC Board Layout Recommendations
  11. 11Device and Documentation Support
    1. 11.1 Community Resources
    2. 11.2 Related Links
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

10 Layout

10.1 Layout Guideline

LM25117 LM25117-Q1 LM25117_layout_snvs714.gifFigure 38. Layout Example

10.1.1 PC Board Layout Recommendations

In a buck regulator the primary switching loop consists of the input capacitor, NMOS power switches and current sense resistor. Minimizing the area of this loop reduces the stray inductance and minimizes noise and possible erratic operation. High quality input capacitors should be placed as close as possible to the NMOS power switches, with the VIN side of the capacitor connected directly to the high-side NMOS drain and the ground side of the capacitor connected as close as possible to the current sense resistor ground connection.

Connect all of the low power ground connections (RUV1, RT, RFB1, CSS, CRES, CCM, CVIN, CRAMP) directly to the regulator AGND pin. Connect CVCC directly to the regulator PGND pin. Note that CVIN and CVCC must be as physically close as possible to the IC. AGND and PGND must be directly connected together through a top-side copper pattern connected to the exposed pad. Ensure no high current flows beneath the underside exposed pad.

The LM25117 has an exposed thermal pad to aid power dissipation. Adding several vias under the exposed pad helps conduct heat away from the IC. 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 IC 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 NMOS switches with exposed pads aids the power dissipation of these devices.