SNVS574G July   2008  – July 2019 LM3421 , LM3423

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Boost Application
  4. 4 Revision History
  5. 5 Device Comparison
  6. 6 Pin Configuration and Functions
    1.     Pin 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 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  Current Regulators
      2. 8.3.2  Predictive Off-Time (PRO) Control
      3. 8.3.3  Average LED Current
      4. 8.3.4  Analog Dimming
      5. 8.3.5  Current Sense and Current Limit
      6. 8.3.6  Overcurrent Protection
      7. 8.3.7  Zero Current Shutdown
      8. 8.3.8  Control Loop Compensation
      9. 8.3.9  Start-Up Regulator
      10. 8.3.10 Overvoltage Lockout (OVLO)
      11. 8.3.11 Input Undervoltage Lockout (UVLO)
        1. 8.3.11.1 UVLO Only
        2. 8.3.11.2 PWM Dimming and UVLO
      12. 8.3.12 PWM Dimming
      13. 8.3.13 LM3423 Only: DPOL, FLT, TIMR, and LRDY
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Inductor
      2. 9.1.2 LED Dynamic Resistance
      3. 9.1.3 Output Capacitor
      4. 9.1.4 Input Capacitors
      5. 9.1.5 Main MOSFET / Dimming MOSFET
      6. 9.1.6 Re-Circulating Diode
      7. 9.1.7 Boost Inrush Current
      8. 9.1.8 Switching Frequency
    2. 9.2 Typical Applications
      1. 9.2.1 Basic Topology Schematics
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1  Operating Point
          2. 9.2.1.2.2  Switching Frequency
          3. 9.2.1.2.3  Average LED Current
          4. 9.2.1.2.4  Inductor Ripple Current
          5. 9.2.1.2.5  LED Ripple Current
          6. 9.2.1.2.6  Peak Current Limit
          7. 9.2.1.2.7  Loop Compensation
          8. 9.2.1.2.8  Input Capacitance
          9. 9.2.1.2.9  N-channel FET
            1. 9.2.1.2.9.1 Boost and Buck-Boost
          10. 9.2.1.2.10 Diode
          11. 9.2.1.2.11 Output OVLO
          12. 9.2.1.2.12 Input UVLO
          13. 9.2.1.2.13 PWM Dimming Method
          14. 9.2.1.2.14 Analog Dimming Method
      2. 9.2.2 LM3421 Buck-Boost Application
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
          1. 9.2.2.2.1  Operating Point
          2. 9.2.2.2.2  Switching Frequency
          3. 9.2.2.2.3  Average LED Current
          4. 9.2.2.2.4  Inductor Ripple Current
          5. 9.2.2.2.5  Output Capacitance
          6. 9.2.2.2.6  Peak Current Limit
          7. 9.2.2.2.7  Loop Compensation
          8. 9.2.2.2.8  Input Capacitance
          9. 9.2.2.2.9  N-channel FET
          10. 9.2.2.2.10 Diode
          11. 9.2.2.2.11 Input UVLO
          12. 9.2.2.2.12 Output OVLO
        3. 9.2.2.3 Application Curve
      3. 9.2.3 LM3421 BOOST Application
        1. 9.2.3.1 Design Requirements
        2. 9.2.3.2 Detailed Design Procedure
      4. 9.2.4 LM3421 Buck-Boost Application
        1. 9.2.4.1 Design Requirements
        2. 9.2.4.2 Detailed Design Procedure
      5. 9.2.5 LM3423 Boost Application
        1. 9.2.5.1 Design Requirements
        2. 9.2.5.2 Detailed Design Procedure
      6. 9.2.6 LM3421 Buck-Boost Application
        1. 9.2.6.1 Design Requirements
        2. 9.2.6.2 Detailed Design Procedure
      7. 9.2.7 LM3423 Buck Application
        1. 9.2.7.1 Design Requirements
        2. 9.2.7.2 Detailed Design Procedure
      8. 9.2.8 LM3423 Buck-Boost Application
        1. 9.2.8.1 Design Requirements
        2. 9.2.8.2 Detailed Design Procedure
      9. 9.2.9 LM3421 SEPIC Application
        1. 9.2.9.1 Design Procedure
        2. 9.2.9.2 Detailed Design Procedure
  10. 10Power Supply Recommendations
    1. 10.1 General Recommendations
    2. 10.2 Input Supply Current Limit
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Related Links
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

9.1.8 Switching Frequency

An external resistor (RT) connected between the RCT pin and the switch node (where D1, Q1, and L1 connect), in combination with a capacitor (CT) between the RCT and AGND pins, sets the off-time (tOFF) as shown in Figure 31. For boost and buck-boost topologies, the VIN proportionality ensures a virtually constant switching frequency (fSW).

For a buck topology, RT and CT are also used to set tOFF, however the iinput voltage (VIN) proportionality does not ensure a constant switching frequency. Instead, constant ripple operation can be achieved. Changing the connection of RT in Figure 31 from VSW to VIN provides a constant ripple over varying VIN. Adding a PNP transistor as shown in Figure 32 provides constant ripple over varying VO.

The switching frequency is defined:

Buck (Constant Ripple vs. VIN)

Equation 31. LM3421 LM3423 30067389.gif

Buck (Constant Ripple vs. VO)

Equation 32. LM3421 LM3423 30067388.gif

Boost and Buck-boost

Equation 33. LM3421 LM3423 30067387.gif

For all topologies, the CT capacitor is recommended to be 1 nF and should be located very close to the LM34xx-Q1.

LM3421 LM3423 30067399.gifFigure 31. Off-timer Circuitry for Boost and Buck-boost Regulators
LM3421 LM3423 30067301.gifFigure 32. Off-timer Circuitry for Buck Regulators