SNVS508D October   2007  – September 2016 LM3405A

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
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 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 Boost Function
      2. 7.3.2 Setting the LED Current
      3. 7.3.3 Output Voltage
      4. 7.3.4 Enable Mode / Shutdown Mode
      5. 7.3.5 LED PWM Dimming
      6. 7.3.6 Undervoltage Lockout
      7. 7.3.7 Current Limit
      8. 7.3.8 Overcurrent Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Thermal Shutdown
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Inductor (L1)
      2. 8.1.2 Input Capacitor (C1)
      3. 8.1.3 Output Capacitor (C2)
      4. 8.1.4 Feed-Forward Capacitor (C4)
      5. 8.1.5 Catch Diode (D1)
      6. 8.1.6 Boost Diode (D2)
      7. 8.1.7 Boost Capacitor (C3)
      8. 8.1.8 Power Loss Estimation
    2. 8.2 Typical Applications
      1. 8.2.1 VBOOST Derived from VIN (VIN = 5 V, IF = 1 A)
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Calculate Duty Cycle (D)
          2. 8.2.1.2.2 Choose Capacitor Values (C1, C2, C3, and C4)
          3. 8.2.1.2.3 Set the Nominal LED Current (R1)
          4. 8.2.1.2.4 Choose Diodes (D1 and D2)
          5. 8.2.1.2.5 Calculate the Inductor Value (L1)
        3. 8.2.1.3 Application Curve
    3. 8.3 System Examples
      1. 8.3.1 VBOOST Derived from VOUT (VIN = 12 V, IF = 1 A)
      2. 8.3.2 VBOOST Derived from VIN through a Series Zener Diode (D3) (VIN = 15 V, IF = 1 A)
      3. 8.3.3 VBOOST Derived from VIN through a Shunt Zener Diode (D3) (VIN = 18 V, IF = 1 A)
      4. 8.3.4 LED MR16 Lamp Application (VIN = 12-V AC, IF = 0.75 A)
      5. 8.3.5 VBOOST Derived from VOUT through a Series Zener Diode (D3) ( VIN = 18 V, IF = 1 A )
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resource
    4. 11.4 Trademarks
      1. 11.3 Community Resource
      2. 11.4 Trademarks
      3. 11.5 Electrostatic Discharge Caution
      4. 11.6 Glossary
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information
  13. 12Mechanical, Packaging, and Orderable Information

Package Options

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

10 Layout

10.1 Layout Guidelines

When planning the layout there are a few things to consider when trying to achieve a clean, regulated output. The most important consideration when completing the layout is the close coupling of the GND connections of the input capacitor C1 and the catch diode D1. These ground ends should be close to one another and be connected to the GND plane with at least two vias. Place these components as close to the IC as possible. The next consideration is the location of the GND connection of the output capacitor C2, which should be near the GND connections of C1 and D1.

There should be a continuous ground plane on the bottom layer of a two-layer board.

The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup that causes inaccurate regulation. The LED current setting resistor R1 should be placed as close as possible to the IC, with the GND of R1 placed as close as possible to the GND of the IC. The VOUT trace to LED anode should be routed away from the inductor and any other traces that are switching.

High AC currents flow through the VIN, SW and VOUT traces, so they should be as short and wide as possible. Radiated noise can be decreased by choosing a shielded inductor.

The remaining components should also be placed as close as possible to the IC. See AN-1229 SIMPLE SWITCHER® PCB Layout Guidelines (SNVA054) for further considerations and the LM3405A demo board as an example of a four-layer layout.

10.2 Layout Example

LM3405A snvs508_layout2.gif Figure 30. Layout Example (MSOP-PowerPAD Package, Schematic in Figure 23)