SNVS891H September   2012  – September 2015 LM3642

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 Timing Requirements
    7. 6.7 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 Power Amplifier Synchronization (TX/TORCH)
      2. 7.3.2 Input Voltage Flash Monitor (IVFM)
      3. 7.3.3 Fault and Protections
        1. 7.3.3.1 Fault Operation
        2. 7.3.3.2 Flash Time-Out
        3. 7.3.3.3 Overvoltage Protection (OVP)
        4. 7.3.3.4 Current Limit
        5. 7.3.3.5 Undervoltage Lockout (UVLO)
        6. 7.3.3.6 Thermal Shutdown (TSD)
        7. 7.3.3.7 LED and/or VOUT Fault
    4. 7.4 Device Functional Modes
      1. 7.4.1 Start-up (Enabling the Device)
      2. 7.4.2 Pass Mode
      3. 7.4.3 Flash Mode
      4. 7.4.4 Torch Mode
      5. 7.4.5 Indicator Mode
    5. 7.5 Programming
      1. 7.5.1 I2C-Compatible Interface
        1. 7.5.1.1 Data Validity
        2. 7.5.1.2 Start and Stop Conditions
        3. 7.5.1.3 Transferring Data
        4. 7.5.1.4 I2C-Compatible Chip Address
        5. 7.5.1.5 Transferring Data
    6. 7.6 Register Map
      1. 7.6.1 Register Descriptions
        1. 7.6.1.1 Enable Register (0x0A)
        2. 7.6.1.2 Flags Register (0x0B)
        3. 7.6.1.3 Flash Features Register (0x08)
        4. 7.6.1.4 Current Control Register (0x09)
        5. 7.6.1.5 Input Voltage Flash Monitor (IVFM) Mode Register (0x01)
        6. 7.6.1.6 Torch Ramp Time Register (0x06)
        7. 7.6.1.7 Silicon Revision Register
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Output Capacitor Selection
        2. 8.2.2.2 Input Capacitor Selection
        3. 8.2.2.3 Inductor Selection
      3. 8.2.3 Application Curves
  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 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Trademarks
    4. 11.4 Community Resources
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LM3642 can drive one flash LED at currents up to 1.5 A. The 4-MHz DC-DC boost regulator allows for the use of small value discrete external components.

8.2 Typical Application

LM3642 30178901.gifFigure 17. Typical Application Circuit

8.2.1 Design Requirements

Example requirements based on default register values:

Table 2. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
Input voltage range 2.5 V to 5.5 V
Brightness control I2C Register
LED configuration 1 Flash LED
Boost switching frequency 4 MHz
Flash brightness 1.5 A maximum

Table 3. Application Circuit Component List

COMPONENT MANUFACTURER VALUE PART NUMBER SIZE CURRENT/VOLTAGE RATING (RESISTANCE)
L TOKO 1 µH DFE201610C 2 mm × 1.6 mm × 1 mm 2.5 A
COUT Murata 10 µF GRM188R60J106M 1.6 mm × 0.8 mm × 0.8 mm (0603) 6.3 V
CIN
LED Lumiled PWF-4 VF = 3.6 V, @1.5 A

8.2.2 Detailed Design Procedure

8.2.2.1 Output Capacitor Selection

The LM3642 is designed to operate with at least a 10-µF ceramic output capacitor. When the boost converter is running the output capacitor supplies the load current during the boost converter's on-time. When the NMOS switch turns off the inductor energy is discharged through the internal PMOS switch, supplying power to the load and restoring charge to the output capacitor. This causes a sag in the output voltage during the on-time and a rise in the output voltage during the off-time. The output capacitor is therefore chosen to limit the output ripple to an acceptable level depending on load current and input/output voltage differentials and also to ensure the converter remains stable.

For proper operation the output capacitor must be at least a 10-µF ceramic. Larger capacitors such as a 22-µF capacitor or capacitors in parallel can be used if lower output voltage ripple is desired. To estimate the output voltage ripple considering the ripple due to capacitor discharge (ΔVQ) and the ripple due to the capacitors ESR (ΔVESR) use the following equations:

For continuous conduction mode, the output voltage ripple due to the capacitor discharge is:

Equation 1. LM3642 30178927.gif

The output voltage ripple due to the output capacitors ESR is found by:

Equation 2. LM3642 30178928.gif

In ceramic capacitors the ESR is very low so a close approximation is to assume that 80% of the output voltage ripple is due to capacitor discharge and 20% from ESR. Table 4 lists different manufacturers for various output capacitors and their case sizes suitable for use with the LM3642.

8.2.2.2 Input Capacitor Selection

Choosing the correct size and type of input capacitor helps minimize the voltage ripple caused by the switching of the LM3642 device’s boost converter, and reduces noise on the boost converter's input terminal that can feed through and disrupt internal analog signals. In the typical application circuit a 10-µF ceramic input capacitor works well. It is important to place the input capacitor as close as possible to the LM3642 device’s input (IN) pin. This reduces the series resistance and inductance that can inject noise into the device due to the input switching currents. Table 4 lists various input capacitors that are recommended for use with the LM3642.

Table 4. Recommended Input and Output Capacitors (X5R/X7R Dielectric)

MANUFACTURER PART NUMBER VALUE CASE SIZE VOLTAGE RATING
TDK Corporation C1608JB0J106M 10 µF 0603 (1.6 mm × 0.8 mm × 0.8 mm) 6.3 V
TDK Corporation C2012JB1A106M 10 µF 0805 (2 mm × 1.25 mm × 1.25 mm) 10 V
TDK Corporation C2012JB0J226M 22 µF 0805 (2 mm × 1.25 mm × 1.25 mm) 6.3 V
Murata GRM188R60J106M 10 µF 0603 (1.6 mm × 0.8 mm × 0.8 mm) 6.3 V
Murata GRM21BR61A106KE19 10 µF 0805 (2 mm × 1.25 mm × 1.25 mm) 10 V
Murata GRM21BR60J226ME39L 22 µF 0805 (2 mm × 1.25 mm × 1.25 mm) 6.3 V

8.2.2.3 Inductor Selection

The LM3642 is designed to use a 1-µH or 0.47-µH inductor. Table 5 lists various inductors and their manufacturers that can work well with the LM3642. When the device is boosting (VOUT > VIN) the inductor will typically be the largest area of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible series resistance is important. Additionally, the saturation rating of the inductor should be greater than the maximum operating peak current of the LM3642. This prevents excess efficiency loss that can occur with inductors that operate in saturation. For proper inductor operation and circuit performance, ensure that the inductor saturation and the peak current limit setting of the LM3642 are greater than IPEAK in the following calculation:

Equation 3. LM3642 30178929.gif

where ƒSW = 4 MHz, and efficiency can be found in the Typical Characteristics plots.

Table 5. Recommended Inductors

MANUFACTURER L PART NUMBER DIMENSIONS (L×W×H) ISAT RDC
TOKO 1 µH DFE252010C 2.5 mm × 2 mm × 1 mm 2.7 A 78 mΩ
TOKO 1 µH DFE252012C 2.5 mm × 2 mm × 1.2 mm 3 A 59 mΩ
TOKO 0.47 µH DFE201612C 2 mm × 1.6 mm × 1.2 mm 3.4 A 82 mΩ
TOKO 1 µH DFE201610C 2 mm × 1.6 mm × 1 mm 2.5 A 79 mΩ

8.2.3 Application Curves

LM3642 30178993.gifFigure 18. Flash LED Efficiency vs. VIN
VLED = 3.8 V, ILED = 1.5 A
LM3642 30178950.gifFigure 20. Input Voltage Flash Monitor Stop and Hold Mode with Default Settings
LM3642 30178995.gifFigure 19. Torch LED Efficiency vs. VIN
VLED = 3.7 V, ILED = 375 mA
LM3642 30178951.gifFigure 21. Flash Mode to Torch Mode Transition
LM3642 30178954.gifFigure 22. Torch Mode to Flash Mode Transition
LM3642 30178957.gifFigure 24. VLED Short Fault
LM3642 30178956.gifFigure 26. TX Transition Plot Behavior of LED Current Shown on Enabling the Part in a TX Event and Upon TX Interrupting During a Flash
LM3642 30178955.gifFigure 23. Indicator Mode - Torch Mode - Flash Mode Transitions
LM3642 30178958.gifFigure 25. VOUT Short Fault