SNVS555D January   2008  – December 2014 LM3881

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 Handling 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 Adjustable Timing
      2. 7.3.2 Enable Circuit
    4. 7.4 Device Functional Modes
      1. 7.4.1 Power Up with EN Pin
      2. 7.4.2 Power Down with EN Pin
      3. 7.4.3 Noninverted Output Mode
      4. 7.4.4 Inverted Output Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Open-Drain Flags Pullup
      2. 8.1.2 Enable the Device
      3. 8.1.3 Timing Adjust
    2. 8.2 Typical Application
      1. 8.2.1 Simple Sequencing of Three Power Supplies
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Sequencing Using Independent Flag Supply
  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 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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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

8.1.1 Open-Drain Flags Pullup

The Simple Power Sequencer contains three open-drain output flags which need to be pulled up for proper operation. 100-kΩ resistors can be used as pullup resistors.

8.1.2 Enable the Device

See Enable Circuit.

8.1.3 Timing Adjust

See Adjustable Timing.

8.2 Typical Application

8.2.1 Simple Sequencing of Three Power Supplies

The Simple Power Sequencer is used to implement a power-up (1 - 2 - 3) and power-down (3 - 2 - 1) sequence of three power supplies.

LM3881TypicalAppCircuit.pngFigure 18. Typical Application Circuit

8.2.1.1 Design Requirements

For this design example, use the parameters listed in Table 1 as the input parameters.

Table 1. Design Parameters

Design Parameter Example Value
Input Supply voltage range 2.7 V to 5.5 V
Flag Output voltage, EN high Input Supply
Flag Output voltage, EN low 0 V
Flag Timing Delay, TD1 10.8 ms - 12.0 ms
Flag Timing Delay, TD2 and TD3 9.6 ms
Power-Up Sequence 1 - 2 - 3
Power-Down Sequence 3 - 2 - 1

8.2.1.2 Detailed Design Procedure

Table 2. Evaluation Board Bill of Materials

Ref Des Description Case Size Manufacturer Manufacturer P/N
U1 LM3881 Sequencer MSOP-8 Texas Instruments LM388
R1 100 kΩ 0603 Vishay Dale CRCW06031003F-e3
R2 100 kΩ 0603 Vishay Dale CRCW06031003F-e3
R3 100 kΩ 0603 Vishay Dale CRCW06031003F-e3
CADJ 10 nF ±10% X7R 16 V 0603 Murata GRM188R71C103KA01

A timing capacitor of CADJ = 10 nF generates typical time delays TD2 and TD3 of 9.6 ms and TD1 of between
10.8 ms and 12.0 ms. The INV pin is tied to GND so that the output flags are active high. See Adjustable Timing for calculating the value for CADJ.

8.2.1.3 Application Curves

LM3881AppCurve2.pngFigure 19. Power-Up Sequence, CADJ = 10 nF
LM3881AppCurve3.pngFigure 21. Power-Up Sequence, TADJ Open
LM3881AppCurve1.pngFigure 20. Power-Down Sequence, CADJ = 10 nF
LM3881AppCurve4.pngFigure 22. Power-Down Sequence, TADJ Open

8.2.2 Sequencing Using Independent Flag Supply

For applications requiring a flag output voltage that is different from the VCC, a separate Flag Supply may be used to pullup the open-drain outputs of the Simple Power Sequencer. This is useful when interfacing the flag outputs with inputs that require a different voltage than VCC. The designer must ensure the Flag Supply voltage is within the range specified in the Recommended Operating Conditions.

LM3881IndFlag Supply.pngFigure 23. Sequencing Using Independent Flag Supply