SNAS512K September   2011  – October 2025 LMK00301

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison
  6. 5 Pin Configuration and Functions
  7. 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
  8. 7 Parameter Measurement Information
    1. 7.1 Differential Voltage Measurement Terminology
  9. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 VCC and VCCO Power Supplies
    4. 8.4 Device Functional Modes
      1. 8.4.1 Clock Inputs
      2. 8.4.2 Clock Outputs
        1. 8.4.2.1 Reference Output
  10. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
        1. 9.2.1.1 Driving the Clock Inputs
        2. 9.2.1.2 Crystal Interface
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Termination and Use of Clock Drivers
          1. 9.2.2.1.1 Termination for DC Coupled Differential Operation
          2. 9.2.2.1.2 Termination for AC Coupled Differential Operation
          3. 9.2.2.1.3 Termination for Single-Ended Operation
      3. 9.2.3 Application Curves
    3. 9.3 Power Supply Recommendations
      1. 9.3.1 Power Supply Sequencing
      2. 9.3.2 Current Consumption and Power Dissipation Calculations
        1. 9.3.2.1 Power Dissipation Example #1: Separate VCC and VCCO Supplies with Unused Outputs
        2. 9.3.2.2 Power Dissipation Example #2: Worst-Case Dissipation
      3. 9.3.3 Power Supply Bypassing
        1. 9.3.3.1 Power Supply Ripple Rejection
      4. 9.3.4 Thermal Management
    4. 9.4 Layout
      1. 9.4.1 Layout Guidelines
      2. 9.4.2 Layout Example
  11. 10Device and Documentation Support
    1. 10.1 Documentation Support
      1. 10.1.1 Related Documentation
    2. 10.2 Receiving Notification of Documentation Updates
    3. 10.3 Support Resources
    4. 10.4 Trademarks
    5. 10.5 Electrostatic Discharge Caution
    6. 10.6 Glossary
  12. 11Revision History
  13. 12Mechanical, Packaging, and Orderable Information

9.2.1.2 Crystal Interface

The LMK00301 has an integrated crystal oscillator circuit that supports a fundamental mode, AT-cut crystal. Figure 9-5 shows the crystal interface.

LMK00301 Crystal InterfaceFigure 9-5 Crystal Interface

The load capacitance (CL) is specific to the crystal, but typically on the order of 18pF to 20pF. While CL is specified for the crystal, the OSCin input capacitance (CIN = 4pF typical) of the device and PCB stray capacitance (CSTRAY approximately 1pF to 3pF) can affect the discrete load capacitor values, C1 and C2.

For the parallel resonant circuit, the discrete capacitor values can be calculated as follows:

Equation 2. CL = (C1 × C2) / (C1 + C2) + CIN + CSTRAY

Typically, C1 = C2 for optimum symmetry, so Equation 2 can be rewritten in terms of C1 only:

Equation 3. CL = C12 / (2 × C1) + CIN + CSTRAY

Finally, solve for C1:

Equation 4. C1 = (CL – CIN – CSTRAY) × 2

Electrical Characteristics provides crystal interface specifications with conditions that verify start-up of the crystal, but the electrical characteristics do not specify crystal power dissipation. The designer needs to verify the crystal power dissipation does not exceed the maximum drive level specified by the crystal manufacturer. Overdriving the crystal can cause premature aging, frequency shift, and eventual failure. Drive level must be held at a sufficient level necessary to start-up and maintain steady-state operation.

The power dissipated in the crystal, PXTAL, can be computed by:

Equation 5. PXTAL = IRMS2 × RESR × (1 + C0/CL)2

where

  • IRMS is the RMS current through the crystal.
  • RESR is the maximum equivalent series resistance specified for the crystal
  • CL is the load capacitance specified for the crystal
  • C0 is the minimum shunt capacitance specified for the crystal

IRMS can be measured using a current probe (for example, Tektronix CT-6 or equivalent) placed on the leg of the crystal connected to OSCout with the oscillation circuit active.

As shown in Figure 9-5, an external resistor, RLIM, can be used to limit the crystal drive level, if necessary. If the power dissipated in the selected crystal is higher than the drive level specified for the crystal with RLIM shorted, then a larger resistor value is mandatory to avoid overdriving the crystal. However, if the power dissipated in the crystal is less than the drive level with RLIM shorted, then a zero value for RLIM can be used. As a starting point, a suggested value for RLIM is 1.5kΩ.