SNAS635E December   2013  – January 2022 LMK00334

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, Propagation Delay, and Output Skew
    7. 6.7 Typical Characteristics
  7. 7 Parameter Measurement Information
    1. 7.1 Differential Voltage Measurement Terminology
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Crystal Power Dissipation vs. RLIM
      2. 8.3.2 Clock Inputs
      3. 8.3.3 Clock Outputs
        1. 8.3.3.1 Reference Output
    4. 8.4 Device Functional Modes
      1. 8.4.1 VCC and VCCO Power Supplies
  9. 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
        2. 9.2.2.2 Termination for DC-Coupled Differential Operation
        3. 9.2.2.3 Termination for AC-Coupled Differential Operation
      3. 9.2.3 Application Curve
  10. 10Power Supply Recommendations
    1. 10.1 Current Consumption and Power Dissipation Calculations
      1. 10.1.1 Power Dissipation Example: Worst-Case Dissipation
    2. 10.2 Power Supply Bypassing
      1. 10.2.1 Power Supply Ripple Rejection
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
    3. 11.3 Thermal Management
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Support 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

9.2.1.2 Crystal Interface

The LMK00334 has an integrated crystal oscillator circuit that supports a fundamental mode, AT-cut crystal. The crystal interface is shown in Figure 9-5.

GUID-F4622648-8221-4701-8F30-3C69530DA3DB-low.gifFigure 9-5 Crystal Interface

The load capacitance (CL) is specific to the crystal, but usually on the order of 18 to 20 pF. While CL is specified for the crystal, the OSCin input capacitance (CIN = 1 pF typical) of the device and PCB stray capacitance (CSTRAY is approximately around 1 to 3 pF) 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 1. CL = (C1 × C2) / (C1 + C2) + CIN + CSTRAY

Typically, C1 = C2 for optimum symmetry, so Equation 1 can be rewritten in terms of C1 only:

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

Finally, solve for C1:

Equation 3. C1 = (CL – CIN – CSTRAY) × 2

Section 6.5 provides crystal interface specifications with conditions that ensure start-up of the crystal, but it does not specify crystal power dissipation. The designer must ensure 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 should 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 4. 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 (Tektronix CT-6 or equivalent, for example) 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 1.5 kΩ.