SCAS892C February   2010  – December 2016 CDCE937-Q1 , CDCEL937-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Device Comparison Table
  7. 7 Pin Configuration and Functions
  8. 8 Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Electrical Characteristics
    6. 8.6 Timing Requirements
    7. 8.7 Typical Characteristics
  9. 9 Parameter Measurement Information
  10. 10Detailed Description
    1. 10.1 Overview
    2. 10.2 Functional Block Diagram
    3. 10.3 Feature Description
      1. 10.3.1 Control Terminal Setting
      2. 10.3.2 Default Device Setting
    4. 10.4 Device Functional Modes
      1. 10.4.1 SDA and SCL Serial Interface
    5. 10.5 Programming
      1. 10.5.1 Data Protocol
      2. 10.5.2 Command Code Definition
      3. 10.5.3 Generic Programming Sequence
      4. 10.5.4 Byte Write Programming Sequence
      5. 10.5.5 Byte Read Programming Sequence
      6. 10.5.6 Block Write Programming Sequence
      7. 10.5.7 Block Read Programming Sequence
      8. 10.5.8 Timing Diagram for the SDA and SCL Serial Control Interface
      9. 10.5.9 SDA and SCL Hardware Interface
    6. 10.6 Register Maps
      1. 10.6.1 SDA and SCL Configuration Registers
  11. 11Application and Implementation
    1. 11.1 Application Information
    2. 11.2 Typical Application
      1. 11.2.1 Design Requirements
      2. 11.2.2 Detailed Design Procedure
        1. 11.2.2.1 Spread-Spectrum Clock (SSC)
        2. 11.2.2.2 PLL Multiplier or Divider Definition
        3. 11.2.2.3 Crystal Oscillator Start-Up
        4. 11.2.2.4 Frequency Adjustment With Crystal Oscillator Pulling
        5. 11.2.2.5 Unused Inputs and Outputs
        6. 11.2.2.6 Switching Between XO and VCXO Mode
      3. 11.2.3 Application Curves
  12. 12Power Supply Recommendations
  13. 13Layout
    1. 13.1 Layout Guidelines
    2. 13.2 Layout Example
  14. 14Device and Documentation Support
    1. 14.1 Documentation Support
      1. 14.1.1 Related Documentation
    2. 14.2 Related Links
    3. 14.3 Receiving Notification of Documentation Updates
    4. 14.4 Community Resources
    5. 14.5 Trademarks
    6. 14.6 Electrostatic Discharge Caution
    7. 14.7 Glossary
  15. 15Mechanical, Packaging, and Orderable Information

Package Options

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

11 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.

11.1 Application Information

The CDCE937-Q1 device is an easy-to-use, high-performance, programmable CMOS clock synthesizer which can be used as a crystal buffer, clock synthesizer with separate output supply pin. The CDCE937-Q1 device features an on-chip loop filter and spread-spectrum modulation. Programming can be done through the I2C interface, or previously saved settings can be loaded from on-chip EEPROM. The pins S0, S1, and S2 can be programmed as control pins to select various output settings. This section shows some examples of using the CDCE937-Q1 device in various applications.

11.2 Typical Application

Figure 14 shows the use of the CDCE937-Q1 device in an infotainment system, such as in head unit or telematics applications, using a 1.8-V single supply. Note that bypass capacitors are not shown in this schematic.

CDCE937-Q1 CDCEL937-Q1 CDCE937_Q1_typical_application_diag.gif Figure 14. Single-Chip Solution Using a CDCE937-Q1 Device for Generating Clocking Frequencies

11.2.1 Design Requirements

The CDCE937-Q1 device supports spread-spectrum clocking (SSC) with multiple control parameters:

  • Modulation amount (%)
  • Modulation frequency (>20 kHz)
  • Modulation shape (triangular, hershey, and others)
  • Center spread or down spread (± or –)

For sample calculations of PLL constants, see PLL Multiplier or Divider Definition.

CDCE937-Q1 CDCEL937-Q1 mod_freq_mod_amount.png Figure 15. Modulation Frequency (fm) and Modulation Amount

11.2.2 Detailed Design Procedure

11.2.2.1 Spread-Spectrum Clock (SSC)

Spread-spectrum modulation is a method to spread emitted energy over a larger bandwidth. In clocking, spread spectrum can reduce electromagnetic interference (EMI) by reducing the level of emission from clock distribution network.

CDCE937-Q1 CDCEL937-Q1 comparison_typ_clock_pwr_spec_spred_spec.gif
CDCS502 with a 25-MHz Crystal, FS = 1, fOUT = 100 MHz, and 0%, ±0.5, ±1%, and ±2% SSC
Figure 16. Comparison Between Typical Clock Power Spectrum and Spread-Spectrum Clock

Spread spectrum clocking can be used to help reduce EMI to meet design specifications. For example, a specified EMI threshold of 55 dB/mV would require ±1% spread spectrum clocking to meet this requirement.

11.2.2.2 PLL Multiplier or Divider Definition

At a given input frequency (ƒIN), the output frequency (ƒOUT) of the CDCEx937-Q1 can be calculated with Equation 1.

Equation 1. CDCE937-Q1 CDCEL937-Q1 q1_fout_cas847.gif

where

  • M (1 to 511) and N (1 to 4095) are the multiplier/divide values of the PLL
  • Pdiv (1 to 127) is the output divider

The target VCO frequency (ƒVCO) of each PLL can be calculated with Equation 2.

Equation 2. CDCE937-Q1 CDCEL937-Q1 q2_fvco_cas847.gif

The PLL internally operates as fractional divider and requires the following multiplier and divider settings:

N

P = 4 – int CDCE937-Q1 CDCEL937-Q1 qinl3_p_cas847.gif

Q = int CDCE937-Q1 CDCEL937-Q1 qinl4_q_cas847.gif

R = N′ – M × Q

where

N′ = N × 2P

N ≥ M

100 MHz < ƒVCO > 200 MHz

Example:
for ƒIN = 27 MHz; M = 1; N = 4; Pdiv = 2; for ƒIN = 27 MHz; M = 2; N = 11; Pdiv = 2;
→ fOUT = 54 MHz → fOUT = 74.25 MHz
→ fVCO = 108 MHz → fVCO = 148.50 MHz
→ P = 4 – int(log24) = 4 – 2 = 2 → P = 4 – int(log25.5) = 4 – 2 = 2
→ N′ = 4 × 22 = 16 → N′ = 11 × 22 = 44
→ Q = int(16) = 16 → Q = int(22) = 22
→ R = 16 – 16 = 0 → R = 44 – 44 = 0

The values for P, Q, R, and N’ is automatically calculated when using TI Pro-Clock™ software.

11.2.2.3 Crystal Oscillator Start-Up

When the CDCE937-Q1 or CDCEL937-Q1 device is used as a crystal buffer, crystal oscillator start-up dominates the start-up time compared to the internal PLL lock time. The following diagram shows the oscillator start-up sequence for a 27-MHz crystal input with an 8-pF load. The start-up time for the crystal is on the order of approximately 250 µs compared to approximately 10 µs of lock time. In general, lock time is an order of magnitude less compared to the crystal start-up time.

CDCE937-Q1 CDCEL937-Q1 crystal_osc_startup_vs_pll_locktime.gif Figure 17. Crystal Oscillator Start-Up vs PLL Lock Time

11.2.2.4 Frequency Adjustment With Crystal Oscillator Pulling

The frequency for the CDCE937-Q1 or CDCEL937-Q1 device is adjusted for media and other applications with the VCXO control input Vctr. If a PWM-modulated signal is used as a control signal for the VCXO, an external filter is required.

CDCE937-Q1 CDCEL937-Q1 freq-adj-pwm-input_SCAS918.gif Figure 18. Frequency Adjustment Using PWM Input to the VCXO Control

11.2.2.5 Unused Inputs and Outputs

If VCXO-pulling functionality is not required, Vctr must be left floating. All other unused inputs must be set to GND. Unused outputs must be left floating.

If one output block is not used, TI recommends disabling it. However, TI recommends providing a supply for all output blocks, even if they are disabled.

11.2.2.6 Switching Between XO and VCXO Mode

When the CDCEx937-Q1 device is in the crystal-oscillator or VCXO configuration, the internal capacitors require different internal capacitance. The following steps are recommended to switch to VCXO mode when the configuration for the on-chip capacitor is still set for XO mode. To center the output frequency to 0 ppm:

  1. While in XO mode, put Vctr = VDD / 2
  2. Switch from XO mode to VCXO mode
  3. Program the internal capacitors to obtain 0 ppm at the output.

11.2.3 Application Curves

Figure 19, Figure 20, Figure 21, and Figure 22 show CDCE937-Q1 measurements with the SSC feature enabled. Device configuration: 27-MHz input, 27-MHz output.

CDCE937-Q1 CDCEL937-Q1 scas849_appcv1.png Figure 19. fOUT = 27 MHz,
VCO frequency < 125 MHz, SSC (2% Center)
CDCE937-Q1 CDCEL937-Q1 scas849_appcv3.png Figure 21. Output Spectrum With SSC Off
CDCE937-Q1 CDCEL937-Q1 scas849_appcv2.png Figure 20. fOUT = 27 MHz,
VCO frequency > 175 MHz, SSC (1%, Center)
CDCE937-Q1 CDCEL937-Q1 scas849_appcv4.png Figure 22. Output Spectrum With SSC On,
2% Center