Xin/CLK

Xout

Vctrl

LP

PWM

control

signal

CDCEx925

Copyright © 2016, Texas Instruments Incorporated

27

CDCE925, CDCEL925

www.ti.com

SCAS847I – JULY 2007 – REVISED OCTOBER 2016

Product Folder Links: CDCE925 CDCEL925

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Copyright © 2007–2016, Texas Instruments Incorporated

Typical Application (continued)

Figure 19. Frequency Adjustment Using PWM Input to the VCXO Control

10.2.2.5 Unused Inputs and Outputs

If VCXO pulling functionality is not required, VCtrl should be left floating. All other unused inputs should be set to

GND. Unused outputs should be left floating.

If one output block is not used, TI recommends disabling it. However, TI always recommends providing the

supply for the second output block even if it is disabled.

10.2.2.6 Switching Between XO and VCXO Mode

When the CDCEx925 is in crystal oscillator or in 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 Vctrl = Vdd/2

2. Switch from X0 mode to VCXO mode

3. Program the internal capacitors to obtain 0 ppm at the output.

10.2.3 Application Curves

Figure 20, Figure 21, Figure 22, and Figure 23 show CDCEx925 measurements with the SSC feature enabled.

Device configuration: 27-MHz input, 27-MHz output.

Figure 20. fOUT = 27 MHz, VCO Frequency < 125 MHz, SSC

(2% Center)

Figure 21. fOUT = 27 MHz, VCO Frequency > 175 MHz, SSC

(1%, Center)