In the Si5395-91 & Si5345-40 devices, DCO mode is intended to be used within the range of +/-350ppm. It is possible to use DCO mode beyond this range, but it is important to be aware of the following caveats:
where Fvco is the VCO frequency, NDEN is the N-divider denominator, NNUM is the N-divider numerator, NNUM,DELTA is the value which the numerator is being incremented/decremented by per step, X is the net amount of steps applied, and R is the integer R-divider value. If DCO mode is operated within a narrow frequency range, Fout has an approximately linear relation to X. However, as the frequency range increases, Fout becomes increasingly nonlinear with respect to X. Consequently, the actual frequency step size will be significantly different from the desired step size as the N-divider is continually incremented/decremented.
The following are the recommended solutions to address the above issues:
For more details on DCO mode and FOTF, please refer to the following application notes: