JAJSF03K September 2011 – December 2023 LMK03806
PRODUCTION DATA
Continuing the example above, we are given the target output frequencies of 156.25 MHz, 125 MHz, 100 MHz, and 25 MHz with an OSCin frequency of 20 MHz. As previously calculated, the LCM and Fvco is 2500 MHz.
First, we will consider the PLL reference path. For lowest possible in-band PLL flat noise, we will try to maximize phase detector frequency. In this case, the highest Fpd possible from the reference path is 40 MHz (with the reference doubler enabled, doubling the 20 MHz OSCin). However, since 40 MHz does not divide into 2500 MHz by an integer value (and thus is unable to be reproduced by the PLL feedback path), we are required to use an Fpd of 20 MHz instead, which does divide into 2500 by an integer value of 125. As noted above, when FOSCin and Fpd are equal, the best PLL in-band noise can be achieved with the PLL reference doubler enabled (EN_PLL_REF_2X=1) and the PLL reference divider is 2 (PLL_R =2).
Next, we will consider the PLL feedback path. As determined earlier, Fvco is 2500 MHz and Fpd is 20 MHz, which is 2500 MHz divided by 125. The prescaler and N divider settings together must divide Fvco by 125. Given that the prescaler can be set between 2 to 8 and the N divider can be set between 1 to 262,143, the only setting that would work in this case is a prescaler value of 5 and an N divider value of 25. Note that in a case where multiple configurations are possible, increasing the N divider value will reduce loop filter component sizes.