Si5351 Frequency Calculation


I've been playing with a 5351 breakout board & have a question about the way the frequency is calculated, and its limitations.

Setting the PLL frequency is based on a crystal frequency (nominally 25 or 27 MHz) and a (three part) multiplier.  In the simplest case, the multiplier can be in the range (15...90), which, for a 25MHz crystal, would imply a (nominal) range of 375MHz to 2250MHz.  

In various discussions, it is noted that the PLL frequency is (should be?) limited to a range of 600-900 MHz, implying a multiplier range of 24-36, given a 25MHz crystal.

I make the assumption that multiplier values outside this 24-36 range are there to enable the use of crystals (or other frequency sources) other than the nominal 25/27MHz.

Or is there something I'm missing?

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