The relation will depend on which configuration you are using. The general equation is as follows Rawcount=((2^N − 1)*VREF* FSW* Cp) / IMOD You may find the details of this equation in the design guide in the following link
I believe you are referring to FSW. If so then this can be configured by setting dividers internally. If you using XTAL for IMO then this signal will be derived from that else you may not be able to use external signal for this.