LM25085 temperature dependency of Vout

Hello Rick,   I hope you are doing well.  

The typical temperature coefficient of the FB reference voltage is provided by Figure 22 of the datasheet.   This curve provides VFB vs Tj.   The VOUT variation with Tj is this FB voltage multiplied times the feedback divider gain which is  (RFB1+RFB2)/RFB1. 

Translating Tj variation to Ta involves the thermal resistance of the package and the total dissipation of the IC.  This dissipation is dominated by gate driver current which depends on switching frequency and gate charge of the PFET.   You could estimate the power of the IC by adding the non-switching Iq current with the gate charge current and multiplying the sum by Vin.     The non-switching input current is 1.3mA typical and  the gate charge current is Qg*f.   The chip power dissipation is therefore  Pd =  Vin*(1.3mA + Qg*f) .   

The thermal resistance theta j-a is provided in the datasheet for the 3 supported packages.   Tj =  Ta + Pd*theta