Most TEGs are in the 0.5Ω to 10Ω range for ESR or source resistance. This graph illustrates how the LTC3108’s output current varies with input voltage and TEG source resistance. Thus, with a 1:100 turns ratio transformer, the LTC3108 is perfectly matched to the source when ESR is ~ 3Ω, although it is fairly close to optimum at other ESRs.