What is the burden of producing evidence in a sexual assault case?

In Coffey we held that the state must establish a prima facie case that defendant had suffered a previous conviction; then, the defendant had "the burden of producing evidence that [these] constitutional right[ were] infringed in the prior proceeding...." (People v. Coffey, supra, 67 Cal.2d 204, 217, 60 Cal.Rptr. 457, 430 P.2d 15, italics added.) The use of the term of art, "burden of producing evidence," itself indicates that we intended to give the defendant only the obligation "to introduce evidence sufficient to avoid a ruling against him on the issue." (Evid.Code, 110.) Yet, in case there should be any confusion about the burden that was assigned to defendant, we explained: "Though the burden of proof as to the constitutionality of the charged prior conviction [51 Cal.3d 1311] remains with the prosecution, and the burden of producing evidence rests initially with it, the latter burden shifts to the defendant upon proof of the fact of his having 'suffered' the prior conviction." (Coffey, supra, 67 Cal.2d at p. 217, fn. 15, 60 Cal.Rptr. 457, 430 P.2d 15, italics added.)