"I'm not aware there are many or even any referenda in the US. Does that make the US a tyranny?"

Yup. If you accept the premise.
"The normal course of events, when an amendment to the Constitution has been desired by the people, is for Congress to pass the amendment and for the state legislatures to then ratify. Congressional proposal of the amendment is by a two-thirds majority vote in both houses. State ratification is by three-fourths majority.
The Constitution does provide for one other way to ratify: by convention. A state convention differs from the state legislature in that it is usually an entirely separate body from the legislature. This introduces a different political dynamic into the amendment process.

The only time that conventions have been used was in the case of the 21st Amendment, which overturned the 18th Amendment. The 18th abolished alcohol manufacture or sales on a national scale. The 21st repealed the 18th, stating instead that each state shall have the ability to set its own laws regarding liquor. The text of the 21st specifically stated that it would have to be ratified by conventions held in each state:

3. The article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by conventions in the several States, as provided in the Constitution, within seven years from the date of the submission hereof to the States by the Congress.
Why specify conventions over legislatures, as every other amendment had been ratified up to then? The thought was that the people of the conventions, which would typically be average citizens, would be less likely to bow to political pressure to reject the amendment than elected officials would be. Note that the Supreme Court has ruled that a popular referendum is not a substitute for either the legislature nor a convention, nor can a referendum approve of or disapprove of the legislature's or a convention's decision on an amendment."
http://www.usconstitution.net/consttop_acon.html