Article Five of the United States Constitution
Instead, the Founders gave the job of creating Constitutional amendments to the Legislative Branch.  By requiring that two-thirds of both Houses of Congress, and three-quarters of the states, approve of every Constitutional amendment, they made sure that there would have to be a lot of debate, compromise, and thought before an amendment was approved.  Also, because so many legislators have to approve every amendment, it is very difficult for small "special interest" groups (like people who only care about a certain part of the country, or a certain issue) cannot control the vote.