Legal provisions for optional referendums at national level

Kazakhstan
Constitution
http://www.kazakhstan.orexca.com/kazakhstan_constitution.shtml#2
Article 91. Amendments and additions to the Constitution of the Republic of Kazakhstan may be introduced only by an all-nation referendum held by the decision of the President of the Republic made on his own initiative, at the recommendation of Parliament or the Government. The draft of amendments and additions to the Constitution shall not be submitted to an all-nation referendum if the President decides to pass it to the consideration of Parliament. [...]
http://aceproject.org/ero-en/regions/asia/KZ/kazakhstan-constitutional-law-on-referenum-1995/view
Article 2. Subject of the republican referendum
The subject of referendum shall be:
1) adoption of the Constitution, constitutional acts, laws of the Republic, introduction to them of any changes and additions;
2) solution of other most important issues of the public life of the Republic.
There areno provisions for mandatory referendums in the legislation of the country. All referendums are optional. (See also Article 91 of Constitution. )