Legal provisions for optional referendums at national level

Philippines
1987 CONSTITUTION OF THE REPUBLICOF THE PHILIPPINES
http://www.chanrobles.com/philsupremelaw1.htm
ARTICLE XVII: AMENDMENTSAND REVISIONS
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The Congress shallprovide for the implementation of the exercise of this right.
Sec. 3. The Congress may,by a vote of two-thirds of all its Members, call a constitutional convention,or by a majority vote of all its Members, submit to the electorate the questionof calling such a convention.