Sage Markandeya composed this famous Lord Venkateshwara Vajra Kavacha Stotram.

 శ్రీ వేంకటేశ్వర వజ్ర కవచ స్తోత్రం

The hymn is composed by Totakacharya and it consists of eight verses in praise of his Guru Sri Adi Shankaracharya.