WILSONVILLE, OR -- Investor Carl Icahn upped his stake in Mentor Graphics to almost 15%, making him the firm's second-largest shareholder.

The notorious corporate raider exercised call options on some 8.23 million shares in the past week, according to a US Securities and Exchange Commission filing. He owned 11% at the end of June.

Through his investment company, Icahn now holds more than 16 million shares of the electronics design automation software company.

Ameriprise Financial holds roughly 16.2 million shares, or about 15.2% of the company.

In June, Mentor adopted a poison pill provision that goes into effect if a person, acting alone or as part of a group, accumulates 15% or more of Mentor stock (or announces an offer to do so). The option gives current shareholders the right to purchase an additional share at 50% of the price of the current price. Mentor did not indicate whether it would exercise the provision.