Irvine-based chipmaker Microsemi Corp. announced plans to take a $548.7 million cash bid for Zarlink Semiconductor Inc. to shareholders after directors of the Canadian company turned down two earlier offers.

“Your continued refusal to discuss our proposal compels us to directly inform your shareholders of our attractive proposal,” Microsemi Chief Executive Officer James Peterson wrote in a letter to Zarlink’s board.

The hostile bid aims to extend Microsemi’s lineup in the communications and medical markets. Its offer for Zarlink was a premium of about 40% based on recent prices on the Toronto Stock Exchange.

Microsemi’s chips serve a variety of military, aerospace, consumer and industrial uses. Its products are built into satellites, digital televisions and other devices. Customers include Cisco Systems Inc., Boeing Co., Hewlett-Packard Co., Dell Inc. and Samsung Electronics Co.

Ottawa-based Zarlink counts on the communications sector for about 80% of its business and the medical sector for most of the rest. Its chips are used by telecommunications and cable companies for bundled voice, video, data and mobile services. Its medical business makes ultra low-power radios for devices such as pacemakers.

Microsemi has a market value of about $1.8 billion, with an estimated workforce of about 400 employees here and more than 2,600 employees companywide. It recently announced plans to move its headquarters to Aliso Viejo, about quadrupling the size of its offices to more than 100,000 square feet.

The Zarlink deal is the latest for Microsemi, which earlier this year said it plans to grow revenue from $520 for the 12 months through October to $1 billion. It is now Orange County’s third-biggest chipmaker by sales and reported about $520 million in revenue for the 12 months through October.

Microsemi is a frequent acquirer, and buys are key to the company’s current goal on revenue.

This week it bought the millimeterwave technology and related assets of Florida-based Brijot Imaging Systems Inc., which debuted a hand-held scanner in May that could replace controversial pat-downs at airports and other security check points.

Last month it acquired Sunnyvale-based “fabless” chipmaker ASIC Advantage Inc., building its position in the profitable aerospace and satellite industries, according to company officials.

ASIC designs and makes integrated circuits for the aerospace, automotive, communications, industrial and medical markets.

ASIC’s products include controllers, sensors, analog-to-digital and digital-to-analog converters, and radiation-tested devices.

Last year saw the company make its largest acquisition to date, a $430 million buy of Northern California’s Actel Corp.

Actel focuses on the aerospace industry and was a rival of Microsemi’s.

Microsemi said it expects a deal for Zarlink to add to its earnings immediately.

It said it has financing for the Zarlink offer from Morgan Stanley Senior Funding Inc.

Nicolaus Weisel and Morgan Stanley & Co. is its financial adviser on the deal.