Stock-Based Compensation (Tables)

9 Months Ended

Sep. 26, 2014

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Stock-Based Compensation Expense

The following table summarizes stock-based compensation expense (in thousands):



	Three months ended				Nine months ended
	September 26, 2014		September 27, 2013		September 26, 2014		September 27, 2013
Stock-based compensation in:
Cost of revenue	$	612	$	605	$	1,751	$	1,838
Research and development expense	1,219		1,076		3,589		3,400
Selling, general and administrative expense	2,521		2,264		7,380		6,628
Total stock-based compensation in operating expense	3,740		3,340		10,969		10,028
Total stock-based compensation	$	4,352	$	3,945	$	12,720	$	11,866

Valuation Assumptions for Stock Options

The Company estimated the fair value of all employee stock options using a Black-Scholes valuation model with the following weighted average assumptions:



	Three months ended				Nine months ended
	September 26, 2014		September 27, 2013		September 26, 2014		September 27, 2013
Expected term (years)	4.70		4.70		4.70		4.70
Volatility	40	%	46	%	40	%	51	%
Risk-free interest rate	1.8	%	1.5	%	1.7	%	0.8	%
Expected dividends	0.0	%	0.0	%	0.0	%	0.0	%

Employee Stock Purchase Plan

The call option and put option were valued using the Black-Scholes option pricing model. The weighted average fair value of the Company's ESPP shares at purchase dates was estimated using the following weighted average assumptions during the nine months ended September 26, 2014 and September 27, 2013:



	Purchase Period Ending
	December 31, 2014		June 30, 2014		June 30, 2013
Expected term (years)	0.50		0.50		0.49
Volatility	33	%	28	%	30	%
Risk-free interest rate	0.1	%	0.1	%	0.2	%
Expected dividends	0.0	%	0.0	%	0.0	%
Estimated weighted average fair value per share at purchase date	$1.84		$1.70		$1.23