In December 2019, we issued convertible senior notes due 2026, or the 2019 Notes, in an aggregate principal amount of $86.3 million. The 2019 Notes are our senior unsecured obligations and are governed by the indenture for the 2019 Notes. The 2019 Notes accrue interest at a fixed rate of 2.00% per year, payable semiannually in arrears on June 15 and December 15 of each year, beginning June 15, 2020. Upon conversion, the 2019 Notes will be convertible into cash, shares of our common stock or a combination thereof, at our election. The 2019 Notes will mature on December 15, 2026, unless earlier repurchased, redeemed, or converted in accordance with the terms of the indenture.

Our net proceeds from issuing the 2019 Notes were approximately $83.5 million after deducting fees and expenses. We used a portion of the proceeds to pay the cost of the capped call transactions described below and repay our senior credit facility.

The 2019 Notes are convertible at an initial conversion rate of 28.9415 shares of our common stock per $1,000 principal amount of the 2019 Notes, which is equal to an initial conversion price of approximately $34.55 per share of our common stock, subject to adjustment under certain circumstances in accordance with the indenture. Prior to the close of business on the business day immediately preceding September 15, 2026, holders of the 2019 Notes may convert all or a portion of their 2019 Notes under the following circumstances:

On or after September 15, 2026, until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert all or any portion of the 2019 Notes, in multiples of $1,000 principal amount, at the option of the holder regardless of the foregoing circumstances.

We may redeem the 2019 Notes for cash, at our option, on or after December 20, 2023, if the last reported sale price of our common stock has been at least 130% of the conversion price at least 20 trading days (whether or not consecutive) during any 30 consecutive trading day period at a redemption price equal to 100% of the principal amount of the 2019 Notes being redeemed, plus any accrued and unpaid interest to, but excluding, the redemption date.

Holders of the 2019 Notes who convert their 2019 Notes in connection with certain corporate events that constitute a make-whole fundamental change (as defined in the indenture) are, under certain circumstances, entitled to an increase in the conversion rate. Additionally in the event of a corporate event constituting a fundamental change (as defined in the indenture), holders of the 2019 Notes may require us to repurchase all or a portion of their 2019 Notes at a repurchase price equal to 100% of the principal amount of the 2019 Notes being repurchased, plus any accrued and unpaid interest to, but excluding, the repurchase date.

Certain convertible-debt instruments that may be settled in cash on conversion are required to be separated into a liability and an equity component. The total proceeds are first allocated to the liability component based on the fair value of a similar debt instrument without the conversion option. The remaining proceeds are allocated to the equity component and recognized in additional paid-in capital.

Accordingly, we separated the 2019 Notes into a liability and an equity component. We determined the fair value of the liability component to be $52.5 million calculated as the present value of future cash flows discounted at the borrowing rate for a similar nonconvertible debt instrument based on the expected term. We determined the borrowing rate to be 9.90% based on the market rates for nonconvertible debt instruments issued by other companies with publicly available credit ratings considered to be comparable to us. We recognized the excess of the principal amount of the 2019 Notes over the initial carrying amount of the liability component as a debt discount of $33.8 million and are amortizing it to interest expense over the expected term of the 2019 Notes using the effective interest rate method. We recorded the equity component of $33.8 million as additional paid-in capital, calculated as the difference between the total proceeds of $86.3 million and the initial carrying amount of the liability component. We will not remeasure the equity component as long as it continues to meet the conditions for equity classification.

As of June 30, 2020, the 2019 Notes were not yet convertible and the “if-converted value” did not exceed the remaining principal amount of the 2019 Notes.

We allocated the 2019 Notes total issuance costs of $2.8 million between liability and equity in the same proportion as the allocation of our total proceeds to liability and equity components. We amortize the issuance costs attributable to the liability component of $1.7 million to interest expense over the respective term of the 2019 Notes using the effective interest rate method. We netted the issuance costs attributable to the equity component of $1.1 million against the respective equity component in additional paid-in capital.

The effective interest rate on the liability component of the 2019 Notes is 10.21%. As of June 30, 2020, we have $77,000 of accrued interest related to the 2019 Notes included in accrued liabilities on our condensed consolidated balance sheet.

The following table presents the interest expense related to the 2019 Notes for the periods presented (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Amortization of debt discount	$	886	$	—	$	1,740	$	—
Amortization of debt issuance costs		27		—		53		—
Cash interest expense		436		—		868		—
Total interest expense	$	1,349	$	—	$	2,661	$	—

Our estimated fair value of the 2019 Notes was $90.8 million and $87.0 million as of June 30, 2020 and December 31, 2019, respectively, which we determined through consideration of quoted market prices. The fair value is classified as Level 2, as defined in Note 2.

The following table presents the outstanding principal amount and carrying value of the 2019 Notes as of the date presented (in thousands):

	June 30, 2020
Outstanding principal amount		86,250
Unamortized debt discount and debt issuance costs		(33,581	)
Carrying value		52,669

In connection with our issuance of the 2019 Notes, we entered into privately negotiated capped-call transactions with certain financial counterparties. The capped-call transactions are generally designed to reduce the potential dilution to our common stock upon any conversion or settlement of the 2019 Notes, or to offset any cash payments we are required to make in excess of the principal amount upon conversion of the 2019 Notes, as the case may be, with such reduction or offset subject to a cap based on the cap price. If, however, the market price per share of our common stock exceeds the cap price of the capped-call transactions then our stock would experience some dilution and/or the capped call would not fully offset the potential cash payments, in each case, to the extent the then-market price per share of our common stock exceeds the cap price. The initial cap price of the capped-call transactions is $54.20 per share, which represents a 100% premium over the last reported sale price of our common stock of $27.10 per share on December 11, 2019 subject to certain adjustments under the terms of the capped-call transactions. The capped-call transactions expire over 40 consecutive scheduled trading days ending on December 11, 2026.

The capped-call transactions meet the criteria for classification in equity, are not accounted for as derivatives, and are not remeasured each reporting period. We paid $10.1 million for the capped-call transactions, which we recorded as a reduction to additional paid-in-capital within shareholders’ equity.

On April 20, 2020, we entered into an unsecured promissory note, or the PPP Note, through Silicon Valley Bank under the Paycheck Protection Program, or PPP, a program administered by the Small Business Administration, or SBA, and established as part of the Coronavirus Aid, Relief and Economic Security Act, or CARES Act. The PPP Note had an aggregate principal amount of approximately $5.8 million, and we received the loan proceeds on April 22, 2020. On April 23, 2020, the SBA issued guidance that substantially narrowed the ability of public companies to participate in the PPP program. While we applied for the PPP program in good faith, considering the SBA’s new guidance, we repaid the PPP Note in full on April 30, 2020.