Exhibit 5.1

Wilson Sonsini Goodrich & Rosati

Professional Corporation

650 Page Mill Road

Palo Alto, California 94304

July 18, 2005

Advanced Analogic Technologies Incorporated

830 East Arques Avenue

Sunnyvale, CA 94085

Re: Registration Statement on Form S-1

Ladies and Gentlemen:

We have examined the Registration Statement on Form S-1 (Registration No. 333-123798) filed by you with the Securities and Exchange Commission on July 18, 2005, as amended (the “Registration Statement”), in connection with the registration under the Securities Act of 1933, as amended, of up to 12,190,000 shares of your common stock (the “Shares”). The Shares include an over-allotment option granted to the underwriters of the offering to purchase 1,590,000 shares of common stock. We understand that the Shares are to be sold to the underwriters for resale to the public as described in the Registration Statement and pursuant to the Underwriting Agreement filed as an exhibit thereto. As your legal counsel, we have examined the proceedings taken and are familiar with the proceedings proposed to be taken by you in connection with the sale of the Shares.

In connection with this opinion, we have examined and relied upon the Registration Statement and related prospectus, your Amended and Restated Certificate of Incorporation and Bylaws, as currently in effect, and the originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. Our opinion is expressed only with respect to the laws of the State of Delaware.

On the basis of the foregoing, and in reliance thereon, it is our opinion that the Shares, when sold in the manner described in the Registration Statement, will be validly issued, fully paid and nonassessable.

We consent to the use of this opinion as an exhibit to the Registration Statement, and we consent to the reference of our name under the caption “Legal Matters” in the prospectus forming part of the Registration Statement.

Very truly yours,

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation

/s/ WILSON SONSINI GOODRICH & ROSATI