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                                                                    Exhibit 10.4

[COMERICA LOGO]    ADVANCE FORMULA AGREEMENT

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As of September 5, 2001, this Agreement is made between SILICON
LABORATORIES INC. ("Borrower") and COMERICA BANK-TEXAS, a Texas banking
association ("Bank").

For and in consideration of the loans and other credit which Borrower may now or
hereafter obtain or request from Bank pursuant to the Credit Agreement of even
date herewith between Borrower and Bank (as from time to time amended, modified
restated, the "Credit Agreement") and which are secured pursuant to the Security
Agreement of even date herewith executed by Borrower in favor of the Bank, and
for other good and valuable consideration, Bank and Borrower agree as follows:

1.   REVOLVING LOANS. The Revolving Loans (as defined in the Credit Agreement)
     which Bank may now or hereafter extend to Borrower are subject to the
     limitations of this Agreement and to the conditions and limitations of the
     Credit Agreement.

2.   ADVANCE FORMULA. Borrower warrants and agrees that Borrower's the Facility
     Usage (as defined in the Credit Agreement) shall never exceed the lesser of
     $5,000,000 or the sum of:

     (a)   eighty percent (80%) of the Domestic Eligible Accounts, as
           hereinafter defined; and

     (b)   ninety percent (90%) of the Foreign Eligible Accounts, as hereinafter
           defined.

3.   FORMULA COMPLIANCE. If the limitations in paragraph 2 above are exceeded at
     any time, Borrower shall immediately pay Bank sums sufficient to reduce the
     Revolving Loans by the amount of such excess.

4.   ELIGIBLE ACCOUNT. "Eligible Account" shall mean an Account (as hereinafter
     defined) arising in the ordinary course of a Debtor's business which meets
     each of the following requirements:

     (a)   it is not owing more than ninety (90) days after the date of the
           original invoice or other writing evidencing such Account;

     (b)   it is not owing by an Account Debtor (as hereinafter defined) who has
           failed to pay twenty five percent (25%) or more of the aggregate
           amount of its Accounts owing to a Debtor within ninety (90) days
           after the date of the respective invoices or other writings
           evidencing such Accounts;

     (c)   it arises from the sale or lease of goods and such goods have been
           shipped or delivered to the Account Debtor under such Account; or it
           arises from services rendered and such services have been performed;

     (d)   it is evidenced by an invoice, dated not later than the date of
           shipment or performance, rendered to such Account Debtor or some
           other evidence of billing acceptable to Bank;

     (e)   it is not evidenced by any note, trade acceptance, draft or other
           negotiable instrument or by any chattel paper, unless such note or
           other document or instrument previously has been endorsed and
           delivered by Borrower to Bank;

     (f)   it is a valid, legally enforceable obligation of the Account Debtor
           thereunder, and is not subject to any offset, counterclaim or other
           defense on the part of such Account Debtor or to any claim on the
           part of such Account Debtor denying liability thereunder in whole or
           in part;

     (g)   it is subject to a first priority, properly perfected security
           interest in favor of Bank and is not subject to any sale of accounts,
           any rights of offset, assignment, lien or security interest
           whatsoever other than to Bank;

     (h)   it is not owing by a subsidiary or affiliate of Borrower;

     (i)   it is not owing by an Account Debtor which is the government of any
           foreign country or sovereign state, or of any state, province,
           municipality or other instrumentality thereof;

     (j)   it is not an Account owing by the United States of America or any
           state or political subdivision thereof, or by any department, agency,
           public body corporate or other instrumentality of any of the
           foregoing, unless all necessary steps are taken to comply with the
           Federal Assignment of Claims Act of 1940, as amended, or with any
           comparable state law, if applicable, and all other necessary steps
           are taken to perfect Bank's security interest in such Account;

     (k)   it is not owing by an Account Debtor for which Borrower has received
           a notice of (i) the death of the Account Debtor or any partner of the
           Account Debtor, (ii) the dissolution, liquidation, termination of
           existence, insolvency or business failure of the Account Debtor,
           (iii) the appointment of a receiver for any part of the property of
           the Account Debtor, or (iv) an assignment for the benefit of
           creditors, the filing of a petition in bankruptcy, or the
           commencement of any proceeding under any bankruptcy or insolvency
           laws by or against the Account Debtor;

     (l)   it is not an Account billed in advance, payable on delivery, for
           consigned goods, for guaranteed sales, for unbilled sales, for
           progress billings, payable at a future date in accordance with its
           terms, subject to a retainage or holdback by the Account Debtor or
           insured by a surety company;

     (m)   it is not that portion of all Accounts owing by any single Account
           Debtor (other than PC-Tel) which exceeds twenty-five percent (25%) of
           the aggregate of all Accounts otherwise deemed eligible hereunder
           which are owing to Borrower by all Account Debtors;

     (n)   it is not that portion of all Accounts owing by PC-Tel which exceeds
           fifty percent (50%) of the aggregate of all Accounts otherwise deemed
           eligible hereunder which are owing to Borrower by all Account
           Debtors; and

     (o)   it is not owing by any Account Debtor whose obligations Bank, acting
           in its sole discretion, shall have notified Borrower are not deemed
           to constitute Eligible Accounts.

For purposes of this Agreement, an "Account" shall mean any right of a Borrower
to payment for goods sold or leased or for services rendered, but shall not
include interest or service charges, and "Account Debtor" shall mean a person
who is obligated on or under an Account. An Account which is at any time an
Eligible Account, but which subsequently fails to meet any of the foregoing
requirements, shall forthwith cease to be an Eligible Account.

     "Domestic Eligible Account" shall mean an Eligible Account that is owing by
     an Account Debtor which maintains its chief executive office in the United
     States of America and is organized under the laws of the United States of
     America, or any state thereto.

     "Foreign Eligible Account" shall mean an Eligible Account (a) that is owing
     by an Account Debtor which either does not maintain its chief executive
     office in the United States of America or is not organized under the laws
     of the United States of America, or any state thereto and (b) with respect
     to which either the Account Debtor has delivered an irrevocable letter of
     credit issued or confirmed by a bank satisfactory to Bank and payable only
     in the United States of America and in U.S. dollars, sufficient to cover
     such Account, in form and substance satisfactory to Bank, or such Account
     is subject to credit insurance payable to Bank issued by an insurer and on
     terms and in an amount acceptable to Bank.

5.   INSPECTIONS; COMPLIANCE. Borrower shall permit Bank and its designees from
     time to time to make such inspections and audits, and to obtain such
     confirmations or other information, with respect to any of the Collateral
     or any Account Debtor as Bank is entitled to make or obtain under the
     Credit Agreement or the Security Agreement, and shall reimburse Bank on
     demand for all costs and expenses incurred by Bank in connection with such
     inspections and audits. Borrower shall further comply with all of the other
     terms and conditions of the Credit Agreement and the Security Agreement.

6.   DEFAULT. Any failure by Borrower to comply with this Agreement shall
     constitute a default under the Credit Agreement.

7.   AMENDMENTS; WAIVERS. This Agreement may be amended, modified or terminated
     only in writing duly executed by Borrower and Bank. No delay by Bank in
     requiring Borrower's compliance herewith shall constitute a waiver of such
     right. The rights granted to Bank hereunder are cumulative, and in addition
     to any other rights Bank may have by agreement or under applicable law.
     This Agreement shall supersede and replace in their entirety any prior
     advance formula agreements in effect between Bank and Borrower.
     This Agreement shall be governed by and construed in accordance with the
     internal laws of the State of Texas, without regard to conflict of laws
     principles.

8.   DILUTION OF ACCOUNTS. In the event the Bank, at any time in its sole
     discretion, determines that the dollar amount of Eligible Accounts
     collectable by Borrower is reduced or diluted as a result of discounts or
     rebates granted by Borrower to the respective Account Debtor(s), returned
     or rejected inventory or services, or such other reason or factor as Bank
     deems applicable, Bank may, in its sole discretion, upon five (5) business
     days' prior written notice to Borrower, reduce or other wise modify the
     percentage of Eligible Accounts included within the Advance Formula under
     paragraph 2 above and/or reduce the dollar amount of Borrower's Eligible
     Accounts by an amount determined by Bank in its sole discretion.


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12.  JURY WAIVER. BORROWER AND BANK ACKNOWLEDGE THAT THE RIGHT TO TRIAL BY JURY
     IS A CONSTITUTIONAL ONE, BUT THAT IT MAY BE WAIVED. EACH PARTY, AFTER
     CONSULTING (OR HAVING HAD THE OPPORTUNITY TO CONSULT) WITH COUNSEL OF THEIR
     CHOICE, KNOWINGLY AND VOLUNTARILY, AND FOR THEIR MUTUAL BENEFIT WAIVES ANY
     RIGHT TO TRIAL BY JURY IN THE EVENT OF LITIGATION REGARDING THE PERFORMANCE
     OR ENFORCEMENT OF, OR IN ANY WAY RELATED TO, THIS AGREEMENT OR THE
     INDEBTEDNESS.

13.  SPECIAL PROVISIONS*


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     *None, if left blank.

IN WITNESS WHEREOF, this Agreement has been duly executed as of the day and year
first above written.

Borrower's Chief Executive Office Address:      BORROWER:

4635 Boston Lane                                SILICON LABORATORIES INC.
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Austin, Texas 78735                             By: /s/ Navdeep Sooch
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                                                    SIGNATURE OF

------------------------------------------          Its: Chief Executive Officer
                                                         TITLE (IF APPLICABLE)

                                                By: /s/ John W. McGovern
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                                                    SIGNATURE OF

                                                    Its: Chief Financial Officer

Accepted and Approved:

COMERICA BANK-TEXAS, a Texas banking association

By: /s/ Julie A. Smith
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    SIGNATURE OF JULIE A. SMITH

Its:
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    TITLE: VICE PRESIDENT