javascript function to update dropdownlist on the form

hi
i have a multiline textbox on the form for the user to enter some description. On its OnTextChnaged event, i m using the text in the textbox and populate a dropdownlist with some values.
the textbox's html readd somthing like this
<td><asp:TextBox ID="TxtAText" TextMode="MultiLine" CssClass="activityTxt" runat="server" Text='<%#Bind("Text")%>' OnTextChanged="TxtAText_TextChanged" AutoPostBack="true"></asp:TextBox></td>
also the onTextChnaged Event for the textbox in the code behind is
protected void TxtAText_TextChanged(object sender, EventArgs e) { StringBuilder sb = new StringBuilder(); string text = TxtAText.Text; string dummy = string.Empty; DdlIndex.Items.Clear(); for (int i = 1; i <= 20; i++) { ...