How do I make a Textbox Postback on KeyUp?

I have a Textbox that changes the content of a dropdown in the OnTextChanged event. This event seems to fire when the textbox loses focus. How do I make this happen on the keypress or keyup event?

Here is an example of my code

<asp:TextBox ID="Code" runat="server" AutoPostBack="true" OnTextChanged="Code_TextChanged">                

<asp:UpdatePanel ID="Update" runat="server">
    <ContentTemplate>
        <asp:DropDownList runat="server" ID="DateList" />             
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="Code" />
    </Triggers>
</asp:UpdatePanel>

So in the codebehind, I bind the dropdown on page load. The Code_TextChanged event just rebinds the dropdown. I would like this to happen on each keypress rather than when the textbox loses focus.

I inherited this code recently and this is not the ideal method of doing this for me, but time limitations prevent me from rewriting this in a web servicy method.

I have tried using jQuery to bind the "keyup" event to match the "change" event for the textbox, but this only works on the first key pressed.