click event of button which is outside the update panel cant work....

    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel runat="server" ID="TimedPanel" UpdateMode="Conditional">
        <ContentTemplate>

        <asp:GridView ID="_gvEmployee" runat="server">
           // Grid Code
        </asp:GridView>
        </ContentTemplate>
    </asp:UpdatePanel>
    <br />
    <br />
<asp:Button ID="_btnDownload" runat="server" Text="Pdf" OnClick="_btnDownload_Click" />

when i m clicking on the button its workin properly

but when i m putting this button inside the updatepanel its cant work