Balloonpopup with update panel

I have an update panel that has a set of text boxes that the user may complete with a check bosx that whe n cecked will disable the current textboxes and enable another set. These are all contained in an update panel and triggered by the checkbox. I dont think that this is working quite right since it looks like there is a postback occuring. Secondly, I have a balloon popupthat explains to the user when they should use this option, which i want to trigger withthe same checkbox. The balloon displays mometnarily - then it appears to post back and the balloon closes - too quickly to read.

<asp:UpdatePanel ID="UpdatePanel1" runat="server" ChildrenAsTriggers="true" UpdateMode="Conditional"><ContentTemplate></td><td><div style="margin-top:10px;">
                                                        Total&nbsp;Time:</div><div style="margin-top:10px;">
                                                        Ride&nbsp;Time:</div><div style="margin-top:10px;">
                                                        Multi-Day&nbsp;Event<asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="bikelist" DataTextField="riderbikeid" DataValueField="riderbikeid"></asp:DropDownList></div></td><td colspan="3"><div><asp:TextBox ID="tbthr" runat="server" TextMode="Number" ToolTip="Enter # of Hours" Width="30" />H<asp:TextBox ID="tbtmin" runat="server" Width="30" />&nbsp;M</div><div><asp:TextBox ID="tbh" runat="server" TextMode="Number" ToolTip="Enter # of Hours" Width="30px" />H<asp:TextBox ID="tbm" runat="server" Width="30" />&nbsp;M</div>&nbsp;<div><asp:CheckBox ID="cbmd" runat="server" AutoPostBack="True" OnCheckedChanged="cbmd_CheckedChanged" />&nbsp;<asp:TextBox ID="tbmdd" runat="server" width="30" />&nbsp;D &nbsp;<asp:TextBox ID="tbmdh" runat="server" width="30" />&nbsp;H<asp:TextBox ID="tbmdm" runat="server" width="30" />&nbsp;M</div><ajaxToolkit:BalloonPopupExtender ID="ImageButton3_BalloonPopupExtender" runat="server" 
                                            BalloonPopupControlID="Panel2" BalloonSize="Large" BalloonStyle="Rectangle" Enabled="True" 
                                            ExtenderControlID="" TargetControlID="cbmd" EnableViewState="true"></ajaxToolkit:BalloonPopupExtender></td></ContentTemplate></asp:UpdatePanel>

code behind

protected void cbmd_CheckedChanged(object sender, EventArgs e)
    {
        CheckBox cbmd = (CheckBox)FormView1.FindControl("cbmd");
        TextBox tbmdd = (TextBox)FormView1.FindControl("tbmdd");
        TextBox tbmdh = (TextBox)FormView1.FindControl("tbmdh");
        TextBox tbmdm = (TextBox)FormView1.FindControl("tbmdm");
        TextBox tbm = (TextBox)FormView1.FindControl("tbm");
        TextBox tbh = (TextBox)FormView1.FindControl("tbh");
        TextBox tbth = (TextBox)FormView1.FindControl("tbthr");
        TextBox tbtm = (TextBox)FormView1.FindControl("tbtmin");
        Label lblmday = (Label)FormView1.FindControl("lblmday");
        if (cbmd.Checked == true)
        {
            tbmdd.Enabled = true;
            tbmdh.Enabled = true;
            tbmdm.Enabled = true;
            tbm.Enabled = false;
            tbh.Enabled = false;
            tbth.Enabled = false;
            tbtm.Enabled = false;
            tbm.Text = "";
            tbh.Text = "";
            tbth.Text = "";
            tbtm.Text = "";
            //lblmday.Visi
}