ValidatorCalloutExtender in asp.net

 <asp:DropDownList runat="server" ID="ddlEquipment" AppendDataBoundItems="true"
                        class="class1">
                        <asp:ListItem Value="-1">--Select Equipment--</asp:ListItem>
                    </asp:DropDownList>
                    <asp:RequiredFieldValidator ID="rfv_Equipment" runat="server" ControlToValidate="ddlEquipment"
                        InitialValue="-1" ErrorMessage="Please Select Equipment" Display="None"
                        ValidationGroup="Request">
                    </asp:RequiredFieldValidator>
                    <ajax:ValidatorCalloutExtender ID="ValidatorCalloutExtender5" runat="server" PopupPosition="BottomLeft"
                        TargetControlID="rfv_Equipment" Width="180px" WarningIconImageUrl="~/images/icon_blue.png"
                        CssClass="CustomValidatorCalloutStyle">