You can use update panel
<asp:UpdatePanel runat="server" ID="YourID">
<contenttemplate>
<asp:TextBox ID="txtRegSubGroupDescription" runat="server" class="txtfield2" TextMode="MultiLine" onKeyDown="textMaxCounter(this, 200, cphContent_RegSubGroupDescription)" Height="70px" Width="252px">
</asp:TextBox>
</contenttemplate>
</asp:UpdatePanel>