Master-detail dropdownlist (state-city) and requredfieldvalidators on ajax like page problem

Question

I want to user select one state from dropdownlist and asynchronously load related cities.

<asp:ScriptManager ID="ScriptManager1" runat="server">

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>                

<table style="direction:rtl; margin:20px; ">                    
<tr>...any textbox with requiredfieldvalidator...ddlStates ...ddlCities...
any textbox with requiredfieldvalidator

when i change selected index of ddlStates, dos'n load any city in ddlCities, becuasee another TextBox in the form is empty and so requiredfieldvalidator dont allow any postback.
who can set dropdownlist behaviours independent of validators?
plz help me.