Update A Control Outside Of An Updatepanel?

I am going to show some text in a TextBox, which is located outside of an updatepanel, after checking a CheckBox but I cannot make it work.

Here is my code:

<asp:UpdatePanel runat="server" ID="uplMaster">
<ContentTemplate>
<asp:CheckBox ID="cbShowText" runat="server" Text="Show Some Text" AutoPostBack="true"
OnCheckedChanged="cbShowText_CheckedChanged" />
</ContentTemplate>
</asp:UpdatePanel>
<asp:TextBox ID="txtBox" Text="Empty" runat="server" />

Code Behind:

protected void cbShowText_CheckedChanged(object sender, EventArgs e)
{
txtBox.Text = "Some Text";
}