<TextBlock Text="{Binding Text}">
     <TextBlock.ToolTip>
        <ToolTip
              DataContext="{Binding Path=PlacementTarget, RelativeSource={x:Static RelativeSource.Self}}"
                 Visibility="{Binding Converter={StaticResource toolVisConverter}}">
                 <TextBlock Text="{Binding Text}"/>  <!-- tooltip content -->
       </ToolTip>
   </TextBlock.ToolTip>