ExtJS4: Give 'radiogroup' behavor to a 'buttongroup'

I have a toolbar in a panel.

In that toolbar there's a buttongroup with three button in a toggleGroup.
When I press a button it became pressed and every other pressed buttons became unpressed.

This is the standard behavior of a toggleGroup, but I need a more "radiobutton" like behavior.
In details, I need that a pressed button can't be unpressed by a click, but only by a click on the other buttons of togglerGroup.

Is there a simple way to do this?

This is the initContent function of the Ext.panel.Panel

Ext.applyIf(me, {
  dockedItems: [
    {
      xtype: 'toolbar',
      dock: 'top',
      items: [
        {
          xtype: 'buttongroup',
          itemId: 'typeView',
          columns: 3,
          items: [
            {
              xtype: 'button',
              itemId: 'buttonDetail',
              enableToggle: true,
              text: 'Dettagli',
              toggleGroup: 'typeView'
            },
            {
              xtype: 'button',
              itemId: 'buttonThumb',
              enableToggle: true,
              text: 'Preview',
              toggleGroup: 'typeView'
            },
            {
              xtype: 'button',
              itemId: 'buttonThumbBig',
              enableToggle: true,
              text: 'Preview grande',
              toggleGroup: 'typeView'
            }
          ]
        }
      ]
    }
  ],
  items: [
    ......
  ]
}

PS: I use Sencha Architect to develop

Solution

Use the allowDepress option. This means a pressed button cannot be "unpressed".