Getting rid of ripple from an LCD backlight
The ripple when switching on the LCD backlight leads to one suspicion: The backlight is an electro luminescent (EL) panel, driven by a high frequency inverter.

A typical EL inverter would consist of a switching circuit, producing a square wave load on the supply.

The common way of dealing with this is to decouple that load from the supply rail, at the source of the noise, i.e. right at the backlight driver. A large capacitance between supply and ground rails, placed as close as possible to the backlight driver, would be a start. If that too causes some ripple to remain at the supply, add an inductor between supply rail and backlight driver's supply input.