The IMX28AEC Technical Data PDF says that "Application should include a Schottky diode between BATT and VDD4P2", but it didn't mention any current rating.
In Figure 8 of the AN4199 AppNote the diode is also shown. Although the MBR120VLSFT1G has a maximum rated current of 1A, I think that the diode is only needed for biasing the internal VDD4P2 circuit. Thus, the current would be theoretically negligible.
Does anybody know something more detailed about it?