Alternately, the 3V rail may be used to power the primary side as long as the secondary side is powered with an isolated supply such as the one shown in the illustration. When a voltage between 3V and 3.7V is applied to Vcc, the internal DC-to-DC converter is disabled and the primary side continues to operate normally. However, the secondary must now be powered with an external isolated voltage of 3V to 5.5V through the Viso pin.