Described is an apparatus which comprises: a clamp coupled between a first power supply and a second power supply, the clamp including a plurality of transistors, a circuit to operate with the second power supply; and a control unit to turn on and off the plurality of transistors to adjust the second power supply when the apparatus enters a low power mode. The control unit includes a first comparator to compare the second power supply with a first reference, a second comparator to compare the second power supply with a second reference, and a counter. The counter counts up when the second power supply is higher than the first reference and counts down when the second power supply is lower than the second reference.