Many large scale integrated circuits contain multiple components providing different functionality and require two different supply voltage levels to operate.  One such integrated circuit would include, for example, a core processor and input/output functions on the same silicon substrate, but operating from two different voltage levels.  During startup, steady state, shut down and under fault conditions, the interaction between these voltages must meet strict requirements to ensure proper operation and to prevent damage to the integrated circuit.  The techniques used to ensure proper interaction of these voltage levels all fall under the class of methods known as "Rail Tracking". 
In a dual supply voltage mode scenario, typically, the larger of the two voltage rails will supply the input/output function, and the smaller of the two is used to power the core processor.  The larger of the two voltage levels is supplied to the input/output functionality of the integrated circuit, and to a voltage regulator which derives the second or lower voltage level for use in powering the core processor. 
The task of the voltage regulator consists of keeping the voltage on the output constant in a defined output range.  One form of voltage regulator comprises a switch mode power supply.  A switch mode power supply usually comprises a pulse width modulator (PWM), a power switch, a rectifier and an output filter.  The pulse width modulator controls the power switch which converts an input voltage into pulsed DC voltage with variable duty cycle which in effect maintains constant voltage on the output of the filter circuit.  In conventional voltage regulator voltage to power the PWM circuit is derived from the regulator's input voltage. 
Because the PWM circuit requires a finite period to achieve steady state conditions there is an initial period between the time that the voltage is supplied to the regulator input and the time in which the output is fixed at the second voltage level.  During this time the voltage difference between the input/output voltage and output core voltage may exceed maximum allowable limits causing damage to the integrated circuit. 
A prior art method dealing with this problem is disclosed in Power Trends application note PT5000/6000 SIP Series (Integrated Switching Regulators DC--DC Converters).  In this prior art solution, the voltage regulator is bypassed by a number of series connected diodes and a small resistor which are connected between the input/output voltage level and the core processor voltage.  The series-connected diodes limit the difference between the two voltage levels as will be discussed in greater detail hereinafter. 
There are shortcomings to this prior art method which render it unacceptable in certain circumstances.  For example, as the steady state difference between the input/output voltage and the core processor voltage approaches the maximum allowable voltage difference, the tolerance on the diode voltage drop becomes critical.  This tolerance is difficult to control inasmuch as the voltage drop across the diode junction is highly current and temperature dependent.  Additionally, the tracking voltage difference can be set only with the resolution of each single junction voltage drop which typically equals approximately 0.6 volts or 0.3 volts for Schottky technology.  Additionally, the series-connected diodes bypassing the voltage regulator negate any overcurrent protection provided by the voltage regulator.  In addition, the diodes themselves can be easily damaged if the regulator fails as all of the current associated with the second voltage level will now flow through the diodes.