US Patent 7605644 Integrated circuit power-on control and programmable comparator

An integrated circuit is provided with a main supply rail and a virtual supply rail connected by strong and weak header transistors. A power-on controller controls the switching on of the strong transistors after the virtual supply rail voltage has already been driven up to close to its operating level by the weak transistor. The power-on controller comprises a comparator monitoring a single reference voltage level with its output being latched within a latch and used to switch on the strong transistor. The comparator may be programmable to detect multiple different trigger voltage levels by using opposing charging and discharging transistors with one set of these operating in a saturated regime and the other in a regime in which the current therethrough varies in dependence upon the voltage being sensed. These opposing transistors can be used to charge or discharge a node with the state of that node being taken to generate the sensed output.