Patent ID: 6084390
Filing Date: 2000-07-04
Classification: G05F,H02J,Y02B,Y04S

Abstract:
A power supply switching circuit for switching power supplies of an integrated circuit, the power supply switching circuit comprising:a comparison element that receives a control signal at a control terminal, a first voltage signal, and a second voltage signal, compares the first voltage signal and the second voltage signal, and generates a compare signal, wherein the first voltage signal tracks changes in a primary supply voltage provided by primary power source at a first rate, the second voltage signal tracks changes in the primary supply voltage at a second rate, and the first voltage signal is equal to the second voltage signal at a predetermined value of the primary supply voltage;a current mirror element, coupled to the primary supply voltage and a reference voltage, comprising a reference leg, having a first transistor of a first type coupled to the primary supply voltage and in parallel to a second transistor of the first type and a first transistor of a second type coupled in series to the first and second transistors of the first type and to the reference voltage, that operates as a current source to generate a current and having at least a first mirror leg that operates to mirror the current generated by the reference leg as the control signal to the control terminal of the comparison element, wherein the second transistor of the first type is sized large enough that the current generated by the reference leg when the first and second transistors of the first type are enabled in a back-biased mode is sufficient to stabilize the comparison element;a control element that receives the compare signal generated by the comparison element and selectively enables the second transistor of the first type of the reference leg in accordance with the compare signal, thereby controlling the current generated by the reference leg of the current mirror element according to the compare signal; anda power coupling element controlled by the compare signal generated by the comparison element to selectively couple the primary supply voltage or a secondary supply voltage produced by a secondary power source to an output node of the system that is coupled to a substrate of the integrated circuit,wherein when the primary supply voltage is less than the predetermined value, the first voltage signal is less than the second voltage signal and the compare signal generated by the comparison element transitions from a first logic state to a second logic state, the control element receives the compare signal of the second logic state and enables the second transistor of the first type in order that the current generated by the reference leg of the current mirror element and provided to the control terminal of the comparison element is generated by both the first and second transistors of the first type operating in a back-biased mode, and the power coupling element selectively couples the secondary supply voltage to the output node, andwhen the primary supply voltage is more than the predetermined value, the first voltage signal is more than the second voltage signal and the compare signal generated by the comparison element transitions from the second logic state to the first logic state, the control element receives the compare signal of the first logic state and disables the second transistor of the first type in order that the current generated by the reference leg of the current mirror element is generated by the first transistor of the first type in a normal mode, and the power coupling element selectively couples the primary supply voltage to the output node.