Power Supply Switch with Programmable Switching Decision

A power supply selector circuit comprising:

• a first power-supply input for receiving a first power-supply voltage;
• a second power-supply input for receiving a second power-supply voltage;
• a power pre-selector, connected to the first power-supply input and to the second power-supply input, for comparing the first power-supply voltage to the second power-supply voltage,
• a common supply node generated by the power pre-selector, wherein the power pre-selector connects the first power-supply input to the common supply node when the first power-supply voltage is above the second power-supply voltage, and wherein the power pre-selector connects the second power-supply input to the common supply node when the first power-supply voltage is below the second power-supply voltage;
• an open-loop decision circuit, powered by the common supply node and connected to the first power-supply input, the open-loop decision circuit comparing the first power-supply voltage to a threshold voltage and activating a first switch signal when the first power-supply voltage is above the threshold voltage, and activating a second switch signal when the first power-supply voltage is below the threshold voltage;
• an output supply node for powering a system load;
• a first switch that connects the first power-supply input to the output supply node when the first switch signal is activated, and for isolating the first power-supply input from the output supply node when the first switch signal is not activated;
• and
• a second switch that connects the second power-supply input to the output supply node when the second switch signal is activated, and for isolating the second power-supply input from the output supply node when the second switch signal is not activated;
• wherein the first power-supply input is connected to the output supply node by the first switch when the first power-supply voltage is less than the second power-supply voltage and greater than the threshold voltage;
• wherein the first power-supply input continues to be connected to the output supply node when the second power-supply voltage is greater than the first power-supply voltage, and the first power-supply voltage has not yet fallen below the threshold voltage
• whereby the first power-supply input is switched to the output supply node in response to comparison with the threshold voltage, while the common supply node is switched in response to comparison between the first and second power-supply voltages.