Supply selection circuit with input noise suppression and wide operating range

A system, comprising:

• a first voltage supply terminal coupled to a first current source;
• a second voltage supply terminal coupled to a second current source;
• a first transistor having a gate coupled to the first current source, a drain coupled to the first voltage supply terminal, and a source coupled to a first input of a comparator;
• a second transistor having a gate coupled to the first current source, a drain coupled to the first voltage supply terminal, and a source coupled to a first switch;
• a third transistor having a gate coupled to the second current source, a drain coupled to the second voltage supply terminal, and a source coupled to a second input of a comparator;
• a fourth transistor having a gate coupled to the second current source, a drain coupled to the second voltage supply terminal, and a source coupled to a second switch;
• wherein the comparator has a comparator output coupled to the first switch and a second switch, wherein the first switch and the second switch are coupled to an output voltage terminal;
• wherein the first transistor is configured to transfer a first voltage from the first voltage supply terminal to the first input of the comparator;
• and
• wherein the third transistor is configured to transfer a second voltage from the second voltage supply terminal to the second input of the comparator.

A system, comprising:

• a first voltage supply terminal coupled to a first current source;
• a second voltage supply terminal coupled to a second current source;
• a first transistor having a gate coupled to the first current source, a drain coupled to the first voltage supply terminal, and a source coupled to a first input of a comparator;
• a second transistor having a gate coupled to the first current source, a drain coupled to the first voltage supply terminal, and a source coupled to a first switch;
• a third transistor having a gate coupled to the second current source, a drain coupled to the second voltage supply terminal, and a source coupled to a second input of a comparator;
• a fourth transistor having a gate coupled to the second current source, a drain coupled to the second voltage supply terminal, and a source coupled to a second switch;
• wherein the comparator has a comparator output coupled to the first switch and the second switch, wherein the first switch and the second switch are coupled to an output voltage terminal;
• wherein the first transistor is configured to transfer a first voltage from the first voltage supply terminal to the first input of the comparator;
• wherein the third transistor is configured to transfer a second voltage from the second voltage supply terminal to the second input of the comparator;
• and
• wherein the output voltage terminal is configured to provide an output voltage to a universal serial bus (USB) power delivery controller.

A method, comprising:

• enabling a first voltage supply and a second voltage supply;
• selecting, with a decision comparator, a selected voltage supply from either the first voltage supply or the second voltage supply based on which of the first voltage supply or the second voltage supply ramps up first;
• monitoring the selected voltage supply with the decision comparator;
• and
• responsive to a voltage from the selected voltage supply dropping below a dropout voltage level of a low dropout regulator, switching to an other voltage supply with the decision comparator.