Power switch control between USB and wireless power system
An electrical system can selectively power a load via a USB connection or via another power source, such as a wireless power transfer path. An integrated switch controller determines whether to power the load via the USB connection or the other power sources and controls two external transistors via a single I/O pin connection to implement that determination. The switch controller determines the greater of two voltages: a voltage associated with the USB connection and a voltage associated with the other power source. The switch controller also determines whether there is a valid USB connection. The switch controller circuitry that controls the two external transistors is powered at the greater voltage to ensure that the external transistors are appropriately and securely turned on or off.
1. An apparatus including switch control circuitry that controls Universal Serial Bus (USB) power transfer circuitry used to selectively convey power from a USB connection to a load, wherein the load can also be powered by another power source, the switch control circuitry comprising:
switch on/off circuitry comprising an inverter connected to generate a control signal used to control the USB power transfer circuitry;
voltage selection circuitry that (a) selects the greater of (i) a USB voltage corresponding to the USB connection and (ii) another voltage corresponding to the other power source and (b) powers the inverter of the switch on/off circuitry based on the selected greater voltage; and
USB detection circuitry that determines whether or not the USB connection is valid and controls the switch on/off circuitry accordingly, and
wherein the switch on/off circuitry further comprises:
logic circuitry configured to generate a logic signal in a first voltage domain; and
a level shifter connected to shift the logic signal from the first voltage domain to the voltage domain of the selected greater voltage to drive the inverter.
2. The apparatus of claim 1 , wherein the USB power transfer circuitry comprises two transistors connected between the USB connection and the load.
3. The apparatus of claim 2 , wherein:
the switch control circuitry is implemented on an integrated circuit;
the two transistors are implemented external to the integrated circuit; and
the switch control circuitry controls the two transistors via a single input/output pin connection of the integrated circuit.
4. The apparatus of claim 2 , wherein:
the two transistors are two p-type transistors connected in series; and
bulk terminals of the two transistors are connected to common channel terminals of the two transistors.
5. The apparatus of claim 1 , wherein the other power source is a wireless power transfer path.
6. The apparatus of claim 1 , wherein the voltage selection circuitry comprises:
a differential comparator connected to compare the USB voltage and the other voltage and generate two differential control signals based on the greater of the USB voltage and the other voltage; and
a pair of transistors controlled by the two differential control signals to drive an output node of the voltage selection circuitry to the selected greater voltage.
7. The apparatus of claim 6 , wherein the differential comparator is powered by the selected greater voltage.
8. The apparatus of claim 1 , wherein the USB detection circuitry comprises a first comparator connected to determine whether the USB voltage is above a specified lower USB threshold voltage.
9. The apparatus of claim 8 , wherein the USB detection circuitry further comprises:
a second comparator connected to determine whether the USB voltage is below a specified upper USB threshold voltage; and
logic circuitry connected to determine whether or not the USB voltage is between the lower and upper USB threshold voltages.
10. The apparatus of claim 9 , wherein the USB detection circuitry further comprises:
a first voltage divider connected to generate a first divided voltage from the USB voltage;
a second voltage divider connected to generate a second divided voltage from the USB voltage; and
a reference voltage source connected to provide a voltage reference signal, wherein:
the first comparator compares the first divided voltage to the voltage reference signal to determine whether the USB voltage is above the specified lower USB threshold voltage; and
the second comparator compares the second divided voltage to the voltage reference signal to determine whether the USB voltage is below the specified upper USB threshold voltage.
11. The apparatus of claim 1 , wherein the apparatus further comprises the USB power transfer circuitry and the load.
12. The apparatus of claim 11 , wherein the apparatus is a consumer electronic device.
13. The apparatus of claim 1 , wherein:
the USB power transfer circuitry comprises two transistors connected between the USB connection and the load;
the switch control circuitry is implemented on an integrated circuit;
the two transistors are implemented external to the integrated circuit;
the switch control circuitry controls the two transistors via a single input/output pin connection of the integrated circuit;
the two transistors are two p-type transistors connected in series;
bulk terminals of the two transistors are connected to common channel terminals of the two transistors;
the other power source is a wireless power transfer path;
the voltage selection circuitry comprises:
a differential comparator connected to compare the USB voltage and the other voltage and generate two differential control signals based on the greater of the USB voltage and the other voltage; and
a pair of transistors controlled by the two differential control signals to drive an output node of the voltage selection circuitry to the selected greater voltage;
the differential comparator is powered by the selected greater voltage;
the USB detection circuitry comprises:
a first comparator connected to determine whether the USB voltage is above a specified lower USB threshold voltage;
a second comparator connected to determine whether the USB voltage is below a specified upper USB threshold voltage;
logic circuitry connected to determine whether or not the USB voltage is between the lower and upper USB threshold voltages;
a first voltage divider connected to generate a first divided voltage from the USB voltage;
a second voltage divider connected to generate a second divided voltage from the USB voltage; and
a reference voltage source connected to provide a voltage reference signal, wherein:
the first comparator compares the first divided voltage to the voltage reference signal to determine whether the USB voltage is above the specified lower USB threshold voltage; and
the second comparator compares the second divided voltage to the voltage reference signal to determine whether the USB voltage is below the specified upper USB threshold voltage; and
the apparatus further comprises the USB power transfer circuitry and the load.