Adaptive constant current engine
An adaptive constant current engine (ACCE) to control a current of a target load device at a constant level is disclosed. The ACCE includes a closed feedback loop which generates a sensed current that represents a load current conducted by the target load device and then compares the sensed current to a reference current. Based on this comparison, the circuit can output a voltage to control the current conducted by the target load device. The disclosed ACCE adapts the comparison to an operating voltage of the target load device in order to provide electrical conditions at a reference node receiving a reference current that are the same as electrical conditions at a sensing node receiving a sensing current. Accordingly, the ACCE can generate very accurate comparisons regardless of variations in the operating voltage of the target load device.
1. A circuit comprising:
a reference current conveyer configured to receive a common load voltage at a first input of the reference current conveyer and to receive a reference current at a second input of the reference current conveyer, and configured to generate a reference-feedback voltage at an output of the reference current conveyer based on the common load voltage and the reference current;
a sensing current conveyer configured to receive the common load voltage at a first input of the sensing current conveyer and to receive a sensed current at a second input of the sensing current conveyer, and configured to generate a sensed voltage at an output of the reference current conveyer based on the common load voltage and the sensed current, wherein the sensing current conveyer and the reference current conveyer are coupled to each other in a butterfly configuration, the butterfly configuration characterized by the sensing current conveyer and the reference current conveyer having matching devices and topologies that are substantially the same; and
a differential amplifier configured to receive the reference-feedback voltage at a first input of the differential amplifier and the sensed voltage at a second input of the differential amplifier and configured to generate a control voltage at an output of the differential amplifier that corresponds to a difference between the sensed voltage and the reference-feedback voltage, the difference unaffected by variations of the common load voltage.
2. The circuit according got claim 1 , wherein the butterfly configuration includes the first input of the reference current conveyer coupled directly to the first input of the sensing current conveyer.
3. The circuit according to claim 2 , wherein:
the common load voltage at the first input of the reference current conveyer is transferred to the second input of the reference current conveyer and the common load voltage at the first input of the sensing current conveyer is transferred to the second input of the sensing current conveyer so that the reference current and the sensed current experience a load condition that is substantially the same.
4. The circuit according to claim 1 , wherein:
the reference current conveyer includes a first reference current mirror coupled to a second reference current mirror;
the sensing current conveyer includes a first sensing current mirror coupled to a second sensing current mirror, the first reference current mirror includes transistors that are matched with transistors of the first sensing current mirror; and
the second reference current mirror includes transistors that are matched with transistors of the second reference current mirror.
5. The circuit according to claim 1 , wherein:
the reference-feedback voltage is a voltage across a diode connected transistor of reference current conveyer; and
the sensed voltage is a voltage across a diode connected transistor of the sensing current conveyer.
6. The circuit according to claim 5 , wherein the diode connected transistor of the reference current conveyer is matched to the diode connected transistor of the sensing current conveyer.
7. The circuit according to claim 1 , wherein the sensed current is generated by a load current sensing circuit that includes a sense transistor coupled at its gate to the control voltage and configured to conduct the sensed current based on the control voltage.
8. The circuit according to claim 7 , wherein the sensing current conveyer sinks current from a current mirror coupled to the sense transistor.
9. The circuit according to claim 7 , wherein the sensing current converter sources current to the sense transistor.
10. The circuit according to claim 1 , wherein the differential amplifier is an opamp.
11. The circuit according to claim 10 , wherein the opamp is configured to apply a bias voltage to the control voltage.
12. A method for setting and maintaining a load current in a target load device to a constant level, the method comprising:
receiving a common load voltage from a remote circuit at an adaptive constant current engine including a load current sensing circuit, a sensing current conveyer, a reference current conveyer, and a differential amplifier;
applying the common load voltage to a first input of the sensing current conveyer;
applying the common load voltage to a first input of a reference current conveyer;
obtaining a sensed current using the load current sensing circuit, the sensed current a fraction of the load current;
applying the sensed current to a second input of the sensing current conveyer;
receiving a reference current from a reference current source at the adaptive constant current engine;
applying the reference current to a second input of the reference current conveyer;
generating a sensed voltage at an output of the sensing current conveyer based on the common load voltage and the sensing current;
generating a reference-feedback voltage at an output of the sensing current conveyer based on the common load voltage and the reference current;
comparing the sensed voltage to the reference-feedback voltage to obtain a control voltage; and
applying the control voltage to the target load device to set and maintain the load current at a constant level.
13. The method according to claim 12 , wherein the obtaining a sensed current using the load current sensing circuit includes:
applying the control voltage to a sense transistor to generate the sense current.
14. The method according to claim 13 , wherein the applying the sensing current to a second input of the sensing current conveyer comprises:
sinking the sense current from a current mirror coupled to the sense transistor.
15. The method according to claim 13 , wherein the applying the sensing current to a second input of the sensing current conveyer includes:
sourcing the sense current to the sense transistor.
16. A system for maintaining a constant current, the system comprising:
a target load device in a remote circuit, the target load device having a common load voltage and conducting a load current;
a reference current source providing a reference current proportional to the load current; and
an adaptive constant current engine configured to sense a sense current that is proportional to the load current and compare the sensed current to the reference current to produce a control voltage that is coupled to the target load device to control the load current, the adaptive constant current engine including:
a load current sensing circuit configured to receive the control voltage and generate the sensed current;
a sensing current conveyer operating at a common load voltage from the remote circuit and generating a sensed voltage based on the sensed current;
a reference current conveyer operating at the common load voltage from the remote circuit and generating a reference-feedback voltage based on the reference current; and
a differential amplifier configured to generate the control voltage based on a difference between the sensed voltage and the reference-feedback voltage, the sensing current conveyer and the reference current conveyer being matched so that the difference is independent of the common load voltage.
17. The system according to claim 16 , wherein the remote circuit is an oscillator.
18. The system according to claim 17 , wherein the target load device is a NMOS transistor configured to control a bias of the oscillator.
19. The system according to claim 16 , wherein the adaptive constant current engine is a high side adaptive constant current engine in which the sensing current conveyer sources a current to the load current sensing circuit.
20. The system according to claim 16 , wherein the adaptive constant current engine is a low side adaptive constant current engine in which the sensing current conveyer sinks a current from the load current sensing circuit.
21. The system according to claim 16 , wherein the sensing current conveyer and the reference current conveyer have matching topologies and matching devices, and both the sensing current conveyer and the reference current conveyer are coupled to the common load voltage so that variations in the common load voltage are received by each.