Systems and methods for closed loop control to ensure a constant current output with a changing load resistance
A closed loop control system automatically ensures that an output of a device is constant. The system can receive an input to set a fixed value for a variable (e.g., a current, a heart rate, a tissue perfusion, an ion level, etc.), and this variable can be delivered to a feedback component. The system can also include the device to deliver the variable to a load. The feedback component can be coupled to the delivery device to sample the output of the delivery device at different times. Based on the sampling, the feedback component can vary a property of the delivery device related to the delivery of the variable to the load to ensure that the variable remains constant at the fixed value. In some instances, the system can be implemented as a stimulator that delivers the constant current of a current source and has a low output impedance of a voltage source.
1. A stimulator with a low output impedance, the stimulator comprising:
a voltage source configured to generate an input voltage waveform;
a variable gain voltage amplifier configured to receive the input voltage waveform from the voltage source and provide an output voltage to a load; and
a feedback component, coupled to a current return of the adjustable gain amplifier, comprising a set point voltage source configured to generate a set point voltage and an amplifier configured to compare an output of a current power detector circuit to the set point voltage,
wherein the feedback component is configured to determine the voltage delivered to the load by sampling current into the load over a time period, estimating a power based on the current into the load over the time period, and
wherein a gain of the adjustable gain amplifier is adjusted based on a difference between the output of the current power detector and the set point voltage so the current power output to the load remains constant.
2. The stimulator of claim 1 , wherein the power is estimated by calculating a root mean square of the current into the load.
3. The stimulator of claim 1 , wherein the current power detector comprises a resistor in series with the load to sample the current into the load and a differential amplifier with a constant gain to determine the output of the current power detector.
4. The stimulator of claim 1 , wherein the circuit further comprises a second amplifier configured to receive the output and the set point voltage and provide the difference between the output of the current power detector and the set point voltage to adjust the gain of the adjustable gain amplifier.
5. The stimulator of claim 1 , wherein the stimulator is configured to reduce charge accumulation at a tissue-electrode interface, wherein the load is tissue.