Feedback control of neurostimulation

An implantable device for controllably applying a neural stimulus, the device comprising:

• a plurality of electrodes including one or more stimulus electrodes and one or more sense electrodes;
• a stimulus source for providing a neural stimulus to be delivered from the one or more stimulus electrodes to a neural pathway in order to give rise to an evoked compound action potential on the neural pathway of a patient;
• measurement circuitry for recording a neural compound action potential signal sensed at the one or more sense electrodes;
• and
• a control unit configured to:
• control application of the neural stimulus as defined by a stimulus current parameter;
• measure via the measurement circuitry the compound action potential evoked by the neural stimulus;
• determine from the measured evoked compound action potential a feedback variable;
• and
• implement a feedback controller which completes a feedback loop, the feedback controller configured to use the feedback variable to control a logarithm of the stimulus current parameter so as to maintain the feedback variable at a setpoint.