Improved devices, circuits and methods of operation in implantable stimulus systems. An implantable defibrillator may comprise a charging circuit using a transformer to store and build up energy on an HV capacitor or capacitor stack, with the HV capacitor in turn coupled to an H-bridge output circuit having low and high sides for issuing therapy. In the output current path, a current controlling circuitry is placed between the H-bridge and ground, allowing the greater flexibility in the selection of switching devices, and drivers for such devices, in the H-bridge circuit and/or enabling circuits between the H-bridge and the HV capacitor or other therapy circuit.