An implantable therapy generator that includes sensing circuitry that senses cardiac signals representative of atrial activity and ventricular activity detection circuitry connected to the sensing circuitry control circuitry that controls generation and selective delivery of a multi-stage atrial cardioversion therapy to implanted electrodes, each stage of the therapy including multiple pulses, each pulse including multiple high-frequency sub-pulses and therapy circuitry. The therapy circuitry includes a high-voltage charging circuit charging a storage capacitor to a predetermined voltage a delivery capacitor connectable to the storage capacitor and a control circuit adapted to selectively cause the storage capacitor to be electrically connected to the delivery capacitor so as to charge the delivery capacitor to a predetermined delivery voltage, and to cause a delivery switching circuit to be repeatedly opened and closed at a predetermined rate, thereby causing the sub-pulses to be transmitted to the electrodes.
