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.