The Supertex HV441 is a high voltage, power-efficient, switchmode ring generator IC requiring minimal external components. Using internal FET switches, it is capable of driving a 5 REN load (1400Ω) up to 70VRMS. Using external FETs, 20 RENs or more can be driven.

Source: http://www.supertex.com/pdf/app_notes/AN-H35.pdf