The Supertex HV421 is a high voltage ring generator designed to drive 1 North American REN (ringer equivalent number) from a 5V source. The HV421 has an internal DC-DC converter which converts the 5V DC supply to a nominal 68V DC connected to the VPP pin. The DC-DC converter frequency of the HV421 is set by an external resistor connected between RSW and VDD. The ringing signal is generated by a high voltage H-bridge which produces two square waves which are 180 degrees from each other. The ringing frequency of the H-bridge is set by an external resistor connected between RRING and VDD.