The PSB 2120 is a Pulse Width Modulator (PWM) circuit designed for fixed-frequency switching regulators especially for telephony and ISDN-environments. The PSB 2120 is fully compatible with the CCITT-power recommendations on the "S"-interface. Coupled with a few external components it can provide a stable 5 V DC-supply for subscriber terminals (TE's) or network terminators (NT's). It can also be programmed for higher output voltages, e.g. to supply the S-lines with 40 V. In telephony and ISDN-systems a high conversion yield is crucial to maintain functionality in all supply conditions via "S"- or "U"-interfaces. The PSB 2120 design and technology realizes high conversion efficiency and low power dissipation. It should be recognized that the PSB 2120 can also be used in numerous DC/DC-conversion systems other than ISDN-power supplies.