The PSB 2121 is a pulse width modulator circuit designed for fixed-frequency switching regulators with very low power consumption. 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 2121 design and technology realize high conversion efficiency and low power dissipation. It should be recognized that the PSB 2121 can also be used in numerous DC/DC-conversion systems other than ISDN-power supplies.