Si9122A is a half-bridge controller IC ideally suited to fixed telecom applications where high efficiency is required at low output voltages (e.g. < 3.3 V). Designed to operate within the fixed telecom voltage range of 33 V to 75 V and withstand 100 V, 100 ms transients, the IC is capable of controlling and driving both the low and high-side switching devices of a half bridge circuit and also controlling the switching devices on the secondary side of the bridge. Due to the very low onresistance of the secondary MOSFETs, a significant increase in conversion efficiency can be achieved as compared with conventional Schottky diodes. Control of the secondary devices is by means of a pulse transformer and a pair of inverters. Such a system has efficiencies well in excess of 90 % even for low output voltages. On-chip control of the dead time delays between the primary and secondary synchronous signals keep efficiencies high and prevent accidental destruction of the power transformer. An external resistor sets the switching frequency from 200 kHz to 625 kHz. Si9122A has advanced current monitoring and control circuitry which allow the user to set the maximum current in the primary circuit. Such a feature acts as protection against output shorting and also provides constant current into large capacitive loads during start-up or when paralleling power supplies. Current sensing is by means of a sense resistor on the low-side primary device.