FEATURES
• 28 V to 75 V input voltage range
• Compatible with ETSI 300 132-2
• Integrated ± 1 A half-bridge primary drivers
• Secondary synchronous rectifier control
signals with programmable deadtime delay
• Voltage mode control
• Voltage feedforward compensation
• High voltage pre-regulator operates during start-up
• Current sensing on low-side primary device
• Frequency foldback eliminates constant current tail
• Advanced maximum current control during start-up and
shorted load
• Low input voltage detection
• Programmable soft-start function
• Over temperature protection
APPLICATIONS
• Network cards
• Power supply modules
DESCRIPTION
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.