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.