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We report on the design of a prototype IC called PX90 dedicated for readout of hybrid semiconductor detectors used for X-ray imaging applications. The PX90 has dimensions of 4 mm × 4 mm and was designed in CMOS 90 nm technology with 9 metal layers. The core of the IC is a matrix of 40 × 32 pixels with 100 m × 100 m pixel size. A 60 m × 60 m square passivation opening in each pixel allows connecting PX90 to a semiconductor detector using stud bump bonding technique. Each pixel contains two charge sensitive amplifiers with Krummenacher feedback scheme, two second stage amplifiers, two discriminators and two 16-bit ripple counters. The stages are DC-coupled and the front-end electronics uses a fully differential readout scheme. To minimize the effective threshold spread at the discriminators inputs, one 8-bit and one 7-bit trim DACs are used. The PX90 can operate in continuous readout mode and in readout mode separate from exposure. The readout of each pixel has some additional functionality, like compression mode or readout of only given number of bits from each pixel. The data are read out via a single LVDS output with 200 Mbps rate.