The Jiangmen Underground Neutrino Observatory (JUNO) is a multipurpose neutrino experiment designed to determine neutrino mass hierarchy with a 20-thousand-ton liquid scintillator spherical detector at about 700-meter deep underground. The detection principle in JUNO is based on the observation of the light emitted in the Inverse Beta Decay in the liquid scintillator volume by array of large photomultipliers (LPMT). To reach the expected sensitivity of mass hierarchy, the energy resolution has to be better than 3% at 1 MeV. In order to achieve this target the LPMT will have to cover approximately 75% of the inner surface of the sphere, elevating the number of necessary LPMTs to 18, 000. In order to improve the calorimetry control of the LPMT system 25,000 small 3-inch small photo-multipliers (SPMTs) will be installed between the LPMTs. The size of these PMTs is chosen to operate in photon-counting mode for all events inside the detector. The large number of SPMTs increases the density and the complexity of the detector system. A customized multichannel Application Specific Integrated Circuit (ASIC), named CATIROC, with the integration of all the analog and digital components into a single chip will simplify the electronic system, decreasing the total power consumption and increasing the reliability. CATIROC has been designed in AMS SiGe 0.35 μm technology and it is composed of 16 independent channels that work in triggerless mode, auto-triggering on the single photo-electron (PE). It provides a charge measurement with a charge resolution of 15 fC and a timing information with a precision of 200 ps rms. It processes the analog signals up to digitization and sends out only zero-suppressed digital data to the central processing and storage unit.