Shunt Regulator for the Serial Powering of the ATLAS CMOS Pixel Detector Modules

A shunt regulator was designed to meet the specifications for the serial powering of the CMOS pixel detector modules in compatibility with the next upgrade of the ATLAS detector. Serial powering greatly increases the system's power efficiency when compared to a parallel powering scheme and allows for significant material budget savings in the power cabling. In such a scheme, each pixel detector chip is powered by a shunt regulator that takes in a constant current and produces a regulated output voltage relative to the module's potential ground. The proposed regulator has a modular structure. Each regulator module consists of a shunt regulation submodule followed by a low-dropout voltage regulation submodule and is designed to deliver a nominal output current of 10 mA. The regulator module's schematic is presented along with a theoretical study and stability analysis. A test chip was designed in the 0.18-CMOS technology containing one main shunt regulator composed of 126 modules, as well as two separate regulators each composed of one single module. The characterization measurements show a correct dc startup for various load conditions, as expected by simulations. The output voltage of a single module is regulated with a precision <1%. Moreover, the regulator module works with a low voltage dropout of 200 mV for a large range of input current from 3 to 18 mA. The equivalent series resistance of a 40-module regulator is measured to be 15 including the wire bonding and test bench parasitics. The test chip is successfully tested in the serial mode and in the parallel mode. In the latter mode, the current mismatch between the parallel chips is measured to be less than 3.4% for an input current of 1 A. Moreover, transient measurements performed with an active load show proper functioning with no undershoots or overshoots. Finally, the test chip was irradiated with an X-ray source up to 125 Mrad. Measurements show a stable response of the regulator with an intrinsic output voltage variation of less than 1%.