Speaker
Description
ATLAS ITk Pixel detector modules are operated in serial power mode using a Shunt-LDO circuit inside the ITkPix readout chip. Due to cooling requirements, the system must be operated within stringent power constraints, resulting in only 10% of the current being burned in the shunt during nominal physics operation. Since the current consumption scales with hit activity and there are significant uncertainties on the measured values of the shunt currents, we investigate the module’s operational margins, and focus on establishing production quality control procedures that ensure the operability of modules within the given power-constrained system.
Summary (500 words)
In preparation for the High-Luminosity LHC, the ATLAS experiment is replacing the innermost tracking detector with an advanced silicon Inner Tracker (ITk), composed of both pixel and strip detectors, that can accommodate the expected increase in radiation and particle activity. The ITk pixel system is constructed from planar and 3D silicon sensors bump-bonded to a front-end ASIC chip, called ITkPixV2, developed by the RD53 collaboration. An ITk pixel module contains three or four chips powered in parallel and the entire ITk pixel system is composed of roughly 9400 modules, covering a sensitive area of 13 m2. In order to minimize the amount of material in the ITk pixel tracking volume, the number of power cables is reduced by powering chains of pixel modules in series, with a total of roughly 1000 separate serial power chains. The study presented here focuses on establishing production quality control procedures that ensure operability of modules within the detector power constraints. It was in particular investigated how to measure in-module power consumption, and when and how failure modes manifest in case of exceeding the power limit.
A key component of the ITkPixV2 chip is its shunt low-dropout (SLDO) power regulator, which combines a standard low-dropout voltage regulator with an actively controlled shunt element. The analog and digital circuits of the chip are powered separately, each with their own SLDO regulator. The current draw of the digital circuit scales with the particle hit rate, resulting in dynamic variations in power consumption. With sufficient supply current, such that the shunt current remains non-zero, the SLDO ensures that the regulated voltage powering the analog and digital circuits in the chip remain stable regardless of fluctuations in current consumption.
Unfortunately, guaranteeing sufficient supply current is not straightforward. Due to the detector’s strict cooling and power budget constraints, only about 10% of the total supply current to a chip is expected to be shunted away during normal physics operation. Additionally, the ITkPixV2 chips monitor the total input and shunt current after scaling them down by a design factor of 1/21600 via a transistor mirror. However, this mirror factor has been shown to vary with operating conditions by approximately ±15%, and its nominal value measured during wafer probing does not translate well to operation in the module. As a result, only an estimated value of the shunt current is available. This uncertainty in the shunt current measurement, the small current headroom, and the expectation that the digital current will fluctuate with particle activity, necessitates a detailed understanding of the chip behavior as the shunt current approaches zero.
The results of this study inform a module’s minimum required shunt current, enforced during production quality control, and provides confidence in the robustness of the ITk pixel system and its serial powering scheme.