Power Measurement

Designing Heatsinks and Thermal Solutions for Xilinx Devices (XAPP1377)

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As mentioned earlier, power dissipation is variable in the device, and power estimation is not always accurate and aligned to the actual operation. For this reason, a very important aspect to in-system thermal analysis is accurate measurement of the power being consumed in the device during testing and operation. The device can accurately measure the voltage via the SYSMON circuit. The current, however, cannot be measured directly from the device. This requires upfront considerations during board design to incorporate a means to measure the current being delivered to the power rails of the device so that actual power can be calculated. Many voltage regulators include some telemetry circuitry. Some do not, and some can have compromised accuracy. If the current measurement is either not possible or trustworthy from the regulator, it is suggested to add such telemetry to the board as a means to properly characterize power dissipation to the device. Without this key information, it is difficult to understand if the thermal management is under-performing or the system is simply consuming more power than anticipated. While device current measurement is generally good to have at any point of a product lifetime, it is particularly important at the prototyping, characterization, and bring-up stages. In order to balance system cost and debug visibility, some customers choose to incorporate current measurement capability to the board which can be removed after production runs occur as a cost-saving measure. If the ability to accurately determine power does not exist, it can severely impede the ability to characterize and debug thermal issues.