IoT Digital Power Management and Power Integrity

Article by: Teledyne LeCroy

This article explains how to examine the power supply of an IoT for proper implementation of digital power management and for power integrity.

An Internet of Things (IoT) device draws its power from either a 12 V DC power supply or a battery. In both cases, power is supplied to one or more power rails that operate at different voltages. These rails power the CPU and other functional blocks on the PC board. In this article, we will see how to examine the power supply of an IoT for proper implementation of digital power management and for power integrity.

Each DC-DC converter within an IoT device typically includes multiple discrete DC-DC converters operating in parallel. Each of these converters is called “phases” or, in some circles, “channels”. For example, a 3.3 V rail can be powered by four 3.3 V converters in parallel, each supplying 25% of the total output current to the rail.

Referring to Figure 1, the half-bridge output current is generally referred to as the “inductor current” because it flows through the output inductor. Inductor current increases when Pulse Width Modulation (PWM) signals are on and decreases when they are off. If the phases are turned on or off by the power management integrated circuit (PMIC) based on the varying power requirements of the load, the PWM outputs will be time-interleaved into a single output by the PMIC. Monitoring the inductor current will allow us to capture and characterize any amplitude and phase errors between the different phases, as well as any resulting distortion patterns.

Figure 1: The half-bridge output current of each DC-DC phase is called inductance current.

The key measurement for IoT power management is the transient response of a given DC rail and its associated PMIC, some examples of which are shown in Figure 2. It is important to understand what happens to the rail when a load is added or subtracted instantly. This is a dynamic test best performed on an oscilloscope with extensive acquisition memory, which is essential for correlating bus commands with changes on the power rail. We want to ensure that rail characteristics such as average voltage, ripple, droop, ringing, and settling time are within expected tolerances.

Figure 2: Here are some examples of key measurements of DC power rail transient response, including settling time, droop, and ripple.

To fully assess the transient rail response of an IoT device, you will want to capture multiple rails at once and analyze the behavior of each in response to a load change. An example of multi-rail analysis is shown in Figure 3. By tracking the average power value of each rail, we can clearly see each rail’s response to a load change. In applications of this nature, the value of a eight channel oscilloscope reveals. Instruments such as Teledyne LeCroy’s Motor Drive Analyzer will time-correlate all of these signals to provide a complete view of power rail activity.

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Figure 3: An eight-channel oscilloscope can be an invaluable tool for analyzing the overall picture of an IoT device with multiple power rails.

For a system to boot properly, the DC power rails must turn on in a specific order, with a predetermined latency between each rail being turned on. Conversely, sequence testing is also essential in the power down process. Figure 4 illustrates an example of how a serial data message instructing the PMIC to turn the DC rails on or off is captured and decoded on an oscilloscope. The delay in the rise (or fall) of each power rail can be measured relative to when the message was sent.

Figure 4: Voltage/power sequence testing is an essential aspect of evaluating IoT devices.

Alan A. Seibert