Introduction

The power-delivery networks (PDNs) of complex, deeply embedded systems often comprise numerous power rails at several voltages with widely varying current demands. An important part of system validation and debug is to evaluate a single power rail’s response to the sudden application of a load, and to look into the response of multiple rails in such circumstances. In this Application Note, we will discuss the use of Teledyne LeCroy’s 12-bit High Definition oscilloscopes to analyze power-rail transient response in PDNs.

Why Power-Rail Transient Response Matters

When a DC power source sees an increase in current demand imposed by a load, its voltage output will droop in correlation with the size of the required current. If the load current transient rises and falls slowly, the power supply’s regulation feedback loop will usually be able to regulate and maintain the nominal output voltage. But if the current rises rapidly, the output voltage will drop sharply and take some period of time to recover.

Figure 1: Analysis of power-rail transient response includes steady-state rail behavior (loaded/unloaded) and step response

In the analysis of power-rail transient response, the areas of interest, shown in Figure 1, include:

  • Steady-state rail behavior in no-load condition: mean voltage and ripple amplitude
  • Step response: droop, recovery time, and settling time
  • Steady-state rail behavior with load applied: mean voltage and ripple amplitude

Figure 2: In a typical embedded system, signals of interest might include, for example, the outputs of the buck DC-DC converters at top right

Figure 2 shows a representative block diagram of an embedded system. The signal being probed might be, for example, the output of one of the buck DC-DC converters at top right. To monitor the rail voltage, a good option is Teledyne LeCroy’s RP4030 power/voltage rail probe. For rail current, one might use a current probe, or probe the voltage across a shunt resistor.

Figure 3: Ripple measurements in a steady-state, no-load condition can be made by placing cursors on the observed peaks and subtracting for the difference

There are several approaches available with 12-bit, High Definition oscilloscopes for quantifying and analyzing rail transient response. These include cursors, zoom traces with measurements, and using Teledyne LeCroy’s Digital Power Management application package.

Analysis Using Cursors

Performing steady-state measurements with cursors is simply a matter of placing them in the appropriate positions on the acquired waveform. In Figure 3, for example, ripple is measured in a steady-state, no-load condition by placing the cursors on observed peaks and subtracting for the difference. Cursors may be used in similar fashion to measure steady-state, no-load mean voltage although it is not a precise method. The same holds for measuring ripple and mean voltage in a load-present condition as well as for voltage droop.

Figure 4: Use cursors to take transient measurements such as recovery and settling time

One may also take transient measurements with cursors. For example, the power rail’s recovery time is how long it takes for the rail voltage to return to a given percentage of the final voltage level. In the example of Figure 4, we selected 10%. Note that the cursor delta of 3.1 mV is 10% of a previously measured voltage droop value of 31 mV. Settling time—the time it takes for rail voltage to settle to its final amplitude—can be measured in similar fashion.

Figure 5: Zoom traces provide greater visibility into high-frequency signal behavior, and also enable limiting of parameter measurements to subsets of the full acquisition

Analysis Using Zooms and Parameter Measurements

Using the oscilloscope’s Zoom function facilitates better visibility into high-frequency signal behavior. Also, by homing in on a selected portion of the acquired waveform, zooms enable limiting of parameter measurements to subsets of the full acquisition. For example, Figure 5 shows voltage and current waveforms (top left and bottom left, respectively) and corresponding zoom traces of the highlighted no-load portions of both (top right and bottom right, respectively). With the pk-pk parameter measurement, we see the ripple in the zoomed steady-state portion of the voltage waveform at top right (Z5). Likewise, the mean voltage parameter measurement shows the mean value of only the zoomed portion. This approach enables the sophisticated capabilities available in the oscilloscope’s built-in measurement parameters, while using the Zoom function to precisely select the area of interest for the measurement.

Figure 6: As with the no-load traces shown in Figure 5, zoom traces can be applied to load-present portions of power-rail acquisitions

The same measurements can be taken of the load-present condition by taking the zoom traces from the relevant portion of the acquisition (Figure 6). For the transient portion of the same voltage waveform, multiple zoom traces allow subtraction of the minimum droop measurement from the mean voltage of the no-load portion (Figure 7).

Figure 7: Multiple zoom traces enable subtraction of the minimum droop measurement from the mean voltage of the no-load portion

Figure 8: Zoom traces make it easier to estimate the mean value of a rail’s recovery time

A similar approach can be taken to measurement of the rail’s recovery time (Figure 8). In this case, the mean value of the recovering voltage is somewhat easier to estimate in the zoomed trace; this also applies to measurement of the voltage settling time.

Analysis Using Digital Power Management Application Package

The available Digital Power Management (DPM) application software package for Teledyne LeCroy’s 12-bit High Definition oscilloscopes has numerous benefits, including simplified analysis, improved measurements, and deeper insight into PDN behavior. Typical Voltage Regulator Module (VRM) devices have behaviors such as ripple that behave periodically at the switching frequency. The Digital Power Management software lets users provide a clock or other synchronizing signal to enable easy “per-cycle” measurements and analysis (Figure 9). Within the DPM package, users have easy access to commonly measured parameters for both the rail current and rail voltage such as VRMS, standard deviation, mean, Pk+, Pk-, pk-pk, and frequency (Figure 10).

Figure 9: “Per-cycle” measurements and analysis require a clock or other sync signal

A powerful capability of the DPM package is Zoom+Gate measurements, which enables simple limiting of parameter measurements to the zoomed portion of the acquired waveform. In the example of Figure 11, the Mean and Pk-Pk values shown in the measurement table provide voltage and ripple measurements for the steady-state, no-load region of the waveform. The same measurement may be taken for the steady-state, load-present condition by zooming into that region of the waveform.

Figure 10: Within the DPM package, parameter measurements are readily available

Figure 11: Zoom+Gate measurements limit parameter measurements to zoomed portions of waveforms; in this example, the mean and pk-pk voltage values are for the steady-state, no-load portion

Yet another powerful analysis tool is calculated Waveforms. For example, if one were measuring the mean voltage value of the voltage on a power rail, a calculated Waveform of “per-cycle” measurement values would show how the mean voltages changes over time (Figure 12). “Cycle” is defined by the applied clock or other sync signal. A calculated Waveform makes determination of recovery and settling times much easier and more precise.

Figure 12: With calculated Waveforms, it’s easy to see how a measured value changes over time

Multiple-Rail Analysis

Often, embedded systems comprise multiple power rails whose behaviors may or may not be closely coupled to one another. When one of those rails suddenly experiences a load release that causes a transient event, do the other power rails in the system suffer any effects (Figure 13)?

Figure 13: When one rail voltage spikes because its load turns off, what happens to other rails in the system?

Teledyne LeCroy’s 12-bit High Definitions oscilloscopes, coupled with its available DPM software, delivers deeper insight than is possible with zoom traces by plotting calculated waveforms of “per-cycle” mean voltage values on other rails.

Figure 14: In this example, calculated Waveforms show a change in mean-voltage values due to a transient event, but the mean values change relatively little

In the example of Figure 14, the load on one voltage rail is quickly released – this can be seen in the current waveform on C8 (orange) and the voltage waveform on C5 (light green). The signals on C1 – C4 are the voltages present on three other power rails in the system. The calculated “per-cycle” Waveforms from the C1-C4 rails do show a change in mean-voltage values at time of a transient event, but mean values change less than 1 mV. Meanwhile, noise on the 12V supply (purple C6), measured as the standard deviation of the output voltage, decreases notably as the load is released (Figure 15). The peak-to-peak voltage on the clock Waveform also decreases.

Figure 15: Noise on the 12-V power supply, measured as the standard deviation of the output voltage, decreases notably as the load is released

Conclusion

With tools such as cursors, zoom traces and parameter measurements, and the available Digital Power Management software package that provides Zoom+Gate capabilities and calculated waveforms based on “per-cycle” values, Teledyne LeCroy’s 12-bit High Definition oscilloscopes deliver powerful insight into the transient behavior of power-delivery networks in embedded systems.