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.
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 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.
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.
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
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.
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).
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).
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.
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.
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)?
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
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.
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.