Transient response is the response of a system to a change of equilibrium. Manufacturers can use these charts to illustrate the voltage and current responses of the DC-DC converter upon startup and how it responds to changes in line and load, highlighting the DC-DC converter’s overshoot/undershoot responses and settling time.
Line-Transient Response: Line-transient responses demonstrate how the voltage and current on the output pins of the DC-DC converter respond to changes in the input voltage. As you increment and then decrement the input voltage, you can monitor the voltage to acquire a graph such as the one in the top right of Figure 7.
Load-Transient Response: Conversely, load-transient responses, on the top left of Figure 7, illustrate the time it takes for the output voltage to settle to the specified accuracy after the output current load changes. You can test different amplitude steps for a complete understanding of the load-transient response, which is critical for testing mobile phones and digital consumer products.
Startup Waveforms: With the PXIe-4139’s maximum sample rate of 1.8 MS/s, you can measure the minimum ON time of the DC-DC converter. This is the settling time, or the time it takes for the output voltage to reach the specified accuracy when the output is at full load. For instance, the minimum ON time is determined when the input voltage is increased from zero to the nominal voltage and a measurement is made of the time it takes the output to settle. The minimum ON time of the TPS54360 can also measured by enabling the EN pin while Vin is present. This can be seen in the bottom graphs of Figure 7.
Noise and Ripple: Noise and ripple are AC measurements made at the output of a DC-DC converter in either millivolts RMS or millivolts peak-to-peak. The output ripple voltage is a series of small pulses with high frequency content and is therefore typically specified in mV peak-to-peak. The two major sources of ripple and noise as seen on the output of a DC-DC converter are the switching noise generated by the converter and the line ripple from the source. For the line ripple, a DC-DC converter source provides some level of ripple rejection; any remaining ripple the converter sees appears at the load. The most common method of filtering output ripple is to add inductance in series and capacitance in parallel at the output of the converter, commonly referred to as an “LC network.” Because of the high-frequency content of the noise and ripple, you should use a digitizer with a high bandwidth for the measurement so that all significant harmonics of the ripple spikes are included.
Figure 8. Load-Transient Response as seen in the TPS54360 Data Sheet (top left);
Line-Transient Response as seen in the TPS54360 Data Sheet (top right);
Start Up Waveforms as seen in the TPS54360 Data Sheet (bottom);
Building Your Test System
Traditional methods for performing transient and noise tests require probing the input and output lines of the DC-DC converter with an oscilloscope. However, the 1.8 MS/s sampling rate of the PXIe-4139 SMU is often fast to characterize line and load transients without the complication and cost of adding another instrument. Figure 8 shows the PXIe-4139 SMU measuring the load and line transient behavior of the TPS54360. In these tests, the SMU is acting as a precision DC power source, external load, and oscilloscope. The external load steps from 25% to 75% of its maximum current in a 500 µs pulse, and the SMU measures both the current draw and voltage output of the DC converter.
Note: Achieving a 500 µs pulse with a fast rise time and no overshoot or oscillation was made possible by using NI SourceAdapt technology, which is a digital control loop technology that allows you to control the transient behavior of the SMU.
Figure 9. Load and Line Transient Characteristics of the TPS54360 using NI SMUs
For higher speed acquisitions or spectral analysis, you can easily add a high-speed oscilloscope to your system in the peripheral slots of the PXI chassis. NI provides a wide range of PXI oscilloscopes, allowing you to optimize for high resolution or high speed measurements up to 24-bits of vertical resolution or up to 5 GS/s sampling in a single PXI slot. For example, by probing the input and output of the DC-DC Converter with the PXIe-5162 4-channel, 5 GS/s, 10-bit oscilloscope, you can examine frequency content of the noise using the Soft Front Panel. In this case, you can see several millivolts of switching noise around 600 kHz.
Figure 10. 2-Channel Frequency Plot using the PXIe-5162 Oscilloscope