The TPS61081 is used in this example to determine the crossover frequency. This example can be applied to other internally compensated dc-dc converters which recommend external feedforward capacitors in the feedback network.

After using the data sheet guidelines to choose all appropriate external components, remove the feedforward capacitor, and measure the converters crossover frequency by using transient analysis or a network analyzer. Note that to determine the crossover frequency, f_nocff, the feedforward capacitor must be left open as shown in Figure 2-5.

Figure 2-5 Internally Compensated Converter Without Feedforward Capacitor

Figure 2-6 shows the tip and barrel measurement method set up for transient analysis. A transient load is connected to the output of the power supply circuit, while a current probe measures the transient load current, and a tip and barrel voltage probe measures the voltage deviation during transient load conditions on the output.

Figure 2-6 Tip and Barrel Measurement Technique

The tip and barrel measurement technique in Figure 2-6 is used to minimize coupling magnetic fields and obtain a more accurate voltage waveform during transient load transitions. TP14 is connected to the measured signal whereas TP15 is connected to ground. TP14 and TP15 are not shown in Figure 2-5. If the power supply does not include the appropriate test points, the test points can be strategically placed using bus wire. It is recommended that the bus wire test points be tacked onto the converter output capacitor closest to the load. Figure 2-7 shows the TPS61081 transient response as measured with the tip and barrel technique. The plots are taken using the TPS61081EVM-147 with Vin = 5 V, Vout = 12 V, and a load transient from 0 mA to 160 mA.

Figure 2-7 Voltage Transient in Response to Load Transient Without Feedforward Capacitor

About 0.9 V of output voltage deviation from the dc voltage set point is observed. The voltage waveform in Figure 2-7 provides insight to the converter crossover frequency as described in Evaluation and Performance Optimization of Fully Integrated DC/DC Converters (Topic 7 of the 2006 Portable Power Design Seminar). The frequency of the voltage deviation waveform in response to a load transient is related to the crossover frequency of the converter. Using the oscilloscope's cursors, the crossover frequency is approximated. The frequency of the transient ripple in this example is approximately 15 kHz. Note that the voltage deviation begins to correct 30 μs after the transient occurs. As the crossover frequency of the converter is increased, it is confirmed that the converter response is improved as the voltage deviation begins to correct in less time, resulting in less voltage deviation.

Figure 2-8 Step Response vs Loop Phase Margin

Using Figure 2-8 from the 2006 Portable Power Design Seminar topic paper Evaluation and Performance Optimization of Fully Integrated DC/DC Converters, the phase margin of the loop can be adequately approximated. Comparing the two plots, the TPS61081 measured the transient response most resembling the number of oscillations of the blue trace with just slightly more oscillation. This means that the measured loop has just slightly less than 30° of phase margin.