3 Measurement and Analysis

The designed filters on the input side and the output side of the DC-DC step down converter TPS6281x-Q1 2.75-V to 6-V Adjustable-Frequency Step-Down Converter data sheet are connected in the configuration shown in Figure 3-1. After adding the designed filters on the input side and output side of the converter, the measured output impedance, load transient response and bode plot response are shown next.

The output impedance can be measured by the How to Measure Impedance of a Power Distribution Network of a DC-DC Converter application note. In the output impedance plot in Figure 3-1 , the first resonance peak can be seen at the frequency of 83.4 kHz, near to resonant frequency of the connected second stage of the output filter that is 79 kHz. At the same time, the gain and phase response of control loop is also measured, as shown in Figure 3-2, which gives us the phase margin of around 26°. This low phase margin can cause unwanted oscillations in the output voltage, which can badly affect the operation of voltage-sensitive applications. The transient output voltage response becomes oscillatory as the phase margin decreases which is also noticeable in the transient output voltage response in Figure 3-3.

This gives an idea about the relationship between output impedance of the converter and stability of a control loop. A high-quality factor of the resonance peak in the output impedance curve can cause the lower phase margin when the resonance peak is triggered by load current, which in result turns into poor transient response of a system. Next section discusses the damping methods for the filters that can help in solving the device stability issue by damping the resonance peaks, which are caused by addition of the filters.