The extremely low noise and low ripple levels of the TPS6291x necessitate a low-noise test setup for accurately measuring the output voltage ripple. The SMA connectors, J4 and J5, should be used to measure the output voltage ripple, before and after the second LC filter. Do not use a normal 10x oscilloscope probe with a high-impedance termination to the oscilloscope. Instead, connect the SMA connector directly to the oscilloscope with a coaxial (coax) cable through a DC blocker. A DC blocker enables the use of the smallest V/div setting on the oscilloscope to view the ripple. To prevent noise pickup and block reflections on the coax cable, the oscilloscope should be set to full bandwidth (BW) and DC coupling with a 50-Ω termination.

Figure 2-1 and Figure 2-4 show the correct measurement settings and output voltage ripple result, while Figure 2-2, Figure 2-3, Figure 2-5, and Figure 2-6 show common measurement methods and settings that cannot accurately measure the very low output voltage ripple. The measurements in Section A show the higher ripple that results when the inductor is not installed in its low-noise orientation.

V_IN = 12 V, V_OUT = 1.2 V, I_OUT = 1000 mA, JP2 Open, Full BW, DC Coupling, 50-Ω Termination With DC Blocker

Figure 2-1 Output Voltage Ripple, Measured at J4 .

V_IN = 12 V, V_OUT = 1.2 V, I_OUT = 1000 mA, JP2 Open, Full BW, DC Coupling, 50-Ω Termination With DC Blocker

Figure 2-2 Output Voltage Ripple, Measured across C4 With 1x Probe.

V_IN = 12 V, V_OUT = 1.2 V, I_OUT = 1000 mA, JP2 Open, Full BW, AC Coupling, High-Impedance Termination Without DC Blocker

Figure 2-3 Output Voltage Ripple, Measured Across C4 With 10x Probe.

V_IN = 12 V, V_OUT = 1.2 V, I_OUT = 1000 mA, JP2 Open, Full BW, DC Coupling, 50-Ω Termination With DC Blocker

Figure 2-4 Output Voltage Ripple, Measured at J5.

V_IN = 12 V, V_OUT = 1.2 V, I_OUT = 1000 mA, JP2 Open, Full BW, DC Coupling, 50-Ω Termination With DC Blocker

Figure 2-5 Output Voltage Ripple, Measured Across C7 With 1x Probe.

V_IN = 12 V, V_OUT = 1.2 V, I_OUT = 1000 mA, JP2 Open, Full BW, AC Coupling, High-impedance Termination Without DC Blocker

Figure 2-6 Output Voltage Ripple, Measured Across C7 With 10x Probe.