LED drive power noise detection design

In this paper, three schemes are used to measure the power supply noise of the CPU on the motherboard. The best solution is a passive 50-ohm transmission line probe with 1× attenuation.

When using a passive 50 ohm transmission line probe, the oscilloscope's channel is set to DC50 ohms; since the power supply noise is measured at the board level, an oscilloscope with a bandwidth of 500 MHz or more can be used; the sampling rate is 2.5 GSa/s, and 1.25 G can be achieved. Quest bandwidth: The oscilloscope time base is set to 1ms/div, which can capture the signal of 10ms duration at a time, and completely measure the noise within 1 power frequency cycle (AC 50Hz after AC-DC-DC conversion, after rectification and voltage regulation) It is 100Hz with a period of 10ms). At this time, the oscilloscope has a memory depth of 25Mpts, as shown in Figure 1.

Passive 50 ohm transmission line probe

In the LED driver power supply noise test, the probe point is usually close to the IC's power supply and ground pad, such as the two pins of the decoupling capacitor near the IC. One method is to directly solder the coaxial cable to the POWER and GND pads to test the power supply noise. This is a low-cost solution. The disadvantage is that re-welding is required every time it is measured, which is inefficient;

Another alternative is to use a custom passive transmission line probe. As shown in Figure 2, the custom passive transmission line probe can directly measure the two ends of the 0603 capacitor, and it is very convenient and quick to measure the power supply noise. The probe is available in both DC-coupled and AC-coupled versions. If the oscilloscope used must be isolated to measure power supply noise, the latter can be used.