Designing a pi filter (C-L-C) for the output of a TPS62740 buck converter

Question

We are using a TPS62740 IC as DC-DC converter.

We are getting ripple at the output in the range of 10 mV to 12 mV, so we are thinking to add a pi (C-L-L) filter at the output of the DC-DC converter.

As you can see, the peak-to-peak voltage is around 12 mV, but the frequency is not stable, varying by up to 100 kHz.

The DC-DC converter switches at up to 2 MHz, V(in) = 3.6 V, we are operating with a small load (50 mA) and V(out) = 2.5 V. We want to design a pi filter for it.

Please help me out with this problem.

Answer 1

These diagrams look like a regular PFM operation for me, especially after you've mentioned variable frequency. In my practice, there are two nice approaches besides passive filtering:

1. If power consumption isn't critical - you may add a constant load at your output (fixed resistor) to shift the converter to the PWM mode. It will allow you to get a fixed frequency, plus often it will decrease ripple amplitude. Other part numbers often do have a dedicated "Forced PWM" pin, but with this one, I'm afraid it's the only option
2. You may push your load point a bit higher (e.g., 2.6 V) and supply it to the 2.5V LDO with high PSRR, or even a voltage reference if the absolute value of the voltage matters (pay attention to the fact that for your output 25 mV is already 1% accuracy!). I've used such an approach for systems with capacitive touch, which are unpleasantly sensitive to such ripple, and with good LDO, it has higher efficiency compared to the first option

In case these options aren't enough and you still need a passive filter - I believe any online calculator (e.g. this one) will do the job, the number of stages will depend on your specs (how much attenuation you'll need)