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# PMLKBUCKEVM: How to calculate the input current ripple? & Why the output voltage ripple is discontinuous?

Part Number: PMLKBUCKEVM
Other Parts Discussed in Thread: TPS54160

Hi, team

Can you help me solve the following problems when I measure the PMLKBUCKEVM board? Thank you

1）How to understand the formula calculating the input current ripple in the page of 39?

Can you tell me how these formulas are derived?

2）How to understand the current waveform, on the page of 38,  flowing through the input capacitor in off-state?

During the toff, the current flowing through the input capacitor should be the same as the current draw from the Vin. However, the waveform of these currents is differential.

3) Why the output voltage ripple is discontinuous when I do the Test#1 in Experiment 2?

Jumpers set-up follows the instructions of the manual(page 42)

Board: PMLKBUCKEVM - TPS54160

Input voltage:15V

The discontinuous ripple waveform is as follows.

Yellow: Switch node voltage

Purple: Inductor current ripple

Blue: Output voltage ripple

• Sidong,

1. I will try to find the derivation of these equations and get back to you soon.

2. During TON, IQ ramps up so ICIN ramps up (negative due to the defined direction of ICIN). The current from CIN causes CIN to lose some charge. During TOFF, all that lost charge is replenished so current must flow into CIN. Initially it's lots of current because VIN is steady and VCIN is low from the lost charge. As CIN charges, VCIN rises, and ICIN ramps down. The discrepancy you see between IIN and ICIN is a good catch. It looks like the ICIN waveform was shown using a high-ESR cap but the IIN waveform was shown using a low-ESR cap. I'll confirm.

3. This discontinuity is due to the ESR of the capacitor.

-Sam

As to the third question 3) Why the output voltage ripple is discontinuous when I do the Test#1 in Experiment 2?

My understanding is that the sudden drop of current through the ESR of the 220uF capacitor C16  causes the output ripple to be discontinuous. However,

considering the continuity of the inductor current ripple and the load current ripple, why does the current flowing through the capacitor suddenly drop?

You can refer to the following information.

The schematic is as shown below.

I remeasure the load current ripple as shown in the purple curve below. Its peak-to-peak is about 20mA

The inductor current ripple as shown in the purple curve below. Its peak-to-peak is about 380mA. In this situation, the output voltage ripple drop is about 20mV.

• Question 3 Update

I compared the output ripple measured in 2 places(A, B), and the shape is shown below.

1. Inductor current ripple - Purple curve
2. The voltage measured across the output capacitor C16 – Blue curve, test point B
3. The voltage measured across the output port that connected to the electronic load. – Yellow curve, test point A

There are significant differences between the output voltage results measured across the output port of the two boards. The result of board#1 is the third question I mentioned in my post.

It seems that the ground plane fluctuated at the switching time in the output port of board#1, which cause the output voltage ripple measured across the output port to jump.

• Hi Sidong,

Where and how the data is taken matters when measuring output voltage ripple. Using a test point with a ground clip creates an inductive loop which can cause the artificial data. We recommend using a tip and barrel across the output capacitor. You should AC couple and not bandwidth limit your waveform.

As for the discrepancies between the boards, capacitor tolerance and component variance can occur to cause different output voltage ripple.

Regards,

Ethan

• Ethan

The waveshape is so clean if using a tip and barrel to measure the output ripple. Thanks for your reply.