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LM25118-Q1: Output Capacitance at low input voltages

Part Number: LM25118-Q1
Other Parts Discussed in Thread: LM5118

Hello,

I recently created a circuit on Ti webench using the LM25118-Q1. I've attached the circuit below. What I've noticed is that even though the LM25118-Q1 is rated to go down to 3V input provided that the output first starts out at 5V or higher, the signal output can become unstable. A good example of this is if the input voltage drops down to 3.2V, the ripple on the output will increase until the circuit powers down. I thought of two solutions to this problem. The first is a low pass filter back to VCCX since I thought that the ripple may be causing the RMS voltage to drop below what VCCX needs to keep things running below 5V. The filter did not seem to have an effect though even though it reduced the ripple into VCCX. The only solution that I could find is adding another bulk capacitor to the output capacitance which already seems to be quite high. My question is if there is something I'm not truly understanding about the LM25118-Q1, or if the answer is simply that a large output capacitance is required for this circuit to operate at low voltages with high loads on its output. Is it possible that the webbench simply underestimated how much capacitance I needed? I expect to be running down to 3V input with up to 5V and 3A  on the output. As I stated, I thought it was the signal going into VCCX not being stable enough, but even after smoothing out this signal, without enough bulk capacitance the circuit powers down at voltages higher than 3V.

Best,

Kyle

  • Hi Kyle,

    Thanks for reaching out with your questions and for using the LM5118 in your design.

    I'm sorry to hear that you are having issues with the LM5118. Based on your observations it seems like the loop might not be stable. Can you let me know the maximum load current so I can double check the values? Can you post the waveform of the VCCX pin voltage?

    Also here is a reference design that I did seems to have similar specifications to your application.

    Please let me know if you have any questions.

    Thanks,

    Garrett