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LMR16006: frequency shift with large (>10.000uF) output capacitor

Part Number: LMR16006

I want to use lmr16006x to charge a bank of four 10.000uF capacitors. The capacitors are used for bridging power supply in case of power fail. The idea was to use the FB-Pin to set the final voltage and to use the internal peek current limit to set the charging current. I have tested it with a modified LMR16006XEVM (R4->1M5, Uout=58V) and it works - in principle.

What I do not understand is the behavior of the LMR16006 most of the time during charging the capacitors (current limit operation). It seems that the controller is operating not with the specified frequency of 700kHz but with round about 140kHz only.

- Can you explain this behavior?

- Has it something to do with not further explained section called "Frequency Shift" of the block diagram?

- How should I select the inductor in this special case?

- If I use lmr16006y instead what frequency will I get during current limit operation?

- Does operating the part in this way for a short time (<10s) cause any damage to the device?

  • The switching frequency could be in foldback mode in the current limiting condition. There's a frequency shift block in the block diagram in the datasheet. I will confirm with the application engineer of this part to confirm the frequency behavior at OC conditions.
  • I recieved feedback from the applications engineer of this part. Yes, the switching frequency will fold back if FB voltage drops. The frequency reduction vs. FB voltage curve is not linear. It is monotonic. The Y version folds back as well. What frequency you get depends on the over loading condition. And I don't think the curve is trimmed. So you will have part to part variation even under the same test condition.

    The frequency foldback under over current condition is to limit the maximum DC load current, thus reduce the power dissipation on the IC, to protect the IC from over heating under such fault conditions.

    The part should be fine to operate under such condition given it is not over heated. Please design the PCB with proper heat dissipation and protect the IC from over heating.

    When you have 58V output voltage, what is the Vin? it is very close to the max Vin of the part. Over voltage could damage the IC.