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LM3150 Instabillity in overcurrent mode

Michal Kiczka
Prodigy 50 points
Other Parts Discussed in Thread: LM3150

Hi,

I have a problem with LM3150 converter.

My design parameters is: Vin = 18..32V, Vo = 15V, Io = 1.5A. The switching freq is about 200kHz.

For the voltage mode (when the output voltage is constant = 15V) there is no problem. When the output current increases the output voltage of the converter goes down, and for the Vin = 32V, and Vout ~12V the working freq is stable and the waveforms of the SW voltage (yellow) and inductor current (red) are pictured below:

Instrument screen

But when I start to decrease the input voltage then I can see some "stable instability" (see below)

Vin = 24.5V

Instrument screen

Vin = 20V

Instrument screen

Vin = 18.8V

Instrument screen

Can you please explain what is going wrong with the circuit. I can't find any description in the datasheet nor in app notes why it is happening. According to the datasheet when the overcurrent occures I should see stable waveform on the scope (see page 12 of the LM3150 datasheet).

Regards

Michał