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LMP8645: Saw tooth oscillation on the output when connected to a DCDC

arnaud Herbert
Prodigy 20 points
Part Number: LMP8645

Dear All,

In a WPT project, I'm making a feedback with a LMP8645 on the feedback pin of a DCDC buck regulator. The voltage on this pin is Vfb=0.6V during regulation

The objective is to increase or decrease the DCDC voltage accordingly to the current used by the load.

In open loop the system works well. I can see the output of the LMP rising or falling with the current requested by the load.

When I connect the output pin of the LMP via a 15k resistor to the Vfb (feedback voltage) of the DCDC, the output starts to oscillate with a saw tooth shape at 100kHz.

What could be the issue? is there some load constraint to pay attention with? 

Any idea to fixe this issue?

Thank you for your help.

Best regards,

Arnaud

  • 0
    Guru 140200 points

    Hi Arnaud,

    the only that can be said without a schematic is that the LMP circuitry seems to add phase lag and to erode the phase margin of feedback loop. You could try to add phase lead compensation to restore the phase margin again. One method could be to put a suited cap in parallel to the 15k resistor. But again, without seeing any schematic...

    Kai

  • 0
    TI__Mastermind 30810 points

    Hi Arnaud,

    It does seem to be a loop stability problem like Kai mentioned. You'd probably want to look in this direction for the root cause.

    The 15KOhm load should not be a problem to drive, I don’t think LMP8645 load constraint is the issue.

    Regards, Guang

  • 0
    Prodigy 20 points

    Thank you all for your answers. This is not relative to the feedbackloop stability. The PWM of the DCDC is working well and the regulation feedback is clean.

    On the LMP 8645, the gain is made by an external resistor connected to pin RG. This pin is high impedance and generally the Rgain is high.

    I have used a 10nF filtering capacitor between this pin and the ground to filter RF components to the GND.

    The saw tooth oscillation has now disepeared.

    Hope this solution could helps other.

    Thanks again for your very quick replies.

    Best,

    Arnaud