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LMR16006X phase margin

SLP
Prodigy 20 points


Hello,

I try to design a power supply with the following specification:

device: LMR16006XDDCR

Vin_min = 6.0 V
Vin_max = 40.0 V
Vout = 3.3 V
Iout = 0.18 A

Iout_min = 0.01 A

I used WEBENCH tool to select the optimal inductor. See attached report file.

LMR16006X - 100uH WebenchReport.pdf

With the current components: L = 100 uH (DRC = 0.6 Ohm), Cout = 22uF (ESR=0.07 Ohm), the phase margin is 62.4 deg @ Vin = 40 V, but for minimal Vin = 6 V the phase margin is 27.6 deg. I got also the warning of Design Stability Check, with says that the design might be unstable because phase margin is below 35 deg.


I have few questions concerning this:

1. What it really means that the phase margin should be higher  that the specification level?

2. Could you please explain me what exactly is the phase margin and what it depends on?

3. How could I improve the phase margin in my design?

Thank you in advance.

Best regards,
Jacek.

  • 0
    TI__Genius 10115 points
    Hi,
    2) Phase margin is the Controls terminology for the difference is phase of a closed loop system between itself and an oscillator. An oscillator has a phase margin of 0.
    1) 45 degrees phase margin is critically damped and anything greater is overdamped. Anything less is under damped and means that the part can have oscillation as it responds to step changes. This tends to show up as more jitter on the switch node as the part responds to changes in input voltage or load current
    3) The inductor chosen by Webench is really high for the lower input voltages of your design. This causes a drop in phase margin as a high frequency pole moves towards the crossover frequency of your design. You should probably allow more ripple current inyour design by using a lower value of inductor such a 47uH. You should be able to adjust the inductor in the BOM of Webench and it will recalculate your design. Alternatively you can use a higher load current in Webench and it will recalculate with a lower value inductor.

    Regards,
    Marc
  • 0 in reply to Marc Davis-Marsh
    Prodigy 20 points

    Hello,


    thank you for your answers. As you suggested I lowered value of the inductor to 47uH and is far more better. The phase margin is within the range.

    Best regards,
    Jacek.