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LMR62014: Frequency for the Zero fz

Josel Go
Expert 5990 points
Part Number: LMR62014

Hi Experts,

Good day.

I am using LMR62014, and I was calculating the Cf (feed-forward capacitor) with the recommended frequency for the zero (fz) 6KHZ found in the datasheet and simulated it on PSPICE and the response was fluctuating. 

I changed the frequency for the zero to 13.8KHZ and recalculated the Cf (feed-forward capacitor) and the system worked well in the simulation, but I don't know why. 

Can you please help me understand this case?

Notes:

-Input voltage is 5V 

-Output Voltage is 6V 

- current needed between 0.6 and 1.4

-R1 calculated is 53.3K ohm

-Cf (while fz is 6kHZ) is almost 510pF

-Cf (while fz is 13.8kHZ) is almost 220pF

Please find the attached images for both the 220pf (A.1 and A.2) and 510pf (B.1 and B.2) capacitors. 






Please advise

Regards,

Josel

  • +1
    TI__Mastermind 19110 points

    Hi Josel,

    Here is the qualitative explanations for your reference:

    1. In the datasheet there is a typical application circuit, converting from 5V to 12V with CF=220pF and top feedback resistor=117K. This configurations should have a stable Vout.
    2. Based on the general rule, in this condition the feedforward zero should be near the cross frequency.
    3. In your application, the top feedback resistor is changed to 53.3K ohm to regulator the Vout as 6V. Now there will be smaller duty cycle D, higher DC gain, and finally, larger cross frequency compared with the circuit in datasheet which has 12V Vout.
    4. If you make the CF larger to keep the zero unchanged at 6KHz, the zero will be smaller than the cross frequency, which may make the phase margin not big enough and finally result in instable Vout.
    5. The solution is to increase the zero accordingly. As you observed, then CF is 220pF and  zero is 13.8KHz, the Vout is stable.

    To approve this, you may help do a experiment: based on your simulation of the instable Vout conditions(CF=510pF), make the Cout from 10uF to 22uF or 44uF, the Vout should be stable. 

    Just let me know if you see any concerns.

    Regards
    Lei