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WEBENCH® Tools/LM61460-Q1: "Design update failed. This design is not stable due phase margin either smaller than 35 or greater than 120"

Matthew Findlow
Intellectual 880 points
Part Number: LM61460-Q1

Tool/software: WEBENCH® Design Tools

Hi, I'm using LM61460-Q1 in a design as a regulator for charging a 12V lead acid battery from a solar panel. ( I also have a VHF receiver on the board and that is why I chose this IC because of low radiated EMI.)
When the battery is absorbing the panels full capacity, up to 5A, the IC will be in drop out, ie not reaching the regulating voltage (14.4V). Once the battery reaches 14.4V the IC will begin to regulate and the current will progressively drop while the panel's voltage, ie Vin, will begin to rise towards 18V or 19V.  After a while the feedback voltage divider will be adjusted (n-fet closes removing a resistance in parallel with feedback voltage divider's bottom resistor) to reduce the regulating voltage to 13.6V. Iout may drop to very low levels, say 0.1A, while Vin will rise close to the panel's open circuit voltage of 20V.
I don't need a rapid transient response, just stability.  When I adjust the input parameters in Webench sometimes it shows series RC (1K and 10pf) in parallel with the feedback dividers top resistor.  What would be 
the best values for stability when regulating at 14.4 or 13.6 with Vin up to 20V and low load?
If I enter I-out max of less than 0.5A, Webench says "Design update failed. This design is not stable due phase margin either smaller than 35 or greater than 120"
This is a bit of a worry considering once the battery has charged and the IC is regulating,  Iout will drop to levels well below 0.5A. I didn't notice a minimum load requirement mentioned in the data sheet.

  • 0
    TI__Mastermind 27470 points

    Hi Mathew:

    1. RC (1K and 10pf) in parallel with the feedback dividers top resistor is for improve the transient , so if you don't need the fast transient, you can open the RC, it won't affect the output votlage .

    2. for Iout max less than 0.5A, cause it is a 6A devices, if wenbench caculated the output choke with 40% of Iomax, the value will be too large for a 6A converter and make it unstable.because the internal compensation and slope compensation is based on 6A output current.  normally for 0.5A,  the device may work at PFM depend on the output votlage and inductance. so we suggest you still the inductor based on 6A and sim for 0.5A output current condition.

    Thanks

    Daniel Li

  • 0 in reply to Daniel Li14
    Intellectual 880 points

    Thanks Daniel.

    Much appreciated.

    Cheers

    Matt.