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LM25085 VCC current limit

Tuan Pham
Intellectual 645 points
Other Parts Discussed in Thread: LM25085

Hi,

I'm developing a DC-DC using LM25085, I refer to LM(2)5085 quick start calculator excel sheet. Everything seems fine except 1 warning from excel calculator sheet: "VCC current limit, decrease Qg or Fs" at line 75.

Parameter: input: 12.8 to 24V, 14.8V normal. Output: 12V, 2A.

I use PFET FDD4141 from Fairchild, Qg = 50nC.

I see the Excel sheet is calculating Icc = Qg * Fs = 50mA (at cell AA-249). Then it compares with minimum VCC current limit (20mA). Because Icc exceeds this current limit so it generates a warning.

From LM25085 datasheet, VCC regulator is the internal regulator to generate internal voltage bias for Mosfet and internal circuit inside LM25085, its limit output current is 20mA minimum. I can't understand how this current limit effect on driver current to the mosfet, while the datasheet also specify the output driver current is about 1.75A (source) and 1.5A (sink).

I attach here my calculation sheet.3566.LM(2)5085_Quick_Start.xls

Do I miss some important thing? Could you please explain the relationship between Vcc current limit and Mosfet driver current.

Thank you for your help.

  • 0
    TI__Genius 11490 points

    Hi Pham,

    The datasheet spec of source and sink current is the peak current from the driver. The average current from the driver is much lower and should be less than the VCC current limit as the current is ultimately coming from VCC regulator. The VCC cap does the averaging.

    Since your application needs only 2A, you can probably choose a FET with lower Qg(smaller FET) or select a lower switching frequency.

    Hope that helps.

  • 0 in reply to Vijay86929
    Intellectual 645 points

    Hi Vijay,

    Thank you for your help, I'm clear now.

    Actually I'm modifying the existing circuit to optimize transient response. I have to increase Fs and reduce Inductor to have better transient response. Increasing Fs leads to this VCC current limit issue, for now I have to choose another FET with lower Qg.

    Related to load transient response of DC-DC converter, especially COT converter, I cannot find any document that gives a formula or equation to calculate transient overshoot/undershoot voltage as well as transient time, so I calculate by myself, but I'm not very much sure. Can you point me some references?

  • 0 in reply to Tuan Pham
    TI__Genius 11490 points

    Hi,

    COTs typically have faster transient response than PWM converters. But it is also dependent on the ripple injection circuit used.

    I don't have any closed form formulae to quantify the overshoot or undershoot thought. I think webench simulation can give you some idea. Another way it so use the pspice models available on the product webpage of LM25085 in tools section. Final tuning can be done in the lab.

    Thanks and regards,

  • 0 in reply to Vijay86929
    Intellectual 645 points

    Hi Vijay,

    Thank you for your help. Actually I did as same as your suggestion, try to calculate, then do simulation, then test and tuning in lab. It is quite good for now. Just small problem is the audible noise at no load, then I replace the output capacitors from ceramic by aluminum polymer and audible noise disappear.

    Thank you again :)