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UNITY-GAIN STABLE OP AMP

MSP430-Beginner
Intellectual 660 points
Other Parts Discussed in Thread: OPA350, OPA353, TLV4110

Hi,

I'm looking for an op amp, which is internally compensated, to be stable when drawing capacitive load of 10uF. The op amp has to be able to provide an output current of 100mA.

My supply voltage is limited to 5V. While searching in TI library, I could not find any parameter kind of "unity gain stability" or so to set... I came across OPA350 which can come along with cap loads, but its maximum Iout is 40mA.... Actually I will be using the op amp with an amplification factor of 2, so it doesn't really have to be unity-gain stable, but at least two-gain stable :)

Thx

  • 0
    TI__Guru 64530 points

    MSP430 Beginner,

    You came across the right op amp.  OPA350/OPA353 are a unity-gain stable and the only ones I know that can drive directly 10uF loads.  As far as the maximum output current goes, even though they are specified as +/-40mA, they can typically provide up to +/-80mA of short-circuit current (see below).

  • 0 in reply to Marek Lis
    Intellectual 660 points

    Thks Marek for your fast answer.

    I have been using TLV4110 as a regulator to supply MSP430. At the input of MSP430 I have 2 caps: 10uF//100nF, and the MSP430 may draw a current of up to 100mA.

    I set the amplification factor of the TLV4110 to 2. Its output voltage is oscillating. In the data sheet of TLV4110 it is mentioned that a series resistor of max 20 Ohm will solve the problem. However, I can't use this solution, since in case of 100mA, I will have 2V drop across that resistor.

    The data sheet proposes a snubber network at the output of the op amp (page 12). Could you help me determine the values of the elements of the snubber? Would it work without Rnull?

    Figure 17 shows an extremely small phase margin for a load cap of 100nF with Rnull=0. Do you think, it is possible to get TLV4110 stable while driving 10uF//100nF of cap load?

    One more question, the Psice model (TINA) of TLV4110 doesn't model this effect. I tried to simulate it, but the simulation shows that Vout is const. Do u have a clue if I'm doing sthg wrong?

    Thanks!

  • 0 in reply to MSP430-Beginner
    TI__Guru 64530 points

    It seems you have answered your own questions but still try to use TLV4110 to drive 10uF load.  I see why you insist on using this solution (very high output current) but, unfortunately, TLV4110 cannot drive more than 1nF-2nF without, Rnull, series output resistor (see graph below).  For the purpose of stability, any phase margin less than 40 degrees should be considered unstable (due to process variations) thus the TLV4110 can drive directly around 1000pF, no more.

     

    The Snubber network involves using Rnull resistor (see below) that you already said that you can't use because of unacceptably large IR drop (2V) so your only solution is to move to use another part like OPA350 or OPA353 that can drive 10uF with no Rnull needed.  By the way, 10uF||100nF = 10.1uF load.

     

    I have attached within this post a new macro-model I created for OPA350 that you may use to simulate your circuit stability.  The old OPA350 and TLV4110 macro-models currently available on the TI website do NOT model properly circuit stability.

    OPAx350 TINA-TI Reference Design.TSC
  • 0 in reply to Marek Lis
    Intellectual 660 points

    Marek Lis said:
    I have attached within this post a new macro-model I created for OPA350 that you may use to simulate your circuit stability.  The old OPA350 and TLV4110 macro-models currently available on the TI website do NOT model properly circuit stability.

    Thank you for sharing your macro. For documentation reasons I need to generate a graph for OPA350, expressing the phase margin vs cap load, exactly like fig 17 you posted above for TLV4110. Could you tell me how to proceed to do that?

    TI has a technical note which shows how to stabilize an op amp using the snubber network (fig. 9.11), but without Rnull. May be I can follow up the same steps. To determine the values of the cap and the resistor I need the macro model of TLV4110 to simulate the circuit stability. Do you have a model for TLV4110, like the one you posted for OPA350?