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OPA2170: Overshoot vs. phase margin

Brian Wang0928
Genius 12810 points
Part Number: OPA2170
Other Parts Discussed in Thread: OPA2991

In the d/s, there are two plots say the overshoot at certain cap load.

In stability's point of view with system, the phase margin with OPA should be > 45degree. There are two questions here.

1. From Fig23/24, are they able to map to phase-margin value? For example, what's the overshoot is equal to PM=45degree?

2. In the Fig23/24, it says the cap load curve is obtained from 0PF to 1000pF for overshoot. What's the suggestion for the capacitor load value? how to design it to have well stability?  

In customer circuit design below, the output cap load is 0.1uF for buffer configuration. Does it have stability issue with 0.1uF cap?

  • 0
    TI__Mastermind 19277 points

    Hey Brian, 

    1. Yes there is an overshoot to phase margin calculator located in the analog engineer's calculator tool: 

    It can be downloaded here: https://www.ti.com/tool/ANALOG-ENGINEER-CALC

    2. The customer's circuit drives a rather large capacitor value (0.1uF) for the OPA2170. The stability recommendation shown in Figure 23, in which an isolation resistor is used, Rout. The customer may try different values for an isolation resistor in order to increase phase margin. 

    There is also the option to upgrade to OPA2991, but I still think they will need to provide an isolation resistor considering how large the capacitor is: 

    Please also leverage the training on TIPL of stability and the existing models, here is the Stability Introduction from TIPL link. 

    All the best,
    Carolina 

  • 0
    Guru 140200 points

    Hi Brian,

    I think we are talking about DC33? Well, it looks like DU1C doesn't need DC33? For DU1C you could easily insert an isolation resistor and adapt the voltage divider DR8 and DR17. Or you could easily increase DC32.

    So, DC33 is important for the other circuitry above DU1D and DU1C? Can you give some details on the circuitry DU1D also supplies?

    Kai