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TPS61021A: Need to understand internal compensation

Jeff Coletti
Expert 8755 points
Part Number: TPS61021A
Other Parts Discussed in Thread: TLV61225

Hi Support team

Working with a customer on an issue with the TPS61021a. I am working on getting the schematic, however this is hot. They are having an issue getting it to stabilize driving current. Quote from customer

I am currently working with a circuit utilizing TPS61021[A] and in a bit of a fight to try to get stabilization (in regulating current). Can you provide any hints on the mysterious built-in compensation for this part?

 

[I am also trying to run some simulations (which I rarely ever

do/trust) using TINA and sometimes it seems sorta close to what I see with my prototype and other times it does not.]

Can you provide us with any info on the internal compensation network? you can contact me via my TI internal email.

 

Thanks

Jeff,

  • 0
    TI__Mastermind 40080 points

    Hi Jeff,

    before answering the question, I want to know the input voltage, output voltage, output current they used in their application?

  • 0 in reply to Helen chen
    TI__Expert 8755 points

    Hi Helen

    Input voltage ~2.3V

    Output voltage ~3V

    Output current ~1.1A

    Also, from the customer:

    In the meantime, maybe you could clarify on something I didn't see detailed in the datasheet, but would expect it to have a definite tolerance range. During simulation, I ran into the scenario mentioned in section 7.3.6 (Over-Voltage Protection) of the datasheet, where the IC prevents overshoot by limiting the valley switch current to 100mA when the FB is below 0.2V and the output voltage is above 2.9V.  Could you please provide the spec on the min and max values for the "100mA"

    valley current limit, below "0.2V" FB threshold, and above "2.9V" output threshold?


    Thanks

    Jeff

  • 0 in reply to Jeff Coletti
    TI__Mastermind 40080 points

    This kind of phenomenon  (valley current limit, below "0.2V" FB threshold, and above "2.9V" output) will happen in the normal working condition. But will "only " happens when the output voltage dividing resistor has solding issue, thus  the output voltage goes higher than 2.9V, but FB is very low, <0.2V or even 0V.

    They met it in simulation, its strange! is there any connection problem in the schematic?

  • 0 in reply to Helen chen
    TI__Expert 8755 points

    Hi Helen

    Cannot get schematic due to NDA issues,

    however here is what I managed to get from the engineer

    , I isolated everything on my board down to the TLV61225 circuitry, after a bit of frustration. I have an input cap of 10µF and an output cap of 10µF.  4.7µH inductor.

     

    Working with 133 ohm load (~25mA) that has a parallel 1.0µF cap, I found that it has problems regulating when connected in. At lower input voltages (<1.5V), there is some jitter.  Above that, the jumpiness increases (2.0V in) and at 2.5V in [and up to ~3.3V in], the output voltage drops down (roughly 2.4V or so).  Scope shots attached (CH1 is regulator Vout; CH2 is load voltage, which is tied to and therefore essentially the same signal through ~3' of clip leads).

     

     - If the 1µF cap is added first and then the resistor is added, there are no problems.

     - If just the resistor is added, there are no problems.

     - If the resistor and cap are added at the same time, I see this erratic behavior.

     - If the cap is added after the resistor, I see this erratic behavior.

     

    Jon Z.pptx

    Any thoughts

    Regards

    Jeff

  • 0 in reply to Jeff Coletti
    TI__Mastermind 40080 points

    This is a normal problem. Add a 1uF cap suddenly at the output side will cause a big inrush current because of the charge up of the capacitor.  This is similar to the big step output load transient. since it finally gets stable, so it is OK!

    Please add the 1uF cap together with the 10uF output cap first, then add the load.