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Original question:

UNITY-GAIN STABLE OP AMP

TLV4110: Buffering an offboard into high capacitive load

Bryan McLaughlin
Prodigy 20 points
Part Number: TLV4110
Other Parts Discussed in Thread: TPS7B4255, INA826, OPA994

Tool/software:

Hello, 

I need to supply an adjustable voltage 1.7-2.7V to a PCB at end-of-line test, 3.6VAbs Max. This will come from an offboard bench supply but I want to dummy-proof accidental setting to higher voltages than this. These are super expensive PCBs.

I'm considering a tracking LDO like TPS7B4255 to "buffer" the voltage but the output voltage doesn't go quite this low.

Thought about Power Op-Amps but I'm less certain that OPAs are suitable for this. The PCB has about 100uF of ceramic capacitance.

Max DC current is 5mA, typical is <1mA.

Any ideas how to use OPAs for this?  DC buffer, G=1

Thanks!

  • 0
    Prodigy 20 points

    Wondering if an INA826 or something could provide the current needed.   Its inputs are tolerant to +/-40V above the rails and it also serves to "break" the GND loop of hooking to external equipment. 

    Thanks

  • +1 in reply to Bryan McLaughlin
    TI__Genius 14766 points

    Hey Bryan,

    Hope you are doing well!

    You are on the correct path here. A power amp with lots of loop compensation can get the job done, but the power amp itself is driving such a light resistive load that the system will hardly benefit from the high IOUT capacity of the power amp. 

    Fortunately, we make a one chip solution, being OPA994. This device is specially designed to automatically compensate the GBWP based on the load capacitance seen at the output. This means no need for RISO or multi-feedback design. 

    Our curve only goes out to 10uF, but trust me in that this device continues to drive as large of a capacitor as you can practically find. 

    Understand that while this device is 24MHz of BW, this goes down significantly as Cload increases. The model can be used to test with your system. 

    INA826 will unfortunately become destabilized due to the high cap loading. It may be possible however to use the INA826 and the OPA994 together to have ground separation with high voltage input tolerance, and high cap load output drive.  

    Let me know what you think of this solution. 

    Thanks,

    Jacob

  • 0 in reply to Jacob Nogaj
    Prodigy 20 points

    Thanks Jacob, that's a really clever part. I think I'll want a high-VIN OPA or INA on the input to protect against transients or reverse polarity. I could see that being a useful reference design.