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OPA569: Current Limit Set pin

Koshi Ninomiya
Expert 5655 points
Part Number: OPA569


Tool/software:

Dear Team,

My customer has questions about the Current Limit Set pin of the OPA569.

Q1: When the Current Limit Set pin (3-pin) is opened, what is the current limit value? 2.2A or o,2A?

Q2: Behavior when the Current Limit Set pin (3-pin) is conneceted to V+ instead of V-.
       With the maximum rating of  3-pin, the voltage range is from (V-) - 0.5V to (V+) + 0.5V, so there is no fatal damae, right?

Best Regards,

Koshi Ninomiya

  • +1
    TI__Genius 15666 points

    Hi Koshi-san,

    The current limit works by acting as a fixed voltage referenced to V- which has the capacity to measure the current which flows out of the current limit set pin. 

    1. If you open this pin, you will have .2A current limit because the effective Rset = high impedance. 

    2. Correct, no fatal damage, but this will not have a desired operation. This is not good to short this pin to V+ for long periods of time. Since V+ is a stronger node than the Iset pin, you will force large current to flow into this Iset pin. While it will not damage the device, it is not recommended to operate the device like this for extended periods of time. 

    Thanks,

    Jacob

  • 0 in reply to Jacob Nogaj
    TI__Expert 5655 points

    Hi Jacob-san,

    The customer has an additional question about the Current Limit Set pin of the OPA569.

    Q3: The OPA569 datasheet, page 12, FIGURE 2. Setting the Current Limit—Resistor Method. (2), show s the equation below.
              ILIMIT = 9800 • [(1.18V – VADJUST)/RSET]
          How does the OPA569 behave if VADJUST is greater than 1.18V?
          In simple calculations, ILIMIT appears to be a negative limit value.

          

    The background to this question is as follows,
      - Single +5V Operation (V+=5V, V-=0V), RSET=10k Ω,
      - Changed RSET connection to 5V instead of 0V.
      - The calculated ILIMIT value is -3.74A.
      - The operation of the OP569 is fixed output 0V, so the effect is under investigation.

    Best Regards,

    Koshi Ninomiya

  • +1 in reply to Koshi Ninomiya
    TI__Genius 15666 points

    Hi Koshi-san,

    Thanks for the details here, I understand the customer test.

    If the customer forces current in the incorrect direction (I limit = negative value), then the current mirror inside the device will fail to operate as intended. 

    This negative current limit will actually appear as a 0mA current limit, preventing the device from driving any output current. This lack of output current will likely cause the OPA to get stuck at VOH/VOL since the amplifier has no ability to operate linearly. The device is only intended to operate where this equation realizes positive values for output current limit: ILIMIT = 9800 • [(1.18V – VADJUST)/RSET]. 

    Fortunately, this improper voltage setting of the I limit pin will not cause damage to the device, just improper operation. 

    Is the customer concerned that the rset may accidentally be connected to V+ instead of V-?

    Thanks,

    Jacob