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OPA548: OPA548 OFFSET ADJ

vemana shankar raja
Prodigy 60 points

Part Number: OPA548

Hi,

Iam designing a linear power supply (0-32V) using OPA548, need to know how to adjust the offset in OPA548 as output need to be zero volt when input is zero, Kindly do the needful

operating at 3A @ 0-32V using potentiometer, do I need negative voltage to bring the output to zero volt.

Regards

Vemana

  • 0
    TI__Guru* 93105 points

    Hello Vemana,

    The OPA548 does not have offset control pins so any offset cancellation voltage would have to be summed into one of the inputs. How this is accomplished depends on which input the cancellation is being applied. This subject was covered in a TI "The Signal" blog by Bruce Trump, published a few years ago.

    Where are the Trim Pins?
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    The OPA548 input common-mode voltage range extends down to 0 V, but the output cannot swing that low when a single, positive supply is used. The voltage output swing towards the negative rail is limited to about (V-) + 1.3 V @ -0.6 A, and (V-) + 3.7 V @ -3.0 A, per the Electrical Characteristics table. Therefore, for the output to be able to swing down to 0 V, the negative supply pin should be powered by at least -5 V.

    The offset control potentiometer would then be connected between the +32 V, and - 5 V supplies.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 60 points

    Hi Thomas,

    Thank you for your reply,

    How much current does negative 5V shall consume if I am using for 3A @32V Linear power output from OPA548.

    Regards

    Vemana    

  • 0 in reply to vemana shankar raja
    TI__Guru* 93105 points

    Hello Vemana,

    When the OPA548 is operated such that it only sources current to the load, the negative supply current will be approximately equal to the normal negative operating current. This will be close to the quescent current level which is less than -20 mA. The positive supply current is where the load current is sourced from and its current level willbe slightly greater than 2 Amps, when the load current is 2 Amps.

    Below, I show a TINA simulation of the OPA548 sourcing 2 Amps to a load. You can see that the positive supply is sourcing the load current, while the negative supply is simply sinking the OPA548 operating current. Therefore, the negative supply current handling capability doesn't have to be very high if the OPA548 is never called upon to sink high current.

    Regards, Thomas

    Precision Amplifiers Applications Engineering