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about OPA2171 input

Vera Mao
Expert 5065 points
Other Parts Discussed in Thread: OPA2171, LM2904, TINA-TI

dear supporting team,

is there clamp circuit btw IN+ &IN- of OPA2171? customer found when the input voltage difference btw IN+ & IN- is very big, then IN+ and IN- will affect each other.  tks! 

The issue phenomenon is as below:

PIN3 is input of 2.5V reference through 1K Ohm.

PIN2 is the voltage sampled & amplified from output current(OCP)

1. if using OPA2171 in D2, when the brd is unloaded, they will find the voltage of PIN3 is only 2.15V, when output current is increasing, i.e PIN2 voltage will increase as well, PIN3 voltage will also increase. when output current get to 30A, then PIN3 get to 2.5V.  then even they increase the output current, PIN3 keep 2.5V.

2. if remove R125 and C13, pin2 is floating, PIN2 is not 0V.

3.  if change D2 to LM2904, keep other portion the same.  PIN3 will stay at 2.5V, will not change with the output current.  so what cause the different behavior of LM2904 and OPA2171?

  • 0
    TI__Mastermind 27715 points

    Hello Vera,

    The OPAx171, OPAx170 and OPAx172 all have internal diodes across their input pins (also known as back-to-back diodes). This is shown in section 7.2 of the OPAx171 data sheet and copied below for reference. The LM2904 does not have these diodes.

    Because of these diodes, the differential voltage across the input pins of the op amp must not exceed one diode forward voltage drop - typically around 0.5V to 0.7V. If the differential input voltage is greater than one diode forward voltage drop, one of the back-to-back diodes will become forward biased and create a low-impedance path across the inputs. Current will conduct inside the device and circuit operation will become unpredictable. If the current into the op amp exceeds 10mA, fatal damage can occur.

    In linear or small-signal operation, an op amp is able to prevent large differential input voltages from developing by adjusting its output voltage. However, there are certain non-linear or large-signal conditions where this may not be possible, such as:

    1. Large signal inputs with fast transients that place the amplifier in a slew rate limit condition
    2. Inputs which drive the amplifier into overload due to output voltage swing limits
    3. Inputs which drive the amplifier into overload due to output current limits

    I've built up the customer's circuit in TINA-TI. You can download it here: OPA171 Input Clamp.TSC

    Doing a quick DC analysis, I see the same behavior where the positive input is pulled down from 2.5V to around 2.17V. In this case, the reason the op amp can't regulate the voltage at its inputs is because there is no negative feedback path at DC because of C6 in the feedback loop. Since the amplifier's running open loop, the very large open loop gain is causing the amplifier output to sit at the positive rail.

    Can you please explain the intended operation of this circuit? The only thing I can guess is that it's designed to be an AC-coupled error amplifier. What is the typical value for VCC and what is the expected input signal from the OCP node?

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog - Op Amps