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OPA2322: Can I add a clamp on output of OPA2322 to prevent its output saturating.

Colin Gillmor
Mastermind 25570 points
Part Number: OPA2322
Other Parts Discussed in Thread: UCC28950

Hello

A customer wants to prevent the output of an error amplifier using the OPA2322 from saturating to shorten its recovery time. Vcc is 5V

We don't want to use a simple anti-saturation diode from OUT to the IN- pin because of temperature co-efficients and other Vf variations. They are suggesting the circuit below.  The input to the inverting input is not shown - it connects through a resistor to a potential divider measuring Vout which in turn is controlled by the UCC28950 - so the error amp is part of a feedback loop. If Vout is too low then the inverting input will go below the non-inverting input and the clamp should act.

1/ The circuit will clamp the op-amp output effectively but the question is 'how much current will the Error Amp deliver into the clamp, and is this acceptable for this device' -

The DS gives a 90 Ohm open loop output impedance

2/ Is there a better way ?

Thanks
Colin

  • 0
    TI__Genius 15410 points

    Hi Colin,

    Yes this will clamp the output voltage of the Err Amp to 3V but when the Clamp Circuit turns on the Err Amp output is essentially grounded which will cause excessive current to flow. You will likely hit the short circuit current limit of the device and saturate the OPA2322 which is exactly what they don't want to do. So you need to reduce the output current of the Err Amp. This can be done by placing a resistor between the output of the Err Amp output and the connection node of the Clamp Circuit. However, there will be a drop across the resistor during normal operation and when it is clamped so the customer must determine if this is ok for their system. 

    I have attached a TINA simulation circuit for you to test yourself. Since Error Amplifiers need to be used in a control loop I placed the Err Amp as a buffer just to show the functionality.

    Error Amp Clamp Circuit.TSC

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    TI__Mastermind 25570 points

    Hello Tim

    Thanks for your earlier reply - it answered my question nicely.

    I have a supplementary question.

    Is it acceptable to run a OPA2322 device from a 3.3V rail but connect the output to a 5V source through a 2.2k resistor and a schottky diode as shown below. The circuit is such that the op-amp linear operating range is from 0v to about 2.8V. There is also a use case where the output is saturated high because VOUT-1 goes below the 2.45 setpoint - so the op-amp will go up to 3.3V. This means that it could be sinking 1.3V/2.2k into its output, assuming about 0.4V across the schottky.

    Is this a safe operating condition or should we add some clamp diode from the output of the op-amp to VCC - or add a clamp diode from the Error_Amp_Out node to 3.3V.

    Thanks

    Colin

     

  • 0 in reply to Colin Gillmor
    TI__Genius 15410 points

    Hi Colin,

    As long as the current through the diodes from the output pin to the supply does not exceed 10mA damage should not occur due to current flow. However, you may want to consider adding protection (a TVS diodes) to the 3.3V power supply because the power supply likely cannot sink current during this condition. If it cannot sink current the supply pin will float up and potentially increase the supply line that may power other devices. Please see our TI Precision Lab videos on Electrical Overstress (EOS) for more information on this condition. I also recommend reading "The self powering device" for more information on powering other devices.

    Thank you,

    Tim Claycomb

  • 0 in reply to Colin Gillmor
    TI__Guru**** 152366 points

    Colin,

    The typical overload recovery is just 100ns. This is not a big delay even compared to the fast slew rate of OPA2322 and small values for feedback components. Would a soft-start be a better solution?

    The current from the 2.2k resistor will flow between op amp output and V- supply pin when op amp is in linear operation or VOL state. In VOH or unpowered state, the current from the 2.2k resistor will flow between op amp output and V+ supply pin (something on V+ bus will need to use this current). This current should not be harmful to the op amp. Adding a diode from output to 3.3V supply, is still a good idea if 5V is on when 3.3V is off.  

  • 0 in reply to Ron Michallick
    TI__Mastermind 25570 points

    Thank you Ronald, that's clear,

    I'll suggest that the customer use a diode from o/p to VCC.

    BTW: This circuit is being used as part of the Vout regulation loop, there is an additional I_Out regulation loop and the V_Loop saturates when the I_Out loop is regulating. We are concerned about the transition time from Constant Current regulation to Constant Voltage regulation (and vice versa). Reducing Vcc from 5V (in the original clamp circuit) to 3.3V gives us the time reduction we need.

    Regards
    Colin

  • 0 in reply to Colin Gillmor
    TI__Genius 15410 points

    Hi Colin, 

    I'm glad we were able to help. Please let us know if you need any additional support.

    Thank you,

    Tim Claycomb