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OPA551 - Output protection

Arun K S
Intellectual 450 points
Other Parts Discussed in Thread: OPA551

 Hello All,

           I am planning to use OPA551 op-amp at output buffer stage for a test equipment. From the data sheet i came to know that the chip is having internal short circuit and Thermal shutdown protections. In my design,  the op amp will be biased with +12 and -12 Volts. There are two worse conditions, the end user can either short the output pin of this op amp (Since it is used as output buffer)  to the ground or to a voltage of 36 Volts, which is the maximum operating voltage of my test equipment. I want to know in these two conditions, whether the op amp output will be protected by itself or do i need to use some other protection mechanism..?

I cannot use complicated protection mechasims at the output since the tolerance of output voltage is very critical in this case.

This is a very urgent requirement from my customer. So i need a quick solution for this.

Expecting quick responses on this topic. Thanks in advance.

Arun Sreedharan

  • 0
    Guru 21975 points

    Arun,

    Short-circuit to ground will be protected by the internal current limit.

    Connecting the output terminal to +36V will very likely damage the amplifier. The output of the OPA551 cannot be pulled beyond its own power supply voltage by more than approximately 1V without causing damage. The conventional way to protect for this condition is to connect a clamp diode from the output to the power supply terminal (anode on output, cathode on +12V supply).

    Depending on the current capability of the 36V supply, an output short circuit to +36V could pull the +12V supply to a higher voltage which could possibly damage other components in your system. Without a more elaborate protection scheme, one of the power supplies must deviate from their normal voltage. The +36V supply may possibly current limit and be pulled down to 12V.

    Regards, Bruce

  • 0
    TI__Guru 67761 points

    Arun,

    Adding to Bruce's comments, there is also a simpler way to prevent any damage to OPA551 if you can add 240ohm power resistor (>10W) in series with the output terminal.  This would protect the OPA551 output from being damaged, while shorting the output to +36V with OPA551 +/-12V supplies, by ensuring that the OPA551 will NOT be pushed either into a short-circuit current OR thermal shutdown where output enters a high impedance state; in such state, the output would be pulled above the positive supply resulting in very large current (limited only by +36V supply capability) flowing from +36V thru forward-biased internal ESD protection diode into the positive +12V supply pin causing the damage to the output stage. 

    Since the highest sinking current would result in case of the output being driven to a negative rail (-12V) while at the same time being externally shorted to +36V, placing 240ohm power resistor (48V/200mA) in series with the output will prevent the OPA551 from entering a short-circuit condition.  At the same time, you need to make sure that the package thermal resistance is below 20C/W (P=24V*200mA=4.8W) so the package temperature rise is limited to 100C, thus avoiding a thermal shutdown condition - this may be accomplished by soldering tab of the DDPAK-7 package directly to the circuit board copper area.

  • 0 in reply to Marek Lis
    Intellectual 450 points

    Hello Together,

            Thanks for the immediate response. Now I am confused. I am planning to use the op-amp as a buffer in a DAC Output stage of equipment (0-10 Volts Analog Signal Simulation and should be capable of delivering a maximum current up to 100mA). The tolerance should be within 50mV Max. So in this scenario, I cannot use any series resistor in the op amp output  (In case of Diode clamping Protection and Current limiting I need to connect a series resistor) which will drop the voltage and affect the tolerance of output voltage. Another concern is that my Equipment works from 6 to 36 Volts, which internally I am converting into +12 and -12 Voltages (Using DC-DC Converter) which are used as op amp supplies. So user can use my equipment at any supply voltage in the ranges of 6-36 Volts, In such a case, if accidentally he short circuits the supply voltage( 6-36 Volts) to any of the DAC Buffer output pins (Means op amp output pin), it should protect the device being getting damaged same time in normal working condition, the tolerance should be within range (30mV) also. I think now my question is clear to you. So how can i proceed now..?

    Thanks in advance.

    Arun Sreedharan

  • 0 in reply to Arun K S
    Guru 21975 points

    Arun,

    From your original question it appeared that you could not tolerate an output series resistance. The simple solution may be the diode clamp. Without details of the nature of the 36V source and the DC-DC converter it is not possible to predict the success of this approach. Is the 36V supply current-limited? During a short-circuit to +36V, this supply would be connected to the +12V supply through the diode. This could possibly damage the DC-DC converter.

    Another possible solution:  The clamp diode from output to + supply terminal of the OPA551 remains. Add a second diode. Power the OPA551 through a series diode from the +12V supply. An output short circuit to +36V would raise the positive supply of the OPA551 to +35V, or so (which it would tolerate). The DC-DC converter would be protected by the series diode. You would lose approximately 1V of output swing due to the drop on the series diode.

    Marek's concerns about power dissipation remain. A short circuit to power supply creates very high power dissipation. Thermal shutdown would likely protect the device but it is a highly stressful condition.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Intellectual 450 points

    Hello Together,

         Thanks for the valuable support. We are able to achieve the tolerance within the limit by using low value resistance value (30 Ohm) in series with the output. Now the output is within the tolerance range and the current will be limited by the series resistor. The diode protection is also used as per the suggestion from Mr. Bruce. Thanks a lot for your help and support.

    With Regards,

    Arun Sreedharan.