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OPA192: output diode clamp

Vincent Li00
Intellectual 2035 points

Part Number: OPA192
Other Parts Discussed in Thread: OPA172,

Hi Guys

We use opa192/opa172 as high side current sense. common mode voltage is around 12V DC. opa192 power supply we typically use 16V-0V single power. gain is around 200V/V. 

The ourput of Amp will go to the ADC port of MCU directly. and the MCU I/O max voltage is 3.3V.  If normal operation, we will adjust the full scale current at 0-3.3V for output of OPA192.  But if the output get in short operation, the output current will go over the spec and the output of OPA192 will go over than 3.3V. At this time, the MCU AD port can not accept over 3.3V. so sample data will only get max limit current. This processing will affect the MCU action to ocuur some frequency unstable phenomenon. So we want to add a Zener Diode at the output of OPA192.

please see above the schematic. please help to verify:

1.whether OPA192 will support this clamp action at about 6.8V when the output go over to 16V?

2.If this clamp action happened, how much current will be the output of OPA192/172? so far we only find the opa192 short current is 65mA from datasheet. Here we want to use max 500mW spec. please evaluate if we can do it.

3.Do we have any reference design about the output clamp circuit? could you share some design to us?

Thanks

-Vincent

  • 0
    TI__Intellectual 2035 points

    Hi Guys

    I also did some simulation about this application. please review this simulation module.OPA172 Reference Circuit.TSC

    When the output clamps, the OPA current will reach the short current.
    OPA192 short circuit current +/-65mA;
    OPA172 short circuit current +/-75mA;

    At this time, the current flowing through the Zener diode is also the short-circuit current, and the power is about 0.4W, so maybe it should be suitable to select 1W. is it OK?

    In addition, the power consumption added to the op amp at this time is approximately equal to (16-6.8) V*short current ≈ 0.6W. The power consumption of our chip is also very large, the heat will be more serious, and it needs to be confirmed with our experts under the customer's working condition, whether our IC can resist such a large power consumption.

    Your feedback are very appreciated.

    Thanks

    -Vincent

  • 0
    Guru 48395 points

    Why don't you put the zener clamp across the 2.05k using say a 3V zener, that way the output current is much lower in fault condition and the ADC is still protected. 

  • 0 in reply to Vincent Li00
    TI__Guru* 93105 points

    Hi Vincent,

    Is the OPA192 output to the MCU input taken from the common connection point of the 3 K and 2.05 k resistors? If so, Michael's suggestion of placing the 3 V Zener across the 2.05 k resistor will keep the power dissipation much lower and safe. Additionally, a 3.3 V may be a better voltage to use because of  the Zener's voltage tolerance (%). 

    We don't have an output clamp reference design circuit for the OPA192. Your application is quite specific.

    Regards, Thomas

    Precision Amplifiers Applications Engineering