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OPA564 application

Andrew Chin
Prodigy 220 points
Other Parts Discussed in Thread: OPA564

I am using the opa564 to act as a unity gain buffer into a reactive load (a transformer). I am trying to pump a 250khz  triangle wave (could be a sin wave if I wanted) out of the opa564 and into the transformer so I can get a 300V pk to pk wave out of the transformer (ratio is 3.2 ohm to 1.6k ohm). I have succeeded somewhat. The issue is that the chips have been failing half the time.  I followed the tech doc and used the value of Rset that will give me max current. I am using a split rail (12 to -12) and the wave by the time it gets to the opa564 is approx 11.6V pk to pk.The only thing I can think is that maybe my application pushes it too much and some chips maybe being pushed outside it's tolerance level?

  • 0
    TI__Guru 64290 points

    Andrew,

    What does it mean "the chips have been failing half the time"? Do you mean 50% of units did not work as expected?

    What Rset value have you used to set the max output current?  The Rset value MUST be between 55k (Iout=400mA) and 10k (Iout=1.6A) - see below.

    I assume you have made sure that you do not exceed the max input common-mode voltage range?  For +/-12V supplies, the valid range is from -12V to +9V - see below.

    Have you also made sure that the OPA564 does NOT excessively heat up to a point of tripping a thermal shutdown?  When OPA564 junction temperature reaches anywhere between 140 to ~157 deg C, the output will be disabled - see below.

    In order to assure that the OPA564 is not being pushed into thermal shutdown, you need to determine the total RMS power dissipated in OPA564 and use it together with thermal resistance shown below to calculate the junction temperature, Tj:

    Tj=T_ambient + P_rms*

  • 0 in reply to Marek Lis
    Prodigy 220 points

    Mark...

    I have been getting about 50% good and 50% failing chips.

    For the output current, I have it set to 7.5kOhms. According to the data sheet, that is the actual minimum recommended. maybe it is not really recommended at all because I just noticed that it says 1.9A later on (even though the max is 1.5A). This could make lots of sense if this is the issue.....

  • 0 in reply to Andrew Chin
    TI__Guru 64290 points

    Andrew,

    The OPA564 max current limit (short-circuit current) set by Rset is always greater than the recommended maximum output current of 1.5A. 

    Please monitor the Tflag in order to confirm that the device has gone into thermal shutdown (active high) - if so, you need to improve the dissipation of the heat sink used to cool the part.  Also, check the state of Iflag to make sure OPA564 has not entered a short-circuit condition - if so, you need to lower the loading at the output.  BTW, remember to solder the PowerPAD to the PCB - this is required (for physical integrity of the package) even in applications that have low power dissipation.

  • 0 in reply to Marek Lis
    Prodigy 220 points

    Thanks Mark,

    We are actually actively cooling it (heat sink and fan pointed to it). We using the version with the pad on the top. i will high Rset ans see if that fixes the issue.

  • 0 in reply to Andrew Chin
    TI__Guru 64290 points

    Andrew,

    Increasing the value of Rset will lower the short-circuit limit and thus make it even more likely that OPA564 will be pushed into a short-circuit protection state (output become high impedance).  Instead, first you need to check the Iflag (pin13) if it is being activated (high).  If so, you need to decrease the loading by increasing the load resistor OR decreasing the amplitude of the output voltage. 

    But remember to also check the Tflag (pin18) in order to make sure that OPA564 has not been pushed into thermal shutdown (active high) - in such state, the output also becomes high impedance and thus cannot drive any load until OPA564 cools off and the Tflag goes low again - this may lead to on-again off-again intermittent functioning of the part as OPA564 is getting in and out of the thermal shutdown.