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OPA549 E/S Pin Operation

Richard Elmquist
Mastermind 45250 points
Other Parts Discussed in Thread: OPA549

I have a customer asking the following question:

I am planning to use two OPA549s in bridge configuration.  If one of the OPA549s goes into thermal shutdown, it is highly undesirable for the other one to remain operating.  It would seem that if the E/S pins of both OPA549s are connected together and the E/S pin of the OPA549 in thermal shutdown can sink sufficient current to pull the E/S pin of the other OPA549 low, then both OPA549s will be disabled as desired.  I also need to be able to disable the OPA549s in response to an external signal.  To do this, I would connect an open collector NPN to the E/S pins (connected together as above) to pull them down.  What is your assessment of using the OPA549 in this manner?

Would this configuration work? Can you send me a drawing of a configuration that would work?

I downloaded the OPA549 macromodel from the TI website, generated a symbol and ran some simulations in LTspice.  My simulation testbench has the OPA549 configured for a gain of -1 with a 1V input and +/-15V supplies.  I ran a temperature sweep and was expecting to see a -1V output up to 160C and 0V above that (I have a large valued resistor from the OPA549 output to ground).  I did NOT see that for a result (not even close), so I ran a DC operating point simulation at 80C.  The observed output voltage implied that the model had an 80mV input offset at that temperature.  A DC operating point simulation at 27C yielded the expected result.  Pulling down E/S to disable the OPA549 also appeared to work correctly at 27C.  It was my intention to try to use the model to answer the questions I have posed to you about E/S pin behavior.  After seeing the simulation results, I thought that perhaps the macromodel was intended for nominal temperature only. 

Will the macromodel properly simulate the operation of the OPA549 over temperature?

Thanks for your help with this!

Richard Elmquist

  • 0
    TI__Mastermind 45250 points

    The customer stated this:

    The fundamental part of my question regarding direct connection of the E/S pins of two OPA549s revolves around values of E/S pin Ih and Il at Vh and Vl in both normal operation and in thermal shutdown.  They are not given in the datasheet.  If you have those numbers, it should be straightforward to assess whether this should work or not.  It would be preferable to have TI provide these numbers to me rather than spending time in the lab with a heat gun extracting them by measurement.  Simply put, can the E/S pin of an OPA549 in thermal shutdown mode sink enough current to pull down the E/S pin of another OPA549 in normal operation mode?

    Thanks for your help with this!

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    TI__Guru 67971 points

    Richard,

    The information you're requesting regarding inner workings of OPA549 E/S function is actually given in the datasheet.  In the enable mode IE/S pin will typically sink 50uA while in disable mode 55uA – see below. What it means is that thermal shutdown function increases the E/S pin sinking by 5uA; thus one op amp in shutdown condition will NOT be able to pull down E/S pin of another op amp in normal operation as long as what drive E/S pins can source more than 110uA current.  If nothing drives the E/S pins of two op amps, they should NOT be connected together.  In order to ensure that the first op amps thermal shutdown disables the the second op amp, you should use additional op amp to buffer E/S pin signal of one op amp and E/S pin of another op amp - just connecting them together may result in unreliable state.

    The thermal shutdown circuit disables only the output when the junction temperature reaches 160 deg C with all the remaining parts of the circuit being unaffected.  The only way to monitor thermal shutdown is to leave the E/S pin floating in which case the voltage on the pin is typically 3.5V above Ref voltage in normal (enable) operation.  Once thermal shutdown occurs, this E/S voltage will drop to approximately 200mV above Ref voltage.  This is described in Thermal shutdown section of the datasheet- see below.