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OPA561 gets hot when e/s pin is pulled up thru a 402k resistor to +12V

Other Parts Discussed in Thread: OPA561, OPA461

I haved prototyped the OPA561 as a simple inverting amplifier with a negative unity gain. The device is adequately heatsinked. I am operating it with a +12vdc single supply. My input is +5.5vdc which is through a 10k resistor to the negative input. My positive input is also at +5.5vdc. The feedback resistor is 10k. I am pulling up the E/S pin to +12VDC through a 402k resistor so the device will always be on. There is no load. However the device gets hot and eventually goes into thermal shutdown. I have reverified all connections multiple times but everything seems correct. In the data sheet I noticed an application where the E/S pin is pulled down through a 100k resistor to ground. So I tried this configuration instead and the board now runs cool with no thermal problem. In figure 9 of the datasheet where R6=100k is used from to connect E/S to V-, what is the reason for this connection? After all it says on page 8 under the Output Disable section that "The E/S pin should be pulled up toat least 2V above V-" which I was doing in accordance with the prior statemewnt on the same page "To enable the OPA561 permanently, the E/S pin can be tied to V+through a 402k pull-up resistor." If it needs to be pulled up then why the thermal problem I am having and then why does the other application circuit show it with 100k pulled down to V- ?

 

 

  • Les,

    OPA561 specified voltage range for an Enable pin is from (V-)+2V to V(-)+5V regardless of the total supply voltage which can vary from min 7V to max 15V (+/-3.5V to +/-7.5V); E/S pin may NOT be connected directly to a positive supply rail since even with a minimum supply voltage of 7V this would violate the upper voltage limit for the Enable pin of 5V above (V-).

     

    Fig 9 (see below) shows OPA561 operating on dual +/-5V supplies so the valid voltage range for Enable pin is from (V-)+2V = -3V to (V-)+5V=0V and thus connecting E/S to GND is a valid voltage for an enable mode: at 0V,  E/S pin is 5V above (V-).

    Therefore, contrary to your claim, Fig 9 does NOT show E/S connected to (V-) - this would place the OPA561 in a shutdown mode; a negative rail is pin 5 but it is pin 8 that is being grounded.  Fig 9 shows (V-)=-5V and thus connecting E/S to ground pulls up E/S pin 5V above (V-) placing the op amp in the normal operating mode.

    I suspect there is something else at work behind the problem you see: perhaps, you connected pin 5 to a voltage below 0V which would result in E/S pin more than 5V above negative rail.  I would suggest you to measure the voltage between the pin 8 and pin 5 - it should NOT be greater than 5V.

  • Hi Marek,

    Before now I was looking for a way to Reply to the post but I did not see how so I just started another post each time.

    Following your most recent suggestion on this I was just able to see the REPLY button on the bottom left hand side

    of your POSTemail  to answer my original post request. I hope this now conforms to the protocol.

     

    Anyway, in response to your most recent post answer.........

    I see your point about the voltage drop across the 402k resistor due to the internal device current drain. Since I read 5.3v at the E/S pin that would mean 12v-5.3v / 402k = 16.66uA.  I suppose to then be on the safe side I could inrease the 402k to perhaps a 510k resistor for a 8.4999v drop accross the resistor and 12v - 8.4999v =3.5V at the E/S pin.

    My current limit resistor has always been 4.7k. This should set a current limit to 816ma according to the given equation. However while trying to determine the cause of my problem I did at one point connect the Ilim pin to -V (0v, GND) but it did not help matters. The Ilim pin was never left open.

    I am at the point of maybe trying another OP561 device in case I somehow damaged the previous ones but this time using a 510k pullup as I mentioned. Actually we have two quad OPA461 boards built and everything I told you so far applies to all 4 OPA561 devices on two different boards.

    BTW, another symtom I noticed was that there was oscillation on the Amplifier outputs. I got rid of much of it with a 2200pf capacitor accross the feedback resistors but not all. I am using a 47uf in parallel with a .1uf ceramic capacitors across +V and -V close to the device.

    Thank you

    Les

  • Les,

    Below please find my comments:

    I see your point about the voltage drop across the 402k resistor due to the internal device current drain. Since I read 5.3v at the E/S pin that would mean 12v-5.3v / 402k = 16.66uA.  I suppose to then be on the safe side I could inrease the 402k to perhaps a 510k resistor for a 8.4999v drop accross the resistor and 12v - 8.4999v =3.5V at the E/S pin.

    Your calculation is correct. Using 402k pullup resistor permanently enables OPA561.

    My current limit resistor has always been 4.7k. This should set a current limit to 816ma according to the given equation. However while trying to determine the cause of my problem I did at one point connect the Ilim pin to -V (0v, GND) but it did not help matters. The Ilim pin was never left open.

    Connecting Ilim pin to (V-) sets the Iout to max 1.2A so this would not damage the part.

    I am at the point of maybe trying another OP561 device in case I somehow damaged the previous ones but this time using a 510k pullup as I mentioned. Actually we have two quad OPA461 boards built and everything I told you so far applies to all 4 OPA561 devices on two different boards.

    At this point using another part sounds like a good idea.

    BTW, another symtom I noticed was that there was oscillation on the Amplifier outputs. I got rid of much of it with a 2200pf capacitor accross the feedback resistors but not all. I am using a 47uf in parallel with a .1uf ceramic capacitors across +V and -V close to the device.

    Oscillation at the output with any significant resistive or capacitive load would result in OPA561 heating up - see forum post below.

    http://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/t/316794.aspx