• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 50 subscribers
  • Views 1204 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA548 output is not fully turning off when E/S pin is asserted (driven to -VNEG supply)

Doug Billington
Prodigy 50 points
Other Parts Discussed in Thread: OPA548, OPA547

My OPA548 power amp circuit is not fully turning off when the chip is disabled. The output is pulling down to the -Vneg supply when disabled, when enabled, it works properly. I have dual +/-7.5V supplies and I use an open collector transistor to -7.5V on the E/S pin to disable the amp as shown in your appnote. Using a pullup to ground (0V) on the disabled output, I am able to pull the voltage closer to ground. With 1k to ground on the output, the output voltage goes from -7.5V to ~-1.5 volts indicating about 1.5mA of pulldown (leakage?) current. QUESTION: IS THIS EXPECTED? The amp text says pulling E/S down to -VNEG=-7.5V will disconnect the output from the load, but I am still seeing current flow thru the lower transistor to -VNEG??

Is there a way to FULLY turn off the output driver transistors when disabled?

Thanks

 

  • 0
    TI__Guru* 93105 points

    Hi Doug,

    We are setting up an OPA548 on the bench to see if we observe the same shutdown state output behavior you describe. I'll let you know what we find.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Guru* 93105 points

    Hi Doug,

    Our bench testing verified what you have observed regarding the OPA548 output level when set in shutdown mode. The output pulls down to the negative supply rail when it does not have a source of external current available. Once a resistor such as load resistor is connected from the output to ground then current is sourced through the resistor to the negative supply. We did tests with 1k, 100 Ohm and 10 Ohm loads and the voltage moves progressively closer to 0 V at the output as the load resistance decreases. This can be seen in the attached PowerPoint slides.

    I do not know of any way to prevent this from occurring; it appears to be a real behavior of the OPA548 in the shutdown mode. The datasheet wording "An input on the (E/S) pin not only disables the output stage to effectively disconnect the load ..." on Pg. 3, Sec. 5 isn't correct in that there is still a dc path.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 50 points
    Thanks Thomas.
    By any chance, is there a pin compatible replacement for the OPA548 that goes into full tri-state when disabled?
    Also, I could not find the link to the "attached PowerPoint slides". Can you please point this out or resend it?
    Regards.
    Doug
  • 0 in reply to Doug Billington
    TI__Guru* 93105 points

    Hello Doug,

    It appears that I forgot to attach, or when I went to attach the PowerPoint slides it didn't take. I'll add the again here:

    OPA548_Output_disable.pptx

    Unfortunately, none of our other power op amps except for the OPA547 drop into the OPA548 socket. The OPA547 is very similar in design to the OPA548 and its output would behave in the same. I checked to see if any power op amps available from competitors dropped into its socket, but again didn't find anything.

    If a low resistance load is connected to the OPA548 output when the amplifier is placed in shutdown mode, then the output should move close to zero volts. But I do realize that is not the same condition as being disconnected from the load.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0
    Genius 3295 points
    Hi Doug,

    You didn't mention what the load is so I'm spit-balling... if the device is being used to pass an AC signal then a capacitor of the appropriate value at the output would make the load see "zero volts". Of course, this doesn't work for DC...

    Mike T.
  • 0 in reply to Mike Tripoli
    Prodigy 50 points

    Thanks Mike

    Our load is DC. We are driving various haptic actuators in the 10 ohm to 20 ohm range.

    Placing a 1k dummy load to ground on the amp output should solve my problem.

    The "disconnected" output sits at about -1.8V which is acceptable in our design.

    The problem I was having is that a downstream mux chip has an abs max rating of +/-5.5V and the OPA548 was going down to -7.5V.

    Thanks for your follow up.

    Doug