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OPA2277: OPA2277 current consumption

Part Number: OPA2277
Other Parts Discussed in Thread: OPA277

Good day all.

Now I use OPA2277(2CH PKG) power supply source for sensor.

Sensor equivalent circuit is as about 1k ohm pure resistor. (no capacitor exists.)

It is connected between high side and low side.    High side output - 1kohm - Low side output. 

Other conditions

Power supply

   Positive +9V(Generated by SW regulator)

   Negative -9V (Generated by charge pump IC)

Input reference voltage +3.3V (Stable and common for both channels)

Gain  high side X2 (High side output=6.6V)

         low side X(-1) (Low side output=-3.3V)  Total sensor power is 9.9V.

Problem

Under this conditions, negative power supply current is unstable.

By simulation and calculation, negative power current is just only less than 20mA, but it increases to about 100mA by periodically and unstable.

And when we changed output load from 1kohm to 10kohm to reduce output current, negative power source current will be stable.

Is there any possible reason that causes above phenomenon?

Please teach me any hint to solve this issue.

Regards.

  • Hi Junji,

    Thank you for your post. Can you please share a schematic of your circuit with input/output voltage expectations? We will help you analyze this. 

    -Tamara

  • Thank you prompt reply.

    I attached simple circuit diagram.  But it is unclear how to attache the file. If there is any problem about attachement, please let me know it and how to send it.

    RegardsA.pdf

  • Hi Junji,

    Thank you for sharing the block diagram. 

    I don't necessarily see anything wrong with how the op amps are configured. However, it looks like the regulator supplies the +9V and then the charge pump supplies the -9V. This is happening at different times. It is suggested that power be supplied at the same time. I would suggest adding some TVS diodes to the power supplies. TVS diodes rated to 10-12V should be fine. 

    You may also want to try testing this circuit using an external bench power supply. Supply +/-9V at the same time and measure the current consumption. This will allow us to identify if there is a problem with the op amps or the supplies.

    -Tamara

  • Good day Tamara.

    Thank you for your good support.

    I confirmed power up sequence and there was no problem.

    By the way, I found strange phenomenon in SPICE simulation with my circuit.

    If I added additional capacitor on both output of amplifiers and increase output impedance of power source. Then power line current started oscillation.

    (For circuit, please refer to attachment)

    Capacitor value:3500pF (I know it is unreasonable value...)

    Output impidance:200 ohm (Due to charge pump has high output impedance.)

    Only one condition (capacitor or output impedance), it not happen.

    Is it reasonable with OPA277?

    I suspect our problems similar above phenomenon.

    Off course there is no any extra capacitor on our circuit except stray capacitor.CircuitDiagram-2.pdf

    Is there any explanation for them? 

  • Hi Junji,

    Does the circuit have a capacitor on the output? I was under the impression that it was purely resistive. What is the value of the capacitor that is on the output? As you know, these capacitors can lead to unstable circuits which could explain the situation here. 

    Can you share with me a schematic of all components (power, op amps, and passives) including values and part numbers? This will help us identify the issue.

    Also, did you add the TVS diodes?

    -Tamara

  • OP_AMP.pdfGood day Tamara

    Sorry late to reply. (Yesterday I could not log on TI site.)

    In my original circuit, off course no capacitor on output of operational amplifier.

    I added capacitor intentionally to cause similar oscillation by SPICE simulation.

    (I hope it may be any hint to solve this issue.)

    Without that extra capacitor, output is always completely stable. (by SPICE simulation.)

    In this situation, output is stable, but -9V(negative) power line is very unstable.

    I could not find any problem on +9V(positive) power line.

    As additional information, when we added total 800pF capacitor on output, output (-3.3V) became unstable.

    (with actual circuit board, not SPICE.)

    If we removed capacitor, it was stable.

    Is it reasonable phenomenon?

     

    And I attached detail information of our circuit. Please find it.

    Best regards.

  • Hello Junji,

    Thank you for sharing this additional information with me. 

    To clarify, is the 800pF phenomenon on the negative power supply? When you have 800pF on the output, it is unstable but when you remove it, it becomes stable? The NJW4191R recommends a 1µF capacitor, have you followed this?

    As you know, we suggest that our op amp supplies are powered at the same time. 

    -Tamara

  • Thank you for your reply.

    Sorry my information was not enough.

    To duplicate similar phenomenon, I tried below 2 items. (All trial performed with SPICE simulation.)

    Item1 (Already informed)

    I attached extra capacitor at

    Between NEG-B and GND

    Between POS-B and GND

     

    For NEG-B and POS-B and its location, please refer to attachment.)

    So, I did not attach capacitor for charge pump IC. (Original 2.2uF capacitor at output.)

    Item2 (New find out)

    If attached small capacitor (less than 40pF/Extra capacitor 2) between Pin #2 and #3 of OPA2277.

    (500pF capacitor was removed.)

    In this condition, negative power supply current becomes unstable.

    So, it looks like our circuit has no enough margin for any electrical limitation.

    For capacitor location, please refer to attachment.

     

    And I tried some test on actual mounted PCB.

    Add capacitor for OPA2277 and charge pump IC.

    -9V supply from power supply unit.

    But any countermeasure was no effect.

    We fall into this issue for more 2 weeks.

    Any help or suggestions are welcome.

    Best regards.

    Sim_data.pdf

  • Good day.

     

    Below is my new finding.

     

    Exposed pad release from 9V (currently it’s connected with -V(=-9V), strange phenomenon disappeared.

    Or connect exposed pad to PCB GND, we got same result. (improved)

    Only connect to -V, I can see strange oscillation at -9V power supply current.

    But in specification sheets, it is specified to connect with -V.

    It looks that any coupling with -9 causes this trouble.

     

    My questions.

    1. Is it possible to explain above phenomenon? It’s OK just a any possibility.

    2. Is it possible a exposed pad leave float or connect GND?

     

    And I would like to know the internal connection of exposed pad of OPA2277.

    Is it just connected with the back side of silicon chip?

    Best regards.

  • Hi Junji,

    Thank you for the additional information. We recommend that the pad of the OPA2277 be connected to V- for proper use. In your case, that would be connected to -9V. This is shown in the datasheet on page 4 of the datasheet where is shows the pin-out configuration for the OPA2277 DRM package.

    -Tamara