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LMH6551: OP AMP Draw too Much Current

usersdt123
Prodigy 100 points
Part Number: LMH6551
Other Parts Discussed in Thread: TINA-TI

Hi, 

In my new design, the op-amps LMH6551 draw too much current from positive and negative supply.

Here is the schematic of the circuit. (There is 100nF between each VCP pin and GND)

1. First, I couldn't see any output signal, it was always zero.

2. Then, I decrease the gain resistors. Finally, I see output signal at output with a gain value. But opamp continue to draw too much current.

For you information, there is 20 opamp on board. They draw 2A per two supply. 

Do you have any idea what causes this problem?

Thank you for your help.

  • 0
    TI__Genius 15975 points
    Hi Maksut,

    It seems that you may have shorted something on your board (possibly the LMH6551 supply voltage) which could be the reason for drawing 2A per two supply.

    Could you reduce the current limit our your power supplies to 300mA or so? Also, are you making sure the positive of the supply voltage is connected to the +3V node on your board and negative is connected to the -3V node? Would it be possible for you to slowly ramp up the supplies from 0V to +/-3V to better understand at what supply voltage the device starts drawing too much current?

    I simulated your schematic in TINA-TI and it looks good with no potential sources of oscillation. However, I would not rule out the possibility of oscillation given the fact that actual implementation might introduce some unknown parasitic that causes the device to oscillate.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 100 points
    Hi Rohit,

    Thanks for your reply.

    As I see, there is no short at one point on the board. When apply power to board, all op amp's junction temperature is increasing.

    To simplify problem, I will assembly one op amp pair in an empty PCB. Then I will apply +3V0 and -3V0 to the opamps from external supply. If op amps draw normal current, I will think that something went wrong my board and all op amps are broken. If they continue to draw much current, I will try to change something in my design.

    I will write the conclusion of my test.

    Best regards,
  • 0 in reply to Rohit Bhat
    Prodigy 100 points
    Hi Rohit,

    I applied the test which I told about in my second message. I assembled only one op amp pair and applied +3V0 and -3V0 from external supply. The circuit drawn 200mA from each supply.

    Then I remove R616 and R617 from board; applied voltage. The current decreases to 10mA for each supply. Then, I assembled larger resistor (about 10k) to R616 and R617. This time it drawn 100mA from each supply.

    It seems to be a problem about second stage of op amp pair. Basicly op amp inputs don't draw any current as you now, but this one seems to be drawn current.

    Do you have an opinion according to new updates?

    Thank you.
  • 0 in reply to usersdt123
    TI__Genius 15975 points
    Hi Maksut,

    For the second stage, only implementing the R616 & R617 of 10k along-with Rf (R318, R319) and Rg (R316, R317) will not work. You will need to implement the Cf (C479, C480) as well as the Cin (C410, C411) in-order to make sure the circuit work correctly.

    If you have implemented the entire second stage circuit and still see 100mA of supply current, then it is a possibility that the amplifier is oscillating. May I know what is the output load circuit the LMH6551 driving? If you are directly driving a capacitive load without any series resistor in-between the LMH6551 output and capacitor, then there is a high chance that the circuit response is peaked up and you are seeing oscillations.
    In the meanwhile, let me see if I can replicate your circuit on the bench.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    TI__Genius 15975 points

    Hi Maksut,

    I verified the high supply current scenario for the second stage LMH6551 MFB filter circuit, and it seems to be the case because the device is oscillating.

    An easy fix for making the circuit stable is to include a 500pF (or two 1nF cap in series tied together) across the differential inputs, which increases the noise gain at higher frequency. The inclusion of the input cap should have minor impact on the -3dB frequency response from the earlier circuit and I have verified the below schematic to be stable on the bench.

    I am sorry that it is not clearly evident on the TINA-TI simulation, but if you simulate it to a higher frequency you could see bump in gain response across frequency. It is possible that the open-loop frequency response of LMH6551 is not correctly modeled in TINA-TI, or else you should see a peaked up response at the bump frequencies indicating instability.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 100 points
    Hi Rohit.

    Thank you for your help. I could test the board last few days unfortunately. Let me write my notes.

    When I applied DC voltage to first op amp from DAC which amplitude is 400mV or -400mV, I couldn't see 400mV at between R314 and R315. Then I changed the gain resistors of these op amp, which are 20k/100k, I managed to see 400mV between to resistors. Then I saw 2V at output at op amp pins. But I didn't understand why opamp draw current from input and output.

    I think, if I change the gain resistors of 2nd stage op-amp, the circuit will work fine. I will write the conclusion of situation and finally I can mark the topic as solved.