• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 48 subscribers
  • Views 1395 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

OPA376: Poor VOL and VOH under high load (over 10mA)

Eli az
Intellectual 730 points
Part Number: OPA376


Hello,

We are using the OPA376 and we have noticed that when we use relatively high loads (above 10mA) the voltage drops. For example, when drawing 30 mA the voltage drops to 400mV (with supply of +/- 2.75V). This We see this behavior with several devices from two different lots. When testing VOL under load of 50mA, the output was as high as 0V.

Is there an explanation for this behavior? Is this a known issue with the device?

Thank you

Eli

  • 0
    TI__Mastermind 23050 points

    Hi Eli,

    What you are seeing is normal behavior for the OPA376 and is demonstrated in figure 12 of the datasheet, shown below: 

    The higher the output current, the lower the output swing. This is because as you source more current through the output transistors, the drain to source voltage increases due to the on resistance of the transistor. Eventually, the short circuit protection circuitry of the amplifier begins to turn on and limits the output voltage to prevent damage to the device (this is the point in the curves where the output voltage rolls off very quickly). All amplifiers exhibit this behavior to varying degrees. If you can share what is important in your design then I can point you towards a device that will be better suited for your application.

  • 0 in reply to Zak Kaye
    Intellectual 730 points

    Hi Zak,

    Thanks for your response.

    I have looked at the graph in the datasheet, but the problem is that the dip that I see in the output voltage is much lower than the one presented in the datasheet. According to it,the output voltage should be well over 1V at least up to 30mA at 25C, but I get VOUT=0.4V @ 30mA, VS=+/-2.75V. Is this variation from the datasheet graph normal?

    Thank you very much,

    Eli.

  • 0 in reply to Eli az
    TI__Mastermind 23050 points
    Eli,

    There are a few things at play here, but the short answer is yes; the deviation you are seeing is not unexpected and the behavior is still related to the short circuit protection circuitry. The curve above characterizes a typical unit. How much this varies depends on the topology of this internal circuitry, but you could expect a deviation of about 15-20% in the curve above. Additionally, the power you are dissipating in the device will heat the die up, pushing you closer to the 125C curve on the left.
  • 0 in reply to Zak Kaye
    Intellectual 730 points

    Thank you, Zak.

    In regards to the self-heating, I see the exact same VOUT result even if I turn on the device directly to 30mA loads, so it doesn't see, to be linked to the power dissipation, because if I expect it to be linked to the self-heat, it would deteriorate over time as the load persists, but it doesn't - at least not substantially.

    Regarding the expected deviation of 20%, how should I regard to it - in regards to what and how do I calculate the 20% expected deviation?

    Thank you,

    Eli.

  • 0 in reply to Eli az
    TI__Guru 68541 points

    Eli,

    You cannot say that Vout is at 0.4V under 30mA load but there is no self-heating due to power dissipation inside the OPA376 package.  If you use +/-2.75V supplies and the Vout is at 0.4V, this means that there is 2.35V (2.75V-0.4V) drop across the output transistor delivering 30mA current; therefore, the power dissipation inside the package is at least P=V*I=2.35V*30mA=0.07W - for SOT-23-5 package with thermal resistance of 273.8C/W, this would result in about 20 deg C increase in junction temperature above the ambient (45C) - see graph below.