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OPA547: About the variation of the current limit

tsuyoshi tokumoto
Expert 2630 points
Part Number: OPA547

Hello team,

  My customer is designing the current limit value of OPA547 with 56.33mA.
   *Please see the attached file.

OPA547.xlsx

　Please let me ask you two questions about the variation of
  the current limit value.
 
 Q1) Could you please inform me of the variation of the current limit value
        including the temperature influence? 
 
 Q2) Could you please inform me of the variation of internal resistance,
        the internal reference and the gain to estimate the variation of current limit
        value when changing the external resistance value,if possible?


Sincerely,
Tsuyoshi Tokumoto

  • 0
    Guru 140200 points
    Hi Tsuyoshi,

    the current limit feature of OPA547 is not ultra precise but shows some manufacturing tolerances, temperature drift and supply voltage drift. On page 3 of datasheet up to +/-8% current limit tolerance is specified for Rcl = 31k6. And on page 4 of datasheet you will find plots showing the temperature drift and supply voltage drift of current limit. Remember that these data are typicals. From my feel I would add a worst case maximum deviation of about +/-10%.

    Kai
  • 0
    TI__Guru* 93105 points

    Tokumoto-san,

    Kai is certainly correct regarding his statements about the OPA547 current limit function, "the current limit feature of OPA547 is not ultra precise but shows some manufacturing tolerances, temperature drift and supply voltage drift." It is not intended to be a precision function and the exact current limit behavior will be differ somewhat between each OPA547 device, different product runs of OPA547 devices, and with the junction temperature under which each OPA547 device is operates.

    Q1) Could you please inform me of the variation of the current limit value
            including the temperature influence?  

    The Typical Characteristics graphs on datasheet Pg. 4 provide information about how the current limit typically behaves across temperature and across its usable power supply range. We can see from the graph on the left that the current limit has a negative temperature coefficient that changes somewhat with the output current level. The top curve in the left graph has slope of roughly -0.14 mA/°C, while the other two curves are closer to -0.2 mA/°C.


     
     Q2) Could you please inform me of the variation of internal resistance,
            the internal reference and the gain to estimate the variation of current limit
            value when changing the external resistance value,if possible?

    The OPA547 Electrical Characteristics table on Pg. 3 provides the current limit equation, and the one a specific condition, ILIM = +/-375 mA, regarding the Current Limit Tolerance. Keep in mind NOTE 1 indicates, High-speed test at TJ = +25°C. That is the extent of what is specified in the datasheet regarding current limit tolerance. 

    The OPA547 datasheet information is the extent of the information that TI provides about the current limit function.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Expert 2630 points

    Kuehl-san,

    　Thank you very much for your prompt reply and detailed explanation.

    　I was concerned about whether the current limit tolerance on page3
    　and the values on the graph on page4 could be applied to this condition
    　(the current limit setting 56.3mA) as well, because these parameters are described
       on the data sheet under conditions of 200mA or more current limit.

       However, from your answer, I am thinking about estimating the current limit variation of
       Ilim=56.3mA(Rcl=390kΩ,Vs=±15V,Ta=-20～65℃) as follows.

       ①The variation of the current limit at Ta=25℃ and Vs=±30V is estimated at ±8%.
    　  　　　　min:51.8mA、typ:56.3mA、max:60.8mA

       ②The slope of the current limit value due to temperature change is estimated at -0.2mA/℃.
                       -20℃:+9mA、65℃:-8mA

       ③The attenuation of the current limit value(+Ilim) due to the change of Vs from ±30V to ±15V
           is estimated at 15mA.
          The attenuation of the current limit value(-Ilim) due to the change of Vs from ±30V to ±15V
          is estimated at 5mA.

          Note: ② and ③ have a large impact on the variation of the current limit
                    when the current limit setting value is 56.3 mA.


      Is my estimate reasonable?


    Thank you and best regards,
    Tsuyoshi Tokumoto

  • 0 in reply to tsuyoshi tokumoto
    TI__Guru* 93105 points
    Tokumoto-san,

    ①The variation of the current limit at Ta=25℃ and Vs=±30V is estimated at ±8%.
    　 　　　　min:51.8mA、typ:56.3mA、max:60.8mA

    The OPA547 Current Limit Tolerance maximum listed in the Electrical Characteristics table which equates to 8 % is for the specific conditions of VS = ±30V, RCL = 31.6kΩ (ILIM = ±375mA), RL = 50Ω, and NOTES: (1) High-speed test at TJ = +25°C. However, at the different output current level of 56.3 mA, the tolerance may be different than 8 %. Since there isn't a datasheet graph for the current limit tolerance vs. the programmed current limit level it would be best to error to the conservative and use a percentage higher than 8 %. There certainly is the internal 4.75 V reference voltage and precise 31.6 kΩ in the current set circuit so an upper limit of 10 to 12 % is probably a realistic limit for analysis purposes.

    ②The slope of the current limit value due to temperature change is estimated at -0.2mA/℃.
    -20℃:+9mA、65℃:-8mA

    I am not exactly sure where the +9 mA and -8 mA numbers come from, but indeed an -17 mA change would occur with a -0.2mA/℃ current limit vs. temperature line slope.

    ③The attenuation of the current limit value(+Ilim) due to the change of Vs from ±30V to ±15V is estimated at 15mA. The attenuation of the current limit value(-Ilim) due to the change of Vs from ±30V to ±15V is estimated at 5mA.

    The change in the current limit with supply that you have estimated look reasonably close. It might actually be a little less, but using your numbers should be okay.

    I think that your estimations will get you close to the actual OPA547 current limit sensitivities. It will be interesting to see what you actually find when you check them.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Expert 2630 points

    Kuehl-san,

      I appreciate your advice and detailed explanation.
      I understand your reply.
      I will share the estimate result based on your advice with my customer.


    Thank you and best regards,
    Tsuyoshi Tokumoto

  • 0 in reply to tsuyoshi tokumoto
    Guru 140200 points
    Hi Tsuyoshi,

    even if the tolerance might look high for some individuals, the current limit feature of OPA547 is very useful and reliable. Such a tolerance is quite acceptable for the very most applications.

    Kai