To determine the output voltage DC
accuracy, the following sources of error are considered:
- VREF within the Electrical
Characteristics table is the predominant source of error. This
encompasses the error due to the reference voltage and error amplifier offset.
The across temperature minimum of 0.594V, maximum of 0.604V, and typical of 0.6V
results in an accuracy of –1.00% and +0.67%. If this error is instead centered
around an average reference voltage of 0.599V, the accuracy is calculated as
±0.83%.
- The VREF specification in not measured
in a switching, closed-loop configuration. Figure 7-31 can be used to see the effects of output current (load regulation) and
switching. It is seen that across the complete 6A load, there is only a small
deviation of approximately 800µV. This corresponds to +0.13% error.
- The external error due to the resistor tolerance of the RFB_TOP and
RFB_BOT resistors need to be added. Since it is assumed the error
is uncorrelated, it is decided to add the errors as a sum of squares. For the
selected 0.1% tolerance RFB_TOP and RFB_BOT resistors, the
total error is R(error) = sqrt(0.1%2 + 0.1%2 ) =
±0.14%.
Equation 20
is used to calculate the system error for output voltage accuracy.
Equation 20. System(error) = VREF(error) +
RFB(error) + VLOAD(error)
The negative system error calculation is
System(error) = –1.00% – 0.14% =
–1.14% and the positive system error is
System(error) = 0.67% + 0.14% + 0.13% =
0.94%. Therefore, the total system error is calculated to be
–1.14%./+0.94. If the total system error is centered, this comes to ±1.04%. These
each meet the 1.5% target.
Lifetime drift data could similarly be
added. Group C data may be used to aid in this calculation. For this example, it is
assumed the lifetime drift is minimal compared to the other sources of error and it
is therefore not added.