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 0.595V, maximum of 0.603V, and typical
of 0.6V results in an accuracy of –0.83% and +0.5%. If this error is instead
centered around an average reference voltage of 0.599V, the accuracy is
calculated as ±0.67%.
- The VREF specification
in not measured in a switching, closed-loop configuration. Figure 6-32 can be used to see the effects of output current (load regulation) and
switching. However, it is seen that across the complete 12A load, there is only
a +155µV and –43µV deviation. This deviation is considered small enough such
that load regulation is not included in this accuracy calculation. Additionally,
since a differential remote sense connection is provided, it is possible to
regulate the voltage directly across the load, which compensates for load
regulation error due to ground offsets.
- 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)
The negative system error calculation
is System(error) = –0.83% – 0.14% =
–0.97% and the positive system error is
System(error) = 0.5% + 0.14% =
0.64%. Therefore, the total system error is calculated to be
+0.64%/–0.97%. If the total system error is centered, this comes to ±0.81%. These
each meet the ±1% accuracy 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.