Output voltages are falling and voltage regulation requirements are getting tighter. However, your job may not be as difficult as it might seem on the surface. Even though you are forced to design with resistors with tolerance of one percent or worse, you may still be able to provide very precise output voltages.

Figure 1 shows a typical regulation circuit for a power supply. The output is divided down and compared against a reference. The difference is amplified and used to drive the regulation loop. At first glance, you might think that this scheme is limited to an accuracy of twice the resistor tolerances. Luckily, this is not true; the accuracy is also a strong function of the ratio of the output voltage to the reference voltage.

Figure 1: Output accuracy is a function of the divider ratio, reference accuracy and error amp offset.
(Click on image to enlarge)

Three different scenarios are fairly easy to visualize for this ratio:

The first scenario is if there is no division at all. In other words, the output voltage is equal to the reference voltage. Obviously, there is no resistor division error in this case.

The second situation is if the output voltage is much larger than the reference. In this case, R1 is much larger than R2. The divider error is twice the tolerance of the resistors, providing the value of R1 shifted in one direction and R2 shifted in the other direction.

A third point that is easy to visualize is if the output voltage is twice the reference voltage. In this case, the nominal value of the resistors is equal. So if the resistor's tolerance shifted in opposite directions, the top of the divider equation shifts by the tolerance value, while the denominator shift is zero.

Figure 2 shows the output accuracy as a function of the relationship of the reference voltage to the output voltage. (See the detailed derivation in the Appendix.) The simplified answer is that the divider accuracy is:

(1 – Vref/Vout) × 2 × Tolerance,

which correlates with our three data points we derived by inspection. This equation is simplified somewhat, but should be sufficiently accurate for most resistor tolerances.

Figure 2: Output accuracy is simply: (1 – Vref/Vout) × 2 × tolerance (1% resistors shown).
(Click on image to enlarge)

Interestingly, this allows more accuracy for lower voltage outputs. Many IC references are in the 0.6-to-1.25 volt range, which allows one percent accuracies or better as the output voltage falls into these ranges.

Table 1 presents some information you may not want to see. This is a compilation of resistor error terms from a typical resistor data sheet.

Table 1: Resistor tolerances can add up
(Click on image to enlarge)

This list can be hard to comprehend in a design. Most engineers stop at the initial tolerances, but there are error terms in this list that probably should not be ignored. Each one of these elements is subtle in its impact. For instance, there is no range specified on the temperature coefficient while, in reality, both resistors probably will shift in the same direction with temperature and will not be at opposite ends of the extremes. After a brief survey of well seasoned design engineers, it was concluded that assuming 2.5% accuracy for a one percent tolerance resistor provides a reasonable tradeoff between worst case and reasonable costs.

To summarize, providing good accuracy on low-voltage outputs will not be a daunting task, as low divider ratios are inherently accurate.

Please join us next month when we will discuss an interesting power-supply topology for making negative voltages.

For more information about this and other power solutions, visit www.ti.com/power-ca.

About the author

Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

The Power Tips! series:
#1, July 2008: Picking the right operating frequency for your power supply
#2, August 2008: Taming a noisy power supply
#3, September 2008: Damping the input filter–Part 1
#4, October 2008: Damping the input filter–Part 2
#5, November 2008: Buck-boost design uses a buck controller
#6, December 2008: Accurately Measuring Power Supply Ripple
#7, January 2009: Efficiently driving LEDs offline
#8, January 2009: Reduce EMI by varying power supply frequency
#9, March 2009: Estimating Surface Mounted Semiconductor Temperature Rise
#10, April 2009: Simply Estimate Load Transient Response
#11, May 2009: Resolve Power Supply Circuit Losses
#12, July 2009: Maximize Power Supply Efficiency
#13, July 2009: Don't get burned by inductor core losses
#14, July 2009: SEPIC converter makes an efficient bias supply
#15, September 2009: Design a low-cost, high-performance LED driver
#16, September 2009: Snubbing the forward converter
#17, November 2009: Snubbing the flyback Converter