Optimizing the output voltage accuracy of the regulator

While output voltages continue to drop and regulation requirements become more stringent, your task may not be as difficult as it seems. Even if you must design with 1% or greater tolerance resistors , you can still achieve very precise output voltages.

Figure 1 shows a typical power regulation circuit . The output is shunted down and compared to a reference voltage. The difference is amplified and used to drive the regulation loop. At first glance, you might think that this scheme is limited to accuracy of twice the resistor tolerance. Fortunately, this is not the case; 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 amplifier compensation.

This ratio can be most easily illustrated by three different cases. The first case assumes that there is no voltage 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 case assumes that the output voltage is much higher than the reference voltage. In this case, R1 is greater than R2. The voltage divider error is twice the resistor tolerance, resulting in R1 values ​​that vary in one direction and R2 values ​​that vary in the other direction. The third case that is easy to illustrate assumes that the output voltage is twice the reference voltage. In this case, the nominal resistor values ​​are equal. Therefore, if the resistor tolerances vary in the opposite direction, the top of the voltage divider equation varies by that tolerance value, and the denominator becomes zero.

Figure 2 shows the output accuracy as a function of the reference voltage versus the output voltage. Simplified, the voltage divider accuracy is (1–Vref/Vout)*2*tolerance, which is related to the three data points we examined. We simplified this equation a bit, but it should be accurate enough for most resistor tolerances.

Figure 2. Output accuracy is straightforward: (1-Vref/Vout)*2*tolerance (1% resistors shown)

Interestingly, this gives you even better accuracy for low voltage outputs. Many IC references have a voltage range of 0.6 to 1.25V, and output voltages down to this range will give you 1% or better accuracy. Table 1 gives you some information you might want to know, which is a compilation of resistor error terms from a typical resistor data sheet. In a design, this list can be difficult to understand. Most engineers stop at the initial tolerance, but there are some error terms in the list that should probably not be ignored. Each term in the table has its own subtle effects. For example, there is no specific temperature coefficient range specified, and in reality both resistors may change in the same direction with temperature and not at opposite extremes. After a brief survey of some experienced design engineers, it was concluded that assuming 2.5% accuracy for a 1% tolerance resistor is a reasonable compromise between extremes and reasonable cost.

Table 1. Resistor tolerances can be added

In summary, providing good low voltage output accuracy is not a daunting task because low divider ratios are inherently accurate.