How to design a suitable voltage reference for our circuit

    Have you ever had to choose between two of your favorite desserts and wondered, "Why can't I have both?" Well, engineers encounter the same thing every day when designing with programmable voltage references (V REF ).

    A very common goal for engineers is to come up with an ultra-low-power design that performs a certain function: sensing temperature, booting a computer, or even delivering our favorite candy. But did you know that to achieve low-power operation, engineers are also giving up other advantages? To achieve low power, engineers often must design with a VREF that delivers very low current, but suffers from a loss of accuracy over the entire operating temperature range. Is there a way for these engineers to have their cake and eat it too? I think you know the answer.

    First, let's look at what I mean by VREF accuracy and the conditions that directly impact accuracy. For this example, I will use the commonly used TL431 to drive my analysis.

    The ATL431 and ATL432 are three-pin adjustable shunt regulators that meet specified thermal stability over the applicable automotive, commercial, and industrial temperature ranges. Both regulators can set the output voltage to any value between Vref (approximately 2.5V) and 36V using two external resistors. Their output impedance is typically 0.05Ω. The active output circuitry of these devices provides excellent turn-on characteristics, making them an excellent replacement for Zener diodes in many applications, such as on-board regulators, adjustable power supplies, and switching power supplies.

    The cathode current range of the ATL43X is more than 20 times greater than that of its predecessor, the TL43X. Furthermore, stability has been improved, allowing it to support a wider range of load capacitor types and values.

    The ATL431 and ATL432 devices are functionally identical, differing only in pinout and ordering number. The ATL43X is available in A and B grades, with initial tolerances of 1% and 0.5%, respectively, at 25°C. Furthermore, both devices feature low output temperature drift, ensuring excellent stability over the entire temperature range.

    The ATL43xxI devices are specified for operation over the –40°C to 85°C temperature range; the ATL43xxQ devices are specified for operation over the –40°C to 125°C temperature range.

    If we have a circuit similar to Figure 1, we can set R1 and R2 to get the desired VKA output based on VREF. We can find more information on how to do this in this application note.

    Figure 1: Power supply current limiter

    V REF is not always at its nominal value; in fact, it is guaranteed to have an offset based on the device operating conditions. Table 1 shows the specifications that directly affect V REF.

    Table 1: TL431 electrical specifications

    Assuming VKA = 5V and a cathode current of 2mA, we can calculate the effective VREF by adding the collective effects of these parameters (typical values) using Equation 1.

      (1)

    This tells us that for the TL431, the effective VREF is now 2.4899V, or 0.2% accuracy, which is not a significant difference under normal conditions. However, once it reaches its maximum value (which typically occurs at high temperatures), we get an effective VREF of 2.539V, or 1.78% accuracy.

    How does this affect our system?

    In an analog environment, total voltage drift might be the threshold necessary to trigger an op amp; a 44.5mV maximum/6mV minimum offset could mean the difference between regulation and standby, potentially leading to system failure. However, this becomes a greater issue when considering using the TL431 as a reference for an analog-to-digital converter (ADC). The least significant bit (LSB) voltage depends on the converter's bit accuracy. Assuming the same conditions of 5V and an 8-bit ADC, we obtain an LSB of 19.53mV, which should be fine during typical operation, as shown in Equation 2. However, as temperature changes, operation can change, and the system may read erroneous data or perform incorrectly.

    (2)

    So how do we address the accuracy issue and still maintain low power operation? One solution is the ATL431, which offers lower operating power consumption but significantly improved accuracy. Using the ATL431 under the same conditions and design parameters as before, we achieve an effective VREF of 2.499V (0.95mV), which translates to 0.03% accuracy. This provides a much larger margin of error when considering analog operation, but more importantly, we can now use a much higher resolution ADC (Equation 3):

    (3)

    In the end, small changes in the right direction can produce a more significant compromise than our original design around the TL431. The ATL431 is a solution that provides good enough power savings while also improving accuracy without having to sacrifice one for the other. In the end, even with compromises, it's possible to get the best of both worlds.