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Voltage reference fundamentals

Posted: 06 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:embedded systems? DC? voltage reference? Zener diode? temperature coefficients?

So, let us say that we are using a typical 5.1V Zener diode biased using a 12V DC power supply. Let us also say that we are running about 10mA of reverse current through this thing. Let us also say that the Zener impedance is approximately 50 ohms. A 1% variance in the DC power supply will yield a variance of around 120mV. Using the voltage divider relationship means this 1% variance will yield approximately a 8mV variance in the reference voltage. At 5.1V we get roughly .008/5.1 which yields a 0.1% variance in the reference voltage. This equates to approximately a 20 dB improvement in the PSRR for our simple Zener reference.

Figure 3: Temperature coefficient of the various Zener diodes.

Temperature characteristics
There are two effects that affect the temperature dependence of a Zener diode. These two effects have opposite temperature coefficients. The 1st characteristic is the Zener effect, which has a negative temperature effect. The 2nd characteristic is the avalanche breakdown. This effect has a positive temperature effect. In fact, for Zener voltages in the 5.1 to 5.6V range, both of the temperature effects nearly cancel each other out. The net result of this can be clearly seen in figure 3.

VREF generation using a Zener Diode
Zener diodes are relatively inexpensive. Using a reverse biased Zener diode along with an op amp buffer. One can get a reasonable voltage reference for approximately $0.60.

We get an even better PSRR by replacing the bias resistor with a constant current source. This circuit is shown in figure 4. In this circuit, we show how to create a simple constant current source using a cheap 3-terminal adjustable regulator.

Figure 4A shows a simple voltage reference using a current source to bias the Zener diode. In figure 4B, we get a bit fancier by adding a non-inverting gain stage. This allows us to attain a reference voltage different from the Zener breakdown voltage.

The cost of goods in this implementation has increased. In general, it will have increased by around $0.15 to about $0.75 and change.

Figure 4: Voltage reference generated by a constant current biased Zener Diode.

Figure 5: Voltage reference generated by a constant current biased Zener Diode.

We can do away with the constant current source and save the $0.15 for the current regulator. In this case, we must find a way to "self-bias" the Zener diode. A simple way to do this is shown in figure 5.

Before we move on, some explanations of this circuit are in order. First of all, there is a small capacitor in the feedback loop of the amplifier. Doing this will improve the stability of the circuit. We need to do this because we have added a self-bias network on the positive loop of the amplifier. Adding this positive feedback reduces the stability of this circuit and may cause oscillations.

Table of vendors
The RBIAS resistor will self-bias the Zener diode. The appropriate value of this resistor is dependent on the desired Zener current. Equation 1 shows how to properly calculate the value of the bias resistor.

Equation 1: How to calculate the bias resistor value for the self-biased reference network.

The source resistor, RS, is used to 'bootstrap' the circuit into operation. It is also needed to give a 'boost' to the OP AMP current output. The value of the 'bootstrap' resistor is determined by the Zener voltage, (VZ), Zener bias current, (IZ), desired reference voltage, (VREF) and the amount of sourcing current desired, (IS). Equation 2 shows the inequalities needed for calculating the proper value for the 'bootstrap' resistor.

Equation 2: Inequalities needed for calculating the 'bootstrap' resistor value Shunt Voltage References.

Figure 6: Using an LM336-2.5 as a low-power shunt voltage reference.

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