Voltage Reference

The first considerations in choosing a voltage reference are output voltage and initial accuracy. Often overlooked, however, are the various other data-sheet parameters that can assume major importance in specific applications. The following discussion of voltage-reference basics will help you better understand the performance parameters associated with the most common voltage-reference topologies: the two-terminal shunt and the three-terminal series designs.

Two-Terminal (Shunt) Reference

As its name implies, the shunt reference operates in parallel with its load (Figure 1). It can be viewed as a voltage-controlled current sink in which the controlling voltage is applied to its input terminal. With no load applied, the shunt reference sinks just enough current so that the voltage drop across R1 produces the desired output voltage (V_IN - I_REFR1 = V_REF). If, for example, V_IN = 6.0V and the desired V_REF is 5.0V, the reference I_REF creates a 1.0V drop across R1. The reference then makes I_REF adjustments as necessary to maintain 5.0V across its input.

Figure 1. The shunt reference is connected in parallel with its load.

Now apply a load to the reference. I_REF no longer equals I_R1, because load current (I_L) produces part of the voltage drop across R1. The reference automatically reduces its sink current by the amount I_L. Thus, the total current through R1 doesn't change (I_REF + I_L equals the original I_R1). I_R1 is shunted between reference and load, hence the name "shunt reference." A shunt reference regulates the output voltage by adjusting its sink current to oppose changes in load current.

Three-Terminal (Series) Reference

The series reference operates in series with its load (Figure 2). It can be viewed as a voltage-controlled resistance in which V_OUT controls an internal resistance between the reference's input and output terminals. A series reference regulates by creating a voltage drop between its input and output, which is equal to the product of the load current and the controlled internal resistance. With no load applied, the series reference draws a small amount of current (I_Q) through the internal resistance (R) to drop a voltage between input and output necessary to produce the correct V_OUT.

Figure 2. A series reference (its regulating part) is connected in series with its load.

As load current increases, the reference maintains the desired output voltage by changing R as required to produce the correct drop between input and output. Applying Ohm's Law, one notes that to maintain a constant drop between input and output, R must decrease as I_OUT increases.