In a typical shunt regulator configuration (see Figure 7-4), an external resistor, R_S, is connected between the supply and the cathode of the TL4051. R_S must be set properly, as it sets the total current available to supply the load (I_L) and bias the TL4051 (I_Z). In all cases, I_Z must stay within a specified range for proper operation of the reference. Taking into consideration one extreme in the variation of the load and supply voltage (maximum I_L and minimum V_S), R_S must be small enough to supply the minimum I_Z required for operation of the regulator, as given by data sheet parameters. At the other extreme, maximum V_S and minimum I_L, R_S must be large enough to limit I_Z to less than its maximum recommended rating of 12 mA.

R_S is calculated as shown in Equation 1.

Equation 1.

Figure 7-4 Shunt Regulator