From Maxim Integrated Application Note 4003:

A shunt reference design is somewhat more difficult because you must calculate the external resistor value. That value (R1) must ensure that its voltage drop due to reference and load currents equals the difference between supply voltage and reference voltage. R1 must be calculated at minimum supply voltage and maximum load current to ensure operation under this worst-case condition. The following equations calculate the value and power dissipation of R1, and power dissipation in the shunt reference (Figure 3).

R1 = (V_MIN – V_REF )/(I_MO + IL_MAX )

The current and power dissipation in R1 depend only on the power-supply voltage. Load current has no effect, because the sum of currents through load and reference is constant:

I_R1 = (V_SUP – V_REF )/R1

P_R1 = (V_SUP – V_REF )² /R1

P_SHNT = V_REF (I_MO + I_R1 – IL)

The worst-case conditions are maximum power-supply voltage and no load:

WC_I_R1 = (V_MAX – V_REF )/R1

WC_P_R1 = (V_MAX – V_REF )² /R1

WC_P_SHNT = V_REF (I_MO + WC_I_R1)

or

WC_P_SHNT = V_REF (I_MO + (V_MAX – V_REF )/R1)

where:

R1 = external resistor

I_R1 = current flowing through R1

P_R1 = power dissipation in R1

P_SHNT = power dissipation in shunt reference

V_MIN = minimum power-supply voltage

V_MAX = maximum power-supply voltage

V_REF = reference output voltage

I_MO = reference minimum operating current

ILMAX = maximum load current

WC_I_R1 = worst-case current through R1

WC_P_R1 = worst-case power dissipation in R1

WC_P_SHNT = worst case power dissipation in shunt reference

This is the fourth fundamental on references leading up to our Ask the Experts session on Wednesday, May 21, at 1:00 p.m. EDT (10:00 a.m. PDT). You can sign in using this link on May 21, and you can even enter questions ahead of time.