Thermally compensated Zener circuit

Thermal stability is the most demanding performance requirement
Of voltage references

Forward drop of a silicon junction decreases about 2mV for every
degree Celsius increase

The idea behind thermal compensated Zener diode is to connect a
forward-biased diode in series with a Zener diode having an equal
but opposing thermal voltage (TC)

Vref = Vz+Vf = 5.5+0.7 = 6.2 V
Uses Iz = 7.5 mA
R1 = (Vcc-Vref)/Iz = 106.6 ohm