REF80 has on chip heater which is factory programmed to T_SET temperature as per T_SET of Heater to regulate the temperature of the die within ± 1ºC, for the entire operating ambient temperature range. This allows REF80 to achieve ultra low temperature drift of 0.05ppm/ ºC. Heater has dedicated supply pins HEATP and HEATM which run independent of VDD supply pin. The steady state power dissipation of the heater is directly proportional to the difference between ambient temperature T_A and T_SET as per Equation 1. Heater current depends on (HEATP - HEATM) voltage. Heater takes high transient current at the time of start-up to heat up the device from ambient. REF80 has heater indicator pin OP_STBL, which de-asserts when the heater reaches within ±0.1% of T_SET.

V_DD is supply voltage for the chip and I_VDD is the supply current.

R_θJA depends on board thickness and layout. The data sheet value of R_θJA is based on JEDEC standard board.

REF80 offers flexibility to decrease the heater temperature by putting a resistor on the T-SET pin as per T-SET Pin Resistor VS Change in T_SET to save power and reduce the thermal noise for restricted ambient temperature applications. T_SET must be programmed to 45ºC above the max ambient temperature to make sure that the heater is regulating the temperature properly. REF80 samples the value of the resistor at the time of startup and the value of T_SET is modified accordingly. Small drift in resistor value will not impact the T_SET temperature as T-SET pin samples discreet value.