8.2 Temperature Coefficient

The REF35 is designed and tested for a low output voltage temperature coefficient. The temperature coefficient

is defined as the change in output voltage over temperature. The temperature coefficient is calculated using the

box method in which a box is formed by the minimum/maximum values for the nominal output voltage over the

operating temperature range. REF35 has a low maximum temperature coefficient of 12 ppm/°C from –40°C to

+105°C. The box method specifies limits for the temperature error but does not specify the exact shape and

slope of the device under test. Due to temperature curvature correction to achieve low-temperature drift, the

temperature drift is expected to be non-linear. See TI's Analog Design Journal, Precision voltage references, for

more information on the box method. Use Equation 1 for the box method.

REF(MAX)

REF(MIN)

6

REF(25 C)

V

V

Drift

10

V

Temperature Range

q

§

·

u

¨

¸

¨

¸

u

©

¹

(1)

Figure 8-3 shows a typical voltage versus temperature curves for various reference voltages. Figure 8-4 shows

the distribution of temperature coefficients for REF35250 devices.

Temperature (

C)

-40

0

40

80

120

-250

0

250

500

750

1000

VREF = 5V

VREF = 2.5V

VREF = 1.25V

Figure 8-3. Output Voltage Drift Vs Free-Air

Temperature

Temperature Coefficient (ppm/

C)

0

12

24

36

48

60

0

1

2

3

4

5

6

7

8

9

10

11

−40

C to 85C

−40

C to 105C

Figure 8-4. Temperature Coefficient Distribution

8.3 Long-Term Stability

One of the key performance parameters of the REF35 references is long-term stability also known as long-term

drift. The long-term stability value is tested in a setup that reflects standard PCB board manufacturing practices.

The boards are made of standard FR4 material and the board does not have special cuts or grooves around the

devices to relieve the mechanical stress of the PCB. The devices and boards in this test do not undergo high

temperature burn in post-soldering prior to testing. These conditions reflect a real world use case scenario and

common manufacturing techniques.

During the long-term stability testing, precautions are taken to ensure that only the long-term stability drift is

measured. The boards are maintained at 35°C in an oil bath. The oil bath ensures that the temperature is

constant across the device over time compared to an air oven. The measurements are captured every 30

minutes with a calibrated 8.5 digit multimeter.

The typical long-term stability characteristic is expressed as a deviation of the reference voltage output over

time.

Figure 8-5 shows that the typical drift value for the REF35 6-pin SOT-23 package is 40 ppm from 0 to 1000

hours and 55 ppm from 0 to 2000 hours. It is important to understand that long-term stability is not ensured by

design and that the value is typical. The REF35 will experience the highest drift in the initial 1000 hr. Subsequent

deviation is typically lower than first 1000 hr.

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REF35

SNAS809A – DECEMBER 2021 – REVISED AUGUST 2022

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