8.2 Temperature Coefficient

The REF35-Q1 is designed and tested for a low output voltage temperature coefficient, which 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 limits for the nominal output voltage over the operating

temperature range. REF35-Q1 has a low maximum temperature coefficient of 15 ppm/°C from –40°C to +125°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.

Temperature (

C)

-40

-7

26

59

92

125

-1500

-750

0

750

1500

VREF = 1.8V

VREF = 2.048V

VREF = 2.5V

VREF = 3.3V

VREF = 5V

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

8.3 Long-Term Stability

One of the key performance parameters of the REF35-Q1 references is long-term stability also known as

long-term drift. The long-term stability value is tested in a typical 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

being 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 8-4 shows the typical drift value for the REF35-Q1 in 6-pin SOT-23 package is 30 ppm from 0 to 1000

hours. It is important to understand that long-term stability is not ensured by design and that the value is typical.

REF35-Q1

SNAS845 – AUGUST 2023

www.ti.com

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