7.4 Thermal Hysteresis

Thermal hysteresis is measured with the REF54 soldered to a PCB, similar to a real-world application. Thermal

hysteresis for the device is defined as the change in output voltage after operating the device at 25°C, cycling

the device through the specified temperature range, and returning to 25°C. The fist thermal cycle is shown in

Figure 7-8 and second cycle is shown in Figure 7-9. Thermal hysteresis for REF54250CDR settles after first

cycle. Hysteresis can be expressed by Equation 2:

6

PRE

POST

HYST

NOM

| V

V

|

V

10

ppm

V

§

·

u

¨

¸

©

¹

(2)

where

• VHYST = thermal hysteresis (in units of ppm)

• VNOM = the specified output voltage

• VPRE = output voltage measured at 25°C pre-temperature cycling

• VPOST = output voltage measured after the device has cycled from 25°C through the specified temperature

range (eg. 0°C to 70°C ) and returns to 25°C.

Thermal Hysteresis (ppm)

0

20

40

60

80

100

-30

-25

-20

-15

-10

-5

0

5

10

Figure 7-8. REF54250CDR Thermal Hysteresis

Distribution (0°C to 70°C) - Cycle 1

Thermal Hysteresis (ppm)

0

5

10

15

20

25

30

35

-5

-3

-1

0

1

3

5

Figure 7-9. REF54250CDR Thermal Hysteresis

Distribution (0°C to 70°C) - Cycle 2

7.5 Solder Heat Shift

The packaging materials of the REF54 have different coefficients of thermal expansion than the PCB material,

resulting in stress change on the device die when the part is heated during soldering process and cooled down

afterwards. Thermal shock due to reflow and stress change on the device die causes the output voltages to shift,

degrading the initial accuracy performance of the product. Reflow soldering is a common cause of this error.

To quantify the impact, 32 devices were soldered on printed circuit boards using lead-free solder paste and the

paste manufacturer suggested reflow profile to illustrate this effect. The reflow profile is as shown in Figure 7-10.

The printed circuit board is comprised of FR4 material. The board thickness is 1.65 mm and the area is 137 mm

× 168 mm.

For recommended reflow profiles using 'Sn-Pb Eutectic Assembly' or 'Pb-Free Assembly' please refer JEDEC

J-STD-020 standard.

REF54

SNVSCN7 – NOVEMBER 2023

www.ti.com

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