Datasheet

13
LT1461
parts alone. What is more interesting are parts IR soldered
onto a PC board. If the PC board is then temperature cycled
several times from – 40°C to 85°C, the resulting hysteresis
curve is shown in Figure 8. This graph shows the influence
of the PC board stress on the reference.
When the LT1461 is soldered onto a PC board, the output
shifts due to thermal hysteresis. Figure 9 shows the effect
of soldering 40 pieces onto a PC board using standard IR
reflow techniques. The average output voltage shift is
110ppm. Remeasurement of these parts after 12 days
shows the outputs typically shift back 45ppm toward their
initial value. This second shift is due to the relaxation of
stress incurred during soldering.
Figure 9. Typical Distribution of Output Voltage Shift After Soldering Onto PC Board
OUTPUT VOLTAGE SHIFT (ppm)
–300
0
NUMBER OF UNITS
2
4
6
8
12
–200
–100 0 100
1461 F09
200 300
10
HYSTERESIS (ppm)
200
NUMBER OF UNITS
12
8
4
10
6
2
11
7
3
9
5
1
0
160 120 80 40
0 40 80 120 160 200
–40°C TO 25°C85°C TO 25°C
1461 F08
WORST-CASE HYSTERESIS
ON 35 UNITS
Figure 8. –40°C to 85°C Hysteresis of 35 Parts Soldered Onto a PC Board
APPLICATIO S I FOR ATIO
WUUU
The LT1461 is capable of dissipating high power, i.e., for
the LT1461-2.5, 17.5V • 50mA = 875mW. The SO-8
package has a thermal resistance of 190°C/W and this
dissipation causes a 166°C internal rise producing a
junction temperature of T
J
= 25°C + 166°C = 191°C. What
will actually occur is the thermal shutdown will limit the
junction temperature to around 150°C. This high tempera-
ture excursion will cause the output to shift due to thermal
hysteresis. Under these conditions, a typical output shift
is –135ppm, although this number can be higher. This
high dissipation can cause the 25°C output accuracy to
exceed its specified limit. For best accuracy and preci-
sion, the LT1461 junction temperature should not ex-
ceed 125°C.