LT6656 Linear Technology Corporation, LT6656 Datasheet - Page 9

• Manufacturer Part Number: LT6656
• Description: 1?a Precision Series Voltage Reference
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
or the battery is installed backwards. In systems where
the output can be held up by a backup battery with the
input pulled to ground, the reverse output protection of
the LT6656 limits the output current to typically less than
30µA. Should the output be pulled above the input when
the LT6656 is biased, the output will typically sink 4mA.
The current versus reverse voltage is shown in the Typical
Performance Characteristics section.
Long-Term Drift
Long-term drift cannot be extrapolated from accelerated
high temperature testing. This erroneous technique gives
drift numbers that are wildly optimistic. A more realistic
way to determine long-term drift is to measure it over the
time interval of interest. The LT6656 drift data was taken
over 100 parts that were soldered into PC boards similar
to a real world application. The boards were then placed
into a constant temperature oven with T
outputs scanned regularly and measured with an 8.5 digit
DVM. The parts chosen in the Long Term Drift curves in
the Typical Performance Characteristics section represent
high, low and typical units.
Hysteresis
Hysteresis on the LT6656 is measured in two steps, for
example, from 25°C to –40°C to 25°C, then from 25°C to
85°C to 25°C, for the industrial temperature range. This
two-step cycle is repeated several times and the maximum
hysteresis from all the partial cycles is noted. Unlike other
commonly used methods for specifying hysteresis, this
ensures the worst-case hysteresis is included, whether it
occurs in the first temperature excursion or the last.
Results over both commercial and industrial temperature
ranges are shown in Figure 8 and Figure 9. As expected,
the parts cycled over the higher temperature range have
a higher hysteresis than those cycled over the lower
range.
Power Dissipation
The LT6656 will not exceed the maximum junction tem-
perature when operating within its specified temperature
range of –40°C to 85°C, maximum input voltage of 18V
and specified load current of 5mA.
A
= 30°C, their
IR Reflow Shift
The different expansion and contraction rates of the
materials that make up the LT6656 package induce small
stresses on the die that can cause the output to shift during
IR reflow. Common lead free IR reflow profiles reach over
250°C, considerably more than lead solder profiles. The
higher reflow temperature of the lead free parts exacerbates
the issue of thermal expansion and contraction causing
the output shift to generally be greater than with a leaded
reflow process.
The lead free IR reflow profile used to experimentally
measure the output voltage shift in the LT6656-2.5 is
shown in Figure 10. Similar results can be expected using
a convection reflow oven. Figure 11 shows the change
in output voltage that was measured for parts that were
20
18
16
14
12
10
30
25
20
15
10
8
6
4
2
0
5
0
–160 –120
Figure 9. –40°C to 85°C Hysteresis
–60
Figure 8. 0°C to 70°C Hysteresis
V
C
I
L
IN
L
= 0
–40°C TO 25°C
85°C TO 25°C
= 1µF
= 3V
–40
–80
HYSTERESIS (ppm)
HYSTERESIS (ppm)
–20
–40
0
0
40
20
0°C TO 25°C
70°C TO 25°C
80
40
V
C
I
L
120
IN
L
= 0
= 1µF
= 3V
6656 F08
6656 F09
160
60
LT6656

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