LT6656ACS6-1.25 LINER [Linear Technology], LT6656ACS6-1.25 Datasheet - Page 12

• Manufacturer Part Number: LT6656ACS6-1.25
• Description: 1?A Precision Series Voltage Reference
• Manufacturer: LINER [Linear Technology]

LT6656
applicaTions inForMaTion
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
run through the reflow process for 1 cycle and also 3
cycles. The results indicate that the standard deviation
of the output voltage increases with a positive mean shift
of 120ppm. While there can be up to 220ppm of output
voltage shift, additional drift of the LT6656 after IR reflow
does not vary significantly.

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
2.5V OPTION
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
2.5V OPTION
V
C
I
L
IN
L
= 0
= 1µF
40
= 3V
120
6656 F09
6656 F08
160
60
PC Board Layout
The mechanical stress of soldering a surface mount volt-
age reference to a PC board can cause the output voltage
to shift and temperature coefficient to change.
To reduce the effects of stress-related shifts, position
the reference near the short edge of the PC board or in a
corner. In addition, slots can be cut into the board on two
sides of the device. See Application Note AN82 for more
information. http://www.linear.com
The input and output capacitors should be mounted close
to the package. The GND and V
short as possible to minimize the voltage drops caused
by load and ground currents. Excessive trace resistance
directly impacts load regulation.
Figure 10. Lead Free Reflow Profile Due to IR Reflow
Figure 11. Output Voltage Shift Due to IR Reflow,
Peak Temperature = 260°C
225
150
300
75
0
7
6
5
4
3
2
1
0
0
RAMP TO
150°C
2.5V OPTION
V
C
I
T = 150°C
0
L
IN
L
T
= 0
= 1µF
S
20
= 3V
CHANGE IN OUTPUT VOLTAGE (ppm)
= 190°C
T
2
S(MAX)
60
= 200°C
4
380s
MINUTES
120s
T
100
L
= 217°C
T
P
6
140
= 260°C
OUT
130s
40s
30s
t
t
P
L
180
8
3 CYCLES
1 CYCLE
traces should be as
DOWN
RAMP
6656 F11
6656 F10
220
10
6656fa