Datasheet

LT6656
15
6656fc
For more information www.linear.com/LT6656
Figure 13. ∆V
OUT
Due to IR Reflow,
Peak Temperature = 260°C, SOT-23
Figure 14. ∆V
OUT
Due to IR Reflow,
Peak Temperature = 260°C, DFN
APPLICATIONS INFORMATION
short as possible to minimize the voltage drops caused
by load and ground currents. Excessive trace resistance
directly impacts load regulation.
Humidity Sensitivity
Plastic mold compounds absorb water. With changes in
relative humidity, plastic packaging materials change the
amount of pressure they apply to the die inside, which
can cause slight changes in the output of a voltage refer
-
ence, usually on the order of 100ppm. The LS8 package is
hermetic, so it is not affected by humidity
, and is therefore
more stable in environments where humidity may be a
concern. However
, PC board material may absorb water
and apply mechanical stress to the LT6656LS8. Proper
board materials and layout are essential.
For best stability, the PC board layout is critical. Change
in temperature and position of the PC board, as well as
aging, can alter the mechanical stress applied to compo
-
nents soldered to the board. FR4 and similar materials also
absorb water, causing the board to swell. Even conformal
coating or potting of the board does not always eliminate
this effect, though it may delay the symptoms by reduc
-
ing the rate of absorption. Removing power and ground
planes in the PC board under the voltage reference can
improve the stability significantly.
Figure 15a shows a tab cut through the PC board on three
sides of an L
T6656, which significantly reduces stress
on the IC, as described in Application Note 82. For even
better performance, Figure 15b shows slots cut through
the PC board on all four sides. The slots should be as
long as possible, and the corners just large enough to
accommodate routing of traces. It has been shown that
for PC boards designed in this way, humidity sensitivity
can be reduced to less than 35ppm for a change in relative
humidity of approximately 60%. Mounting the reference
near the center of the board, with slots on four sides, can
further reduce the sensitivity to less than 10ppm.
An additional advantage of slotting the PC board is that the
LT6656 is thermally isolated from surrounding circuitry.
This can help reduce thermocouple effects and improve
accuracy.
CHANGE IN OUTPUT VOLTAGE (ppm)
0 6020
NUMBER OF UNITS
7
6
4
2
1
5
3
0
6656 F13
220180140100
SOT-23
3 CYCLES
1 CYCLE
2.5V OPTION
V
IN
= 3V
C
L
= 1µF
I
L
= 0
CHANGE IN OUTPUT VOLTAGE (ppm)
–160
NUMBER OF UNITS
6
8
10
160
6656 F14
4
2
5
7
9
3
1
0
–80
0
80
240
DFN
3 CYCLES
1 CYCLE
2.5V OPTION
V
IN
= 3V
C
L
= 1µF
I
L
= 0
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.
T
o 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
OUT
traces should be as