Datasheet

LT6654
14
6654fa
APPLICATIONS INFORMATION
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 coeffi cient to change. These two
changes are not correlated. For example, the voltage may
shift but the temperature coeffi cient may not.
To reduce the effects of stress-related shifts, mount 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.
The capacitors should be mounted close to the LT6654.
The GND and V
OUT
traces should be as short as possible
to minimize I • R drops, since high trace resistance directly
impacts load regulation.
Figure 12. Lead Free Refl ow Profi le
MINUTES
TEMPERATURE (°C)
0
0
75
RAMP
DOWN
t
P
30s
40s
t
L
130s
120s
150
225
300
2468
6654 F12
10
RAMP TO
150°C
380s
T
P
= 260°C
T
L
= 217°C
T
S(MAX)
= 200°C
T
S
= 190°C
T = 150°C
Figure 13. Output Voltage Shift Due to IR Refl ow (%)
CHANGE IN OUTPUT (ppm)
–140
0
NUMBER OF UNITS
2
4
6
8
10
12
14
–120 –100 –80
–60
6654 F13
–40 0–20
260°C 3 CYCLES
260°C 1 CYCLE
IR Refl ow Shift
The different expansion and contraction rates of the ma-
terials that make up the LT6654 package may cause the
output voltage to shift after undergoing IR refl ow. Lead
free solder refl ow profi les reach over 250°C, considerably
more than with lead based solder. A typical lead free IR
refl ow profi le is shown in Figure 12. Similar profi les are
found using a convection refl ow oven. LT6654 devices run
up to three times through this refl ow process show that the
standard deviation of the output voltage increases with a
s l i g h t n e g a t i v e m e a n s h i f t o f 0 . 0 0 3 % a s s h o w n i n F i g u r e 13 .
While there can be up to 0.014% of output voltage shift,
the overall drift of the LT6654 after IR refl ow does not
vary signifi cantly.