Datasheet

LT6654
17
6654fd
For more information www.linear.com/LT6654
applicaTions inForMaTion
Hysteresis
The hysteresis data is shown in Figure 11. The LT6654 is
capable of dissipating relatively high power. For example,
with a 36V input voltage and 10mA load current applied
to the LT6654-2.5, the power dissipation is P
D
= 33.5V
• 10mA = 335mW, which causes an increase in the die
temperature of 64°C. This could increase the junction
temperature above 125°C (T
JMAX
is 150°C) and may cause
the output to shift due to thermal hysteresis.
PC Board Layout
The mechanical stress of soldering a surface mount volt
-
age reference to a PC board can cause the output voltage
to s
hift and temperature coefficient to change. These two
changes are not correlated. For example, the voltage may
shift but the temperature coefficient 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.
IR Reflow Shift
The different expansion and contraction rates of the ma-
terials that make up the LT6654 package may cause the
ou
tput voltage to shift after undergoing IR reflow. Lead
free solder reflow profiles reach over 250°C, considerably
more than with lead based solder. A typical lead free IR
reflow profile is shown in Figure 12. Similar profiles are
found using a convection reflow oven. LT6654 devices run
up to three times through this reflow process show that the
standard deviation of the output voltage increases with a
slight negative mean shift of 0.003% as shown in Figure 13.
While there can be up to 0.014% of output voltage shift,
t
h
e overall drift of the LT6654 after IR reflow does not
vary significantly.
Figure 12. Lead Free Reflow Profile
MINUTES
TEMPERATURE (°C)
0
0
75
RAMP
DOWN
t
P
30s
40s
t
L
130s
120s
150
225
300
2 4 6 8
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 Reflow (%)
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
LT6654S6
Humidity Sensitivity
Plastic mould 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 reference,
usually on the order of 100ppm. The LS8 package is her
-
metic, so it is not affected by humidity, and is therefore
more stable in environments where humidity may be a
concern.