LT6654 Linear Technology, LT6654 Datasheet - Page 8

• Manufacturer Part Number: LT6654
• Description: SOT-23 Precision Wide Supply High Output Drive Voltage Reference
• Manufacturer: Linear Technology

Long-Term Drift
Long-term drift cannot be extrapolated from accelerated
high temperature testing. This erroneous technique
gives drift numbers that are wildly optimistic. The only
way long-term drift can be determined is to measure it
over the time interval of interest. The LT6654 drift data
was taken on 40 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
their outputs scanned regularly and measured with an
8.5 digit DVM. Long-term drift curves are shown in the
Typical Performance Characteristics section.
Hysteresis
The hysteresis data is shown in the Typical Performance
Characteristics. The LT6654 is capable of dissipating
relatively high power. With a maximum input voltage and
10mA load current applied to the LT6654-2.5 the power
LT6654
APPLICATIONS INFORMATION
Positive or Negative Operation
The LT6654 can be operated in series mode or as a negative
voltage reference. A typical series connection is shown
on the front page of this data sheet.
The circuit in Figure 7 shows an LT6654 confi gured for
negative operation. In this confi guration, a positive volt-
age is required at V
circuitry. This voltage must be current limited with R1
to keep the output PNP transistor from turning on and
driving the grounded output. C1 provides stability dur-
ing load transients. This connection maintains nearly the
same accuracy and temperature coeffi cient of the positive
connected LT6654.
8
Figure 7. Using the LT6654-2.5 to Build a –2.5V Reference
R1 =
V
EE
500μA
6
– V
OUT
IN
LT6654-2.5
(Pin 4) to bias the LT6654 internal
V
EE
1, 2
4
0.1μF
C1
1μF
4.7k
V
OUT
3V
= –2.5
6654 F
A
= 30°C,
dissipation is approximately P
which causes an increase in the die temperature of 64°C.
This could increase the junction temperature above 125°C
(T
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 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
to minimize I • R drops. High trace resistance directly
impacts load regulation.
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 8. Similar profi les are found
using a convection refl ow oven. LT6654 devices run up
JMAX
is 150°C) and may cause the output to shift due
300
225
150
75
0
Figure 8. Lead Free Refl ow Profi le
0
RAMP TO
150°C
T = 150°C
OUT
T
S
= 190°C
2
traces should be as short as possible
T
S(MAX)
380s
= 200°C
4
MINUTES
T
120s
L
= 217°C
D
T
P
= 33.5V • 10mA = 335mW,
6
= 260°C
130s
40s
30s
t
t
L
P
8
RAMP
DOWN
6654 F08
10
6654f