Datasheet

LT1790
22
1790fc
For more information www.linear.com/LT1790
APPLICATIONS INFORMATION
11 shows the shift after 1 cycle, while Figure 12 shows
shift after 3 cycles. In the worst case, shifts are typically
150ppm, but may be as high as 290ppm. Shifts in output
voltage are proportional to temperature and dwell time.
In general, the output shift can be reduced or fully recovered
by a long (12-24 hour) bake of the completed PC Board
assembly at high temperature (100°C to 150C°) after
soldering to remove mechanical stress that has been
induced by thermal shock. Once the PC Boards have cooled
to room temperature, they may continue to shift for up to
3 times the bake time. This should be taken into account
before any calibration is performed.
Assuming 80µA max supply current for the LT1790, a 25µA
load, 120mV max dropout and a 4V to 30V input specifica
-
tion, the largest that R1 can be is (4V – 3.3V – 120mV)/
(80µA + 25µA) = 5.5k. Furthermore, assuming 220mW of
dissipation in the 18V SOT-23 Zener, this gives a max cur
-
rent of (220mW)/(18V) = 12.2mA. So the smallest that R1
should be is (30V – 18V)/12.2mA = 1k, rated at 150mW.
With R1 = 1k, and assuming a 450mV worst-case drop
-
out, the LT1790 can deliver a minimum current of (4V
– 3.3V–450mV)/(1k) = 250µA. In Figure 13, R1 and C1
provide filtering of the Zener noise when the Zener is in
its noisy V-I knee.
There are other variations for higher voltage operation that
use a pass transistor shown in Figures 14 and 15. These
circuits allow the input voltage to be as high as 160V while
maintaining low supply current.
LT1790-3.3
4V TO 30V
C1
0.1µF
BZX84C18
1µF
V
OUT
R1
1790 F13
Figure 13. Extended Supply Range Reference
PPM
2902702502302101901701501301109070
NUMBER OF UNITS
2.0
2.5
3.0
1790 F12
1.5
1.0
0
0.5
3.5
Figure 12. 3X IR Reflow Peak Temperature = 250°C,
Delta Output Voltage (ppm)
LT1790
C1
0.1µF
C2
1µF
ON SEMI
MMBT5551
V
S
6V TO 160V
V
OUT
1790 F14
R2
4.7k
R1
330k
BZX84C12
LT1790
C2
1µF
BAV99
V
OUT
1790 F15
C1
0.1µF
R1
330k
V
S
6.5V TO 160V
ON SEMI
MMBT5551
Figure 14. Extended Supply Range Reference
Figure 15. Extended Supply Range Reference
Higher Input Voltage
The circuit in Figure 13 shows an easy way to increase
the input voltage range of the LT1790. The Zener diode
can be anywhere from 6V to 18V. For equal power shar
-
ing between R1 and the Zener (at 30V), the 18V option
is better. The circuit can tolerate much higher voltages
for short periods and is suitable for transient protection.
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