isl21440 Intersil Corporation, isl21440 Datasheet - Page 12

• Manufacturer Part Number: isl21440
• Description: Micropower Voltage Reference With Comparator
• Manufacturer: Intersil Corporation

Board Assembly Considerations
FGA references provide high accuracy and low
temperature drift but some PC board assembly
precautions are necessary. Normal Output voltage shifts of
100µV to 1mV can be expected with Pb-free reflow
profiles or wave solder on multi-layer FR4 PC boards.
Precautions should be taken to avoid excessive heat or
extended exposure to high reflow or wave solder
temperatures, this may reduce device initial accuracy.
Post-assembly X-ray inspection may also lead to
permanent changes in device output voltage and
should be minimized or avoided. If X-ray inspection is
required, it is advisable to monitor the reference
output voltage to verify excessive shift has not
occurred. If large amounts of shift are observed, it is
best to add an X-ray shield consisting of thin zinc
(300µm) sheeting to allow clear imaging, yet block x-
ray energy that affects the FGA reference.
Special Applications Considerations
In addition to post-assembly examination, there are
also other X-ray sources that may affect the FGA
reference long term accuracy. Airport screening
machines contain X-rays and will have a cumulative
effect on the voltage reference output accuracy.
Carry-on luggage screening uses low level X-rays and
is not a major source of output voltage shift, although
if a product is expected to pass through that type of
screening over 100x it may need to consider shielding
with copper or aluminum. Checked luggage X-rays are
higher intensity and can cause output voltage shift in
much fewer passes, so devices expected to go through
those machines should definitely consider shielding.
Note that just two layers of 1/2 ounce copper planes
will reduce the received dose by over 90%. The lead
frame for the device which is on the bottom also
provides similar shielding.
If a device is expected to pass through luggage X-ray
machines numerous times, it is advised to mount a
2-layer (minimum) PC board over the top of the
package, which along with a ground plane underneath
will effectively shield it from 50 to 100 passes through
the machine. Since these machines vary in X-ray dose
delivered, it is difficult to produce an accurate
maximum pass recommendation.
Typical Applications
Low Battery Detector
Figure 24 shows a typical implementation for the
ISL21440, a low battery detector. The values for R
and R
The input trip point for V
reference voltage, 1.182V, and a resistor divider at the
input sets the Lo
HYST
provide 20mV of hysteresis and 0.5µA I
BAT
trip point at 2.7V. The total
detect
12
is the same as the
ISL21440
REF
REF
.
current draw for the circuit is going to be 1.1µA for
V
Window Comparator
The ISL21440 can be combined with a micropower to
produce a window comparator circuit. The circuit in
Figure 25 uses a 3 resistor divider to produce high and
low trip points, and the ISL28197 (800nA supply current)
comparator is added to give the second output. The two
outputs can be used separately for over or undervoltage
indication, or a gate can be added as shown to report
either in-window or out-of window condition.
The resistors are shown as Equations 3, 4 and 5 as
follows.
Set:
Example: For V
R
The resulting circuit draws about 3µA and works down to
V
R
R
R
FIGURE 24. LOW BATTERY DETECTOR WITH
DD
2
DD
3
2
1
R1
R2
R3
V
= 402k, R
=
=
=
BAT
= 2.2V.
and 0.6µA for V
FIGURE 25. WINDOW COMPARATOR CIRCUIT
1M 1%
R
R
V
3
3
[
(
BAT
(
V
[
V
1.8M
2.4M
H
H
⁄
)
V
⁄
V
20k
2.4M
1
HYSTERESIS
L
REF
–
= 1.82M (can be 1%)
H
IN+
IN-
HYST
REF
1
V
]
= 3.8V, V
BAT
–
1
] R
+
OR V
BAT
-
ISL21440
+
–
-
V-
HYST
REF
IN+
V+
IN-
2
.
ISL28197
)
DD
GND
L
OUT
= 2.7V (3.3V ± 0.5V)
+
-
V
DD
ISL21440
V-
V+
V
GND
V
OUT
HI
LOW
-
-
December 7, 2009
Lo
BAT
V
-
(EQ. 3)
(EQ. 4)
(EQ. 5)
FN6532.0
WINDOW