ISL21440 Datasheet, PDF (12/15 Pages) Intersil Corporation – Micropower Voltage Reference with Comparator

ISL21440
temperature profile should not be exceeded. Expect up
to 1mV drift from the solder reflow process.
FGA references are susceptible to excessive
X-radiation like that used in PC board manufacturing.
Initial accuracy can change 10mV or more under
extreme radiation. If an assembled board needs to be
X-rayed, care should be taken to shield the FGA
reference device.
Hysteresis
The Hysteresis function allows for changing the value of
the reference switchover point depending on the
previouse state of the comparator. This works to remove
the effects of noise or glitches in the voltage detection
input and provide more reliable output transitions.
Hysteresis is added to the ISL21440 by connecting one
resistor between the REF and HYST pins (R_REF), and
another resistor(R_HYST) between the HYST pin and
ground. The hysteresis voltage (V_H) is designed to be
twice the voltage difference between the HYST pin and
REF pin (V_H = 2 * (V_REF - V_HYST)). Since the reference
voltage is 1.182V (V_REF), Equations 1 and 2 for these
two resistors are shown as follows:
R_REF = V_H ⁄ (2^∗I_REF) = (V_REF – V_HYST) ⁄ I_REF
(EQ. 1)
R_HYST = (1.182 – V_H ⁄ 2) ⁄ I_REF = V_HYST ⁄ I_REF
(EQ. 2)
I_REF is chosen to be less than the maximum output of
the reference, usually 5µA is a safe value but for
lowest power, 0.1µA can be used.
If the hysteresis is not used, the HYST pin should be
tied to the REF pin.
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_REF
and R_HYST provide 20mV of hysteresis and 0.5µA I_REF.
The input trip point for V_detect is the same as the
reference voltage, 1.182V, and a resistor divider at the
input sets the Lo_BAT trip point at 2.7V. The total
current draw for the circuit is going to be 1.1µA for
V_DD and 0.6µA for V_BAT.
V_BAT
2.4M
1.8M
20k
VDD
IN+
+
IN- _-
V+
OUT
HYST
REF
ISL21440
LoBAT-
2.4M
V- GND
FIGURE 24. LOW BATTERY DETECTOR WITH
HYSTERESIS
12
FN6532.1
March 2, 2010