ICL7106_14 Datasheet, PDF (7/17 Pages) Intersil Corporation – 31/2 Digit, LCD/LED Display, A/D Converters

ICL7106, ICL7107, ICL7107S
Analog COMMON
This pin is included primarily to set the common mode voltage
for battery operation (ICL7106) or for any system where the input
signals are floating with respect to the power supply. The
COMMON pin sets a voltage that is approximately 2.8V more
negative than the positive supply. This is selected to give a
minimum end-of-life battery voltage of about 6V. However,
analog COMMON has some of the attributes of a reference
voltage. When the total supply voltage is large enough to cause
the zener to regulate (>7V), the COMMON voltage will have a low
voltage coefficient (0.001%/V), low output impedance (15Ω),
and a temperature coefficient typically less than 80ppm/×°C.
The limitations of the on chip reference should also be recognized,
however. With the ICL7107, the internal heating which results from
the LED drivers can cause some degradation in performance. Due
to their higher thermal resistance, plastic parts are poorer in this
respect than ceramic. The combination of reference Temperature
Coefficient (TC), internal chip dissipation, and package thermal
resistance can increase noise near full scale from 25µV to
80µV_P-P. Also the linearity in going from a high dissipation count
such as 1000 (20 segments on) to a low dissipation count such as
1111(8 segments on) can suffer by a count or more. Devices with
a positive TC reference may require several counts to pull out of an
over-range condition. This is because over-range is a low
dissipation mode, with the three least significant digits blanked.
Similarly, units with a negative TC may cycle between overrange
and a non-overrange count as the die alternately heats and cools.
All these problems are of course eliminated if an external
reference is used.
The ICL7106, with its negligible dissipation, suffers from none of
these problems. In either case, an external reference can easily
be added, as shown in Figure 4.
Analog COMMON is also used as the input low return during
auto-zero and deintegrate. If IN LO is different from analog
COMMON, a common mode voltage exists in the system and is
taken care of by the excellent CMRR of the converter. However, in
some applications IN LO will be set at a fixed known voltage
(power supply common for instance). In this application, analog
COMMON should be tied to the same point, thus removing the
common mode voltage from the converter. The same holds true
for the reference voltage. If reference can be conveniently tied to
analog COMMON, it should be since this removes the common
mode voltage from the reference system.
Within the lC, analog COMMON is tied to an N-Channel FET that
can sink approximately 30mA of current to hold the voltage 2.8V
below the positive supply (when a load is trying to pull the
common line positive). However, there is only 10µA of source
current, so COMMON may easily be tied to a more negative
voltage thus overriding the internal reference.
V
REF HI
REF LO
ICL7106
ICL7107
V+
6.8V
ZENER
I_Z
V-
FIGURE 4A.
V+
V
ICL7106
ICL7107
20kΩ
REF HI
REF LO
COMMON
6.8kΩ
ICL8069
1.2V
REFERENCE
FIGURE 4B.
FIGURE 4. USING AN EXTERNAL REFERENCE
TEST
The TEST pin serves two functions. On the ICL7106 it is coupled
to the internally generated digital supply through a 500Ω resistor.
Thus, it can be used as the negative supply for externally
generated segment drivers such as decimal points or any other
presentation the user may want to include on the LCD display.
Figures 5 and 6 show such an application. No more than a 1mA
load should be applied.
V+
1MΩ
ICL7106
TO LCD
DECIMAL
POINT
BP ₂₁
TEST
37
TO LCD
BACKPLANE
FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT
The second function is a “lamp test”. When TEST is pulled high
(to V+) all segments will be turned on and the display should read
“1888”. The TEST pin will sink about 15mA under these
conditions.
CAUTION: In the lamp test mode, the segments have a constant DC
voltage (no square-wave). This may burn the LCD display if maintained for
extended periods.
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FN3082.9
October 24, 2014