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/×
o
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 over-range and a non-over-
range 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 de-integrate. 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
6.8V
ZENER
I
Z
V+
ICL7106
ICL7107
V-
FIGURE 4A.
V+
V
ICL7106
ICL7107
REF HI
REF LO
COMMON
20kΩ
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
TO LCD
DECIMAL
POINT
ICL7106
BP
TEST
21
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 main-
tained for extended periods.
FN3082.8
7
INTERSIL [ INTERSIL CORPORATION ]
