Datasheet ICL7106, ICL7107, ICL7107S (Intersil) - 7

link to page 7 link to page 7 link to page 8 ICL7106, ICL7107, ICL7107S Analog COMMON
V+
This pin is included primarily to set the common mode voltage for battery operation (ICL7106) or for any system where the input
V
signals are floating with respect to the power supply. The
REF HI
COMMON pin sets a voltage that is approximately 2.8V more
REF LO 6.8V
negative than the positive supply. This is selected to give a
ZENER
minimum end-of-life battery voltage of about 6V. However, analog COMMON has some of the attributes of a reference
ICL7106 IZ
voltage. When the total supply voltage is large enough to cause
ICL7107
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.
V-
The limitations of the on chip reference should also be recognized, FIGURE 4A. however. With the ICL7107, the internal heating which results from the LED drivers can cause some degradation in performance. Due
V+
to their higher thermal resistance, plastic parts are poorer in this respect than ceramic. The combination of reference Temperature
V 6.8kΩ
Coefficient (TC), internal chip dissipation, and package thermal
20kΩ
resistance can increase noise near full scale from 25µV to
ICL7106 ICL7107
80µVP-P. Also the linearity in going from a high dissipation count such as 1000 (20 segments on) to a low dissipation count such as
REF HI ICL8069
1111(8 segments on) can suffer by a count or more. Devices with
1.2V REFERENCE
a positive TC reference may require several counts to pull out of an
REF LO
over-range condition. This is because over-range is a low
COMMON
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. FIGURE 4B. All these problems are of course eliminated if an external FIGURE 4. USING AN EXTERNAL REFERENCE reference is used. The ICL7106, with its negligible dissipation, suffers from none of TEST these problems. In either case, an external reference can easily The TEST pin serves two functions. On the ICL7106 it is coupled be added, as shown in Figure 4. to the internally generated digital supply through a 500Ω resistor. Thus, it can be used as the negative supply for externally Analog COMMON is also used as the input low return during generated segment drivers such as decimal points or any other auto-zero and deintegrate. If IN LO is different from analog presentation the user may want to include on the LCD display. COMMON, a common mode voltage exists in the system and is Figures 5 and 6 show such an application. No more than a 1mA taken care of by the excellent CMRR of the converter. However, in load should be applied. 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
V+ 1MΩ
for the reference voltage. If reference can be conveniently tied to analog COMMON, it should be since this removes the common
TO LCD DECIMAL
mode voltage from the reference system.
POINT ICL7106
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
BP 21
below the positive supply (when a load is trying to pull the
TEST
common line positive). However, there is only 10µA of source
37 TO LCD
current, so COMMON may easily be tied to a more negative
BACKPLANE
voltage thus overriding the internal reference. 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. FN3082 Rev 9.00 Page 7 of 17 October 24, 2014