TC7116A(2002) View Datasheet(PDF) - Microchip Technology
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TC7116A Datasheet PDF : 22 Pages
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Analog common is also used as VIN- return during
auto-zero and de-integrate. If VIN- is different from ana-
log 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, VIN- 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 it can be conve-
niently referenced to analog common, it should be.
This removes the Common mode voltage from the
reference system.
Within the IC, analog common is tied to an N-channel
FET, that can sink 30mA or more of current to hold the
voltage 3V below the positive supply (when a load is
trying to pull the analog common line positive). How-
ever, there is only 10µA of source current, so analog
common may easily be tied to a more negative voltage,
thus overriding the internal reference.
3.1.7 TEST
The TEST pin serves two functions. On the TC7117/
TC7117A, it is coupled to the internally generated digi-
tal supply through a 500Ω resistor. Thus, it can be used
as a negative supply for externally generated segment
drivers, such as decimal points, or any other presenta-
tion the user may want to include on the LCD.
(Figure 3-3 and Figure 3-4 show such an application.)
No more than a 1mA load should be applied.
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 10mA under these conditions.
FIGURE 3-3:
SIMPLE INVERTER FOR
FIXED DECIMAL POINT
V+
TC7116
TC7116A
BP
21
TEST
37
V+
4049
GND
To LCD
Decimal
Point
To LCD
Backplane
TC7116/A/TC7117/A
FIGURE 3-4:
EXCLUSIVE “OR” GATE
FOR DECIMAL POINT
DRIVE
V+
V+
BP
TC7116
TC7116A
Decimal
Point
Select
To LCD
Decimal
Point
TEST
4030
GND
3.2 Digital Section
Figure 3-5 and Figure 3-6 show the digital section for
TC7116/TC7116A and TC7117/TC7117A, respectively.
For the TC7116/TC7116A (Figure 3-5), an internal dig-
ital ground is generated from a 6V zener diode and a
large P-channel source follower. This supply is made
stiff to absorb the relative large capacitive currents
when the backplane (BP) voltage is switched. The BP
frequency is the clock frequency 4800. For 3 readings
per second, this is a 60Hz square wave with a nominal
amplitude of 5V. The segments are driven at the same
frequency and amplitude, and are in phase with BP
when OFF, but out of phase when ON. In all cases,
negligible DC voltage exists across the segments.
Figure 3-6 is the digital section of the TC7117/
TC7117A. It is identical to the TC7116/TC7116A,
except that the regulated supply and BP drive have
been eliminated, and the segment drive is typically
8mA. The 1000's output (Pin 19) sinks current from two
LED segments, and has a 16mA drive capability. The
TC7117/TC7117A are designed to drive common
anode LED displays.
In both devices, the polarity indication is ON for analog
inputs. If VIN- and VIN+ are reversed, this indication can
be reversed also, if desired.
© 2002 Microchip Technology Inc.
DS21457B-page 9
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