TC7116ACKW datasheet(9/22 Pages) MICROCHIP

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DS21457B-page 9

TC7116/A/TC7117/A

Analog commonisalsousedasVIN- 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 10

µA 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 500

Ω 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

FIGURE 3-4:

EXCLUSIVE “OR” GATE

FOR DECIMAL POINT

DRIVE

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.

TC7116

TC7116A

TEST

V+

GND

To LCD

Decimal

Point

To LCD

Backplane

4049

TC7116

TC7116A

Decimal

Point

Select

V+

TEST

GND

4030

To LCD

Decimal

Point

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