TC7126 datasheet(11/26 Pages) MICROCHIP

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DS21458C-page 11

TC7126/A

5.0

DIGITAL SECTION

The TC7126A contains all the segment drivers neces-

sary to directly drive a 3-1/2 digit LCD, including an

LCD backplane driver. The backplane frequency is the

external clock frequency divided by 800. For 3 conver-

sions per second, the backplane frequency is 60Hz

with a 5V nominal amplitude. When a segment driver is

in phase with the backplane signal, the segment is

OFF. An out of phase segment drive signal causes the

segment to be ON (visible). This AC drive configuration

results in negligible DC voltage across each LCD seg-

ment, ensuring long LCD life. The polarity segment

driver is ON for negative analog inputs. If VIN+ and VIN-

are reversed, this indicator reverses.

On the TC7126A, when the TEST pin is pulled to V+,

all segments are turned ON and the display reads -

1888

. During this mode, LCD segments have a

constant DC voltage impressed.

The display font and segment drive assignment are

shown in Figure 5-1.

FIGURE 5-1:

Display Font and Segment

Assignment

5.1

System Timing

The oscillator frequency is divided by four prior to

clocking the internal decade counters. The four-phase

measurement cycle takes a total of 4000 counts

(16,000 clock pulses). The 4000-count cycle is

independent of input signal magnitude.

Each phase of the measurement cycle has the following

length:

Auto-Zero Phase: 1000 to 3000 counts

(4000 to 12,000 clock pulses).

For signals less than full scale, the auto-zero

phase

assigned

the

unused

reference

integrate time period.

Signal Integrate: 1000 counts

(4000 clock pulses).

This time period is fixed. The integration period

is:

EQUATION 5-1:

Reference Integrate: 0 to 2000 counts

(0 to 8000 clock pulses).

The TC7126A is a drop-in replacement for the TC7126

and ICL7126, which offer a greatly improved internal

reference

temperature

coefficient.

external

component value changes are required to upgrade

existing designs.

Note:

Do not leave the display in this mode for

more than several minutes. LCDs may be

destroyed if operated with DC levels for

extended periods.

Display Font

1000's

100's

10's

1's

TSI = 4000

FOSC

Where: FOSC is the externally set clock frequency.

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