TC7136ACPI datasheet(11/22 Pages) MICROCHIP

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DS21461B-page 11

TC7136/TC7136A

5.0

DIGITAL SECTION

The TC7136/A contains all the segment drivers neces-

sary to directly drive a 3-1/2 digit LCD. An LCD back-

plane driver is included. The backplane frequency is

the external clock frequency divided by 800. For three

conversions per second, the backplane frequency is

60Hz with a 5V nominal amplitude. When a segment

driver is in phase with the backplane signal, the seg-

ment is OFF. An out-of-phase segment drive signal

causes the segment to be ON, or visible. This AC drive

configuration results in negligible DC voltage across

each LCD segment, ensuring long LCD life. The polar-

ity segment driver is ON for negative analog inputs. If

VIN+and VIN- are reversed, this indicator would

reverse.

On the TC7136/A, when the TEST pin is pulled to V+,

all segments are turned ON. The display reads -1888.

During this mode, the 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 4 prior to clocking

the internal decade counters. The four-phase mea-

surement cycle takes a total of 4000 counts, or 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: 3000 to 2900 counts

(1200 to 11,600 clock pulses)

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

Zero integrator: 11 to 640 counts

The TC7136 is a drop-in replacement for the TC7126

and ICL7126. The TC7136A offers a greatly improved

internal reference temperature coefficient. Minor com-

ponent value changes are required to upgrade existing

designs and improve the noise performance.

6.0

COMPONENT VALUE

SELECTION

6.1

Auto-Zero Capacitor (CAZ)

The CAZ capacitor size has some influence on system

noise. A 0.47

µF capacitor is recommended for 200mV

full scale applications, where 1LSB is 100

µV. A 0.1µF

capacitor is adequate for 2V full scale applications. A

Mylar type dielectric capacitor is adequate.

6.2

Reference Voltage Capacitor

(CREF)

The reference voltage, used to ramp the integrator out-

put voltage back to zero during the reference integrate

phase, is stored on CREF.A 0.1µF capacitor is accept-

able when VREF- is tied to analog common. If a large

Common mode voltage exists (VREF- ≠ analog com-

mon) and the application requires a 200mV full scale,

increase CREF to 1µF. Rollover error will beheldtoless

than 0.5 count. A Mylar type dielectric capacitor is

adequate.

6.3

Integrating Capacitor (CINT)

CINT should be selected to maximize integrator output

voltage swing without causing output saturation. Ana-

log common will normally supply the differential voltage

reference in this case, a ±2V full scale integrator output

swing is satisfactory. For 3 readings per second

(FOSC = 48kHz), a 0.047µF value is suggested. For

one reading per second, 0.15

µF is recommended. If a

different oscillator frequency is used, CINT must be

changed in inverse proportion to maintain the nominal

±2V integrator swing.

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

Where:

tSI = 4000

FOSC









FOSC is the externally set clock frequency.

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