TC818A datasheet(15/24 Pages) MICROCHIP

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DS21475B-page 15

TC818A

3.13

Low Battery Detection Circuit

The TC818A contains a low battery detector. When the

9V battery supply has been depleted to a 7V nominal

value, the LCD low battery annunciator is activated.

The low battery detector is shown in Figure 3-10. The

low battery annunciator remains OFF with the battery

supply greater than 0.7V. The annunciator is ON before

the supply battery has reached 6.3V.

FIGURE 3-10:

LOW BATTERY

DETECTOR

3.14

Triplex Liquid Crystal Display

(LCD) Drive

The TC818A directly drives a triplexed liquid crystal

display (LCD) using 1/3 bias drive. All numeric data,

decimal point, polarity and function annunciator drive

signals are developed by the TC818A.

The LCDs must be driven with an AC signal having

zero DC component, for long display life. The liquid

crystal polarization is a function of the RMS voltage

appearing across the backplane and segment driver.

The peak drive signal applied to the LCD is:

VCC -VDISP

For example, if VDISP is set at a potential 3V below

VCC, the peak drive signal is:

VP =VCC -VDISP =3V

An “OFF” LCD segment has an RMS voltage of VP/3

across it, or 1 volt. An “ON” segment has a 0.63VP

signal across it or 1.92V for VCC -VDISP =3V.

Since the VDISP pin is available, the user may adjust

the “ON” and “OFF” LCD levels for various manufactur-

ers’ displays, by changing VP signal across it, or 1.92V

for VDISP =3V.

“OFF” segments may become visible at high LCD oper-

ating temperatures. A voltage with a -5 to -20mV/°C

temperature coefficient can be applied to VDISP to

accommodate the liquid crystal temperature operating

characteristics, if necessary.

The TC818A internally generates two intermediate

LCD drive potentials (VH and VL) from a resistive

divider (Figure 3-11) between VCC and VDISP. The latter

impedance is approximately 150k

Ω. This drive method

is commonly known as 1/3 bias. With VDISP connected

to digital ground, VP ≈ 5.0V.

The intermediate levels are needed so that drive sig-

nals giving RMS “ON” and “OFF” levels can be gener-

ated. Figure 3-12 shows a typical drive signal and the

resulting waveforms for “ON” and “OFF” RMS voltage

levels across a selected LCD element.

FIGURE 3-11:

1/3 BIAS LCD DRIVE

3.15

Liquid Crystal Displays (LCDs)

Most users will design their own custom LCD. How-

ever, for prototyping purposes, a standard display is

available from Varitronix, Ltd. This prototype display

configuration is shown in Figure 3-13 and Table 3-4.

Varitronix Ltd.

9/F Liven House, 61-63, King Yip Street

Kwun Tong, Hong Kong

Tel: (852)3-410286

Fax: (852)34-39555

Part No.: VIM-328-DP

USA Office:

VL Electronics, Inc.

3171 LOs Feliz Blvd, #303

Los Angeles, CA 90039

Tel: (213) 738-8700

TC818A

–

To LCD

Annunciator

Selection Logic

Low Battery Annunciator Displayed for VS < 7V.

VCC

Low Battery Detector

VZ ≈ 6.2V

VSS

Comparator

VT = 73

= 6.2V

R1 + R2

VCC

50k

Triplex

Segment

Drive Logic

See VDSIP

For Proper VP

with Resistive

Divider

VDISP

VP = VCC - VDISP

"OFF" = VP/3 RMS

"ON" =

VP RMS

TC818A

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