TC815 datasheet(17/26 Pages) MICROCHIP

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DS21474B-page 17

TC815

8.0

LOW BATTERY DETECTION

CIRCUIT

The TC815 contains a low battery detector. When the

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

value, the LCD display low battery annunciator is

activated.

The low battery detector is shown in Figure 8-1. 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 8-1:

LOW BATTERY

DETECTOR

9.0

TRIPLEX LIQUID CRYSTAL

DRIVE

The TC815 directly drives a triplexed liquid crystal dis-

play (LCD) using 1/3 bias drive (see Figure 9-1). All

data, decimal point, polarity and function annunciator

drive signals are developed by the TC815. A direct

connection to a triplex LCD display is possible without

external drive electronics. Standard and custom LCD

displays are readily available from LCD manufacturers.

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.

If VDISP, for example, 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 sig-

nal 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 manufac-

turer’s displays by changing Vp. The liquid crystal

threshold voltage moves down with temperature.

“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 TC815 internally generates two intermediate LCD

drive potentials (VH and VL) from a resistive divider

(Figure 9-1), between VCC (Pin 28) and VDISP (Pin 7).

The ladder 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 9-2 shows a typical drive signal and the

resulting wave forms for “ON” and “OFF.” RMS voltage

levels across a selected LCD element. Also, see

Figure 9-3 and Table 9-1.

FIGURE 9-1:

1/3 BIAS LCD DRIVE

9.1

LCD Displays

Although most users will design their own custom LCD

display, several manufacturers offer standard displays

for the TC815. Figure 9-3 shows a typical display avail-

able from Varitronix.

1. Varitronix Ltd.

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

Kwun Tong, Hong Kong

Tel: (852)2389-4317

Part No.: VIM 310-1 Pin Connector

VIM 310-2 Elastomer Connector

USA OFFICE:

VL Electronics/Varitronix

3250 Wilshire Blvd. Suite 1901

Los Angeles, CA 90010

Tel: (213) 738-8700

2. Adamant Kogyo Co., LTD

16-7, Shinden, 1-Chome, Adachi-Ku,

Tokyo, 123, Japan

Tel: Tokyo 919-1171

TC815

Comparator

–

VCC

VSSA

Low Battery Detector

To LCD

Annunciator

Selection

Logic

≈ 7 X

= 6.2V

VZ 6.2V

R1 + R2

TC815

VSSA

To Triplex

Segment Drive

Logic

Set VDISP

For Proper VP

with Resistive

Divider

50k

VDISP

VCC

VP = VCC – VDISP

"OFF" = VP/3 RMS

"ON" =

VP RMS

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