TC815CBU View Datasheet(PDF) - Microchip Technology
Part Name
Description
Manufacturer
TC815CBU
Microchip Technology
TC815CBU Datasheet PDF : 26 Pages
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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
VCC
R1
R2
Low Battery Detector
TC815
VT +
R3
–
Comparator
To LCD
Annunciator
Selection
Logic
VZ 6.2V
VSSA
R2
VT ≈ 7 X R1 + R2 = 6.2V
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.
© 2002 Microchip Technology Inc.
TC815
“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 150kΩ. 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
VCC
28
TC815 50k
VH
To Triplex
Segment Drive
50k
Logic
VL
50k
VSSA
VP = VCC – VDISP
"OFF" = VP/3 RMS
11
"ON" =
3
3
VP RMS
Set VDISP
For Proper VP
with Resistive
Divider
6
VDISP
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
DS21474B-page 17
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