TC818A Просмотр технического описания (PDF) - Microchip Technology
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TC818A Datasheet PDF : 24 Pages
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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
VCC
Low Battery Detector
R1
TC818A
VT Comparator
+
R3
–
To LCD
Annunciator
Selection Logic
R2
VZ ≈ 6.2V
VSS
VT
=
73
R2
R1 + R2
=
6.2V
Low Battery Annunciator Displayed for VS < 7V.
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.
© 2002 Microchip Technology Inc.
TC818A
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 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 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
VCC
To VH
Triplex
Segment
Drive Logic VL
TC818A
50k
50k
See VDSIP
For Proper VP
with Resistive
50k
Divider
VDISP
VP = VCC - VDISP
"OFF" = VP/3 RMS
"ON" = 11 VP RMS
33
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
DS21475B-page 15
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