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參數(shù)資料
| 型號(hào)： | TC818 |
| 廠商： | Microchip Technology Inc. |
| 英文描述： | Auto-Ranging Analog-to-Digital Converter with 3-1/2 Digit Display |
| 中文描述： | 自動(dòng)測(cè)距模擬到數(shù)字轉(zhuǎn)換器與數(shù)字顯示3-1/2 |
| 文件頁(yè)數(shù)： | 15/24頁(yè) |
| 文件大?。?/td> | 583K |
| 代理商： | TC818 |
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2002 Microchip Technology Inc.
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:
V
CC
- V
DISP
For example, if V
DISP
is set at a potential 3V below
V
CC
, the peak drive signal is:
V
P
= V
CC
- V
DISP
= 3V
An “OFF” LCD segment has an RMS voltage of V
P
/3
across it, or 1 volt. An “ON” segment has a 0.63V
P
signal across it or 1.92V for V
CC
-V
DISP
= 3V.
Since the V
DISP
pin is available, the user may adjust
the “ON” and “OFF” LCD levels for various manufactur-
ers’ displays, by changing V
P
signal across it, or 1.92V
for V
DISP
= 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 V
DISP
to
accommodate the liquid crystal temperature operating
characteristics, if necessary.
The TC818A internally generates two intermediate
LCD drive potentials (V
H
and V
L
) from a resistive
divider (Figure 3-11) between V
CC
and V
DISP
. The latter
impedance is approximately 150k
. This drive method
is commonly known as 1/3 bias. With V
DISP
connected
to digital ground, V
P
≈
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 V
S
< 7V.
V
CC
R
1
R
2
Low Battery Detector
V
T
R
3
V
Z
≈
6.2V
V
SS
Comparator
V
T
= 73 R
1
+ R
2
R
2
V
CC
50k
50k
50k
To
Triplex
Segment
Drive Logic
See V
DSIP
For Proper V
P
with Resistive
Divider
V
DISP
V
H
V
L
V
P
= V
CC
- V
DISP
"OFF" = V
P
/3 RMS
"ON" = 11
P
RMS
3
TC818A
3
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