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3.5- and 4.5-Digit, Single-Chip
ADCs with LCD Drivers
Internal Clock
The MAX1491/MAX1493/MAX1495 contain an internal
oscillator. Using the internal oscillator saves board
X
Y
Z
space by removing the need for an external clock
source. The oscillator is optimized to give 50Hz and
60Hz power supply and common-mode rejection.
Charge Pump
a
a
The MAX1491/MAX1493/MAX1495 contain an internal
f
b
BP1
f
b
charge pump to provide the negative supply voltage for
the internal analog input/reference buffers. The bipolar
input range of the analog input/reference buffers allows
g
g
the devices to accept negative inputs with high source
e
c
BP2
e
c
impedances. For the charge pump to operate correctly,
connect a 0.1μF capacitor from VNEG to GND.
LCD Driver
DP
d
ANNUNCIATOR
d
The MAX1491/MAX1493/MAX1495 contain the neces-
sary backplane and segment driver outputs to drive
3.5-digit (MAX1491) and 4.5-digit (MAX1493/MAX1495)
BP3
DP
ANNUNCIATOR
LCDs. The LCD update rate is 2.5Hz. Figures 4–7 show
the connection schemes for a standard LCD. The
MAX1491/MAX1493/MAX1495 automatically display the
Figure 3. Connection Diagrams for Typical Seven-Segment
Displays
results of the ADC.
with BP2 to form the g segment, and with BP3 to form
Triplexing
An internal resistor string of three equal-value resistors
(52k ? , 1% matching) is used to generate the display
drive voltages. One end of the string is connected to
DV DD and the other end is connected to GND. Note that
V LCD (V LCD = DV DD - GND) should be three times the
threshold voltage for the liquid-crystal material used.
The connection diagram for a typical 7-segment display
font with two annunciators is illustrated in Figure 3 and
Figure 8. The MAX1491/MAX1493/MAX1495 numeric
display drivers (4.5 digits, 3.5 digits) use this configura-
tion to drive a triplexed LCD with three backplanes and
13 segment driver lines (10 for 3.5 digits). Figures 4 and
5 show the assignment of the 4.5-digit display segments
and Figures 6 and 7 show the assignment of the 3.5-
digit display segments.
The voltage waveforms of the backplane lines and y
segment line (Figure 3) have been chosen as an exam-
ple. This line intersects with BP1 to form the a segment,
Table 1. List of LCD Manufacturers
the d segment. Eight different ON/OFF combinations of
the a, g, and d segments and their corresponding
waveforms of the y segment line are illustrated in
Figures 9 and 10. The schematic diagram in Figure 8
shows that each intersection acts as a capacitance
from segment line to common line. Figure 11 illustrates
the voltage across the g segment.
The RMS voltage across the segment determines the
degree of polarization for the liquid-crystal material and
thus the contrast of the segment. The RMS OFF voltage
is always V LCD / 3, whereas the RMS ON voltage is
always 1.92V LCD / 3. This is illustrated in Figure 11. The
ratio of RMS ON to OFF voltage is fixed at 1.92 for a
triplexed LCD.
Figure 12 illustrates contrast vs. applied RMS voltage
with a V LCD of 3.1V. The RMS ON voltage is 2.1V and
the RMS OFF voltage is 1.1V. The OFF segment has a
contrast of less than 5%, while the ON segments have
greater than 85% contrast.
MANUFACTURER
DCI, Inc.
WEBSITE
www.dciincorporated.com
PART NUMBER
04-0924-00
04-0924-01
04-0925-00
04-0925-01
DESCRIPTION
3.5 digit, 5V
3.5 digit, 3V
4.5 digit, 5V
4.5 digit, 3V
The following site has links to other custom LCD manufacturers: www.earthlcd.com/mfr.htm
10
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