MAX1491 Datasheet, PDF (10/25 Pages) Maxim Integrated Products – 3.5- and 4.5-Digit, Single-Chip ADCs with LCD Drivers

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
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
The MAX1491/MAX1493/MAX1495 contain an internal
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
the devices to accept negative inputs with high source
impedances. For the charge pump to operate correctly,
connect a 0.1µF capacitor from VNEG to GND.
LCD Driver
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)
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
results of the ADC.
Triplexing
An internal resistor string of three equal-value resistors
(52kΩ, 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.
Table 1. List of LCD Manufacturers
MANUFACTURER
WEBSITE
DCI, Inc.
www.dciincorporated.com
LXD, Inc.
www.lxdinc.com
Varitronix International Limited www.varitronix.com
The following site has more links to custom LCD
manufacturers: www.earthlcd.com/mfr.htm
X
Y
Z
a
f
b
g
e
c
d
a
BP1
f
b
g
BP2
e
c
d
DP
ANNUNCIATOR
BP3
DP
ANNUNCIATOR
Figure 3. Connection Diagrams for Typical Seven-Segment
Displays
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,
with BP2 to form the g segment, and with BP3 to form
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.
If ghosting is present on the LCD, the RMS OFF voltage
is too high. Choose an LCD with a higher RMS OFF
voltage or decrease DV_DD.
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