The second function is a “lamp test”. When TEST is pulled
high (to V+) all segments will be turned on and the display
should read “1888”. The TEST pin will sink about 15mA
under these conditions.
CAUTION: In the lamp test mode, the segments have a constant DC
voltage (no square-wave). This may burn the LCD display if main-
tained for extended periods.
Digital Section
Figures7and8showthedigitalsectionforthe
,aninternaldigital
ground is generated from a 6V Zener diode and a large
P-Channel source follower. This supply is made stiff to
absorb the relative large capacitive currents when the back
plane (BP) voltage is switched. The BP frequency is the
clock frequency divided by 800. For three readings/sec., this
is a 60Hz square wave with a nominal amplitude of 5V. The
segments are driven at the same frequency and amplitude
and are in phase with BP when OFF, but out of phase when
ON. In all cases negligible DC voltage exists across the
segments.
Figure8istheDigitalSectionofthe
7106exceptthattheregulatedsupplyandback
plane drive have been eliminated and the segment drive has
been increased from 2mA to 8mA, typical for instrument size
common anode LED displays. Since the 1000 output (pin 19)
must sink current from two LED segments, it has twice the
drive capability or 16mA.
In both devices, the polarity indication is “on” for negative
analog inputs. If IN LO and IN HI are reversed, this indication
can be reversed also, if desired.
V+
BP
TEST
DECIMAL
POINT
SELECT
CD4030
GND
V+
TO LCD
DECIMAL
POINTS
FIGURE 6. EXCLUSIVE ‘OR’ GATE FOR DECIMAL POINT DRIVE
7
SEGMENT
DECODE
SEGMENT
OUTPUT
0.5mA
2mA
INTERNAL DIGITAL GROUND
TYPICAL SEGMENT OUTPUT
V+
LCD PHASE DRIVER
LATCH
7
SEGMENT
DECODE
÷200
LOGIC CONTROL
INTERNAL
V
TH
= 1V
7
SEGMENT
DECODE
1000’s 100’s 10’s 1’s
TO SWITCH DRIVERS
FROM COMPARATOR OUTPUT
DIGITAL
GROUND
÷4
CLOCK
40 39 38
OSC 1
OSC 2
OSC 3
BACKPLANE
21
V+
TEST
V-
500Ω
37
26
6.2V
COUNTER
COUNTER COUNTER COUNTER
1
c
a
b
c
d
f
g
e
a
b
a
b
c
d
f
g
e
a
b
c
d
f
g
e
†
† THREE INVERTERS
ONE INVERTER SHOWN FOR CLARITY
7106 DIGITAL SECTION
WS7107, respectively. In the WS7106
