WS7106 / WS7107
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
Figures 7 and 8 show the digital section for the
WS7106 and
WS7107, respectively. In the WS7106
, an internal digital
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.
Figure 8 is the Digital Section of the
WS7107. It is identical
to the WS
7106 except that the regulated supply and back
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.
a
a
b
f
g
e
d
c
b
c
f
g
e
d
c
e
d
21
LCD PHASE DRIVER
7
SEGMENT
DECODE
7
SEGMENT
DECODE
7
SEGMENT
DECODE
V+
V+
BP
WS7106
DECIMAL
POINT
SELECT
TO LCD
DECIMAL
POINTS
TEST
CD4030
GND
FIGURE 6. EXCLUSIVE ‘OR’ GATE FOR DECIMAL POINT DRIVE
a
b
f
a
b
g
c
BACKPLANE
TYPICAL SEGMENT OUTPUT
V+
0.5mA
SEGMENT
OUTPUT
2mA
1000’s
COUNTER
INTERNAL DIGITAL GROUND
TO SWITCH DRIVERS
FROM COMPARATOR OUTPUT
CLOCK
÷
200
LATCH
100’s
COUNTER
10’s
COUNTER
1’s
COUNTER
1
V+
†
†
THREE INVERTERS
ONE INVERTER SHOWN FOR CLARITY
÷
4
INTERNAL
DIGITAL
GROUND
LOGIC CONTROL
6.2V
500Ω
TEST
V
TH
= 1V
37
26
40
OSC 1
OSC 2
39
OSC 3
38
V-
FIGURE 7. WS
7106 DIGITAL SECTION
7
WINGS [ WING SHING COMPUTER COMPONENTS ]
